9th Physics Notes English Medium

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2018

9th PHYSICS Short question answers. All questions from textbook are answered. All the questions from BISE Lahore PHYSICS paper from 2013-18 are answered with suitable picture diagrams.

Exercise Short Questions

All Questions from Past Papers

Available At: hamzaayub333@gmail.com hamzaayub333.blogspot.com issuu.com/hamzaayub333 03234515251 Ayub‫حمزه‬ STARS ACADEMY Shahdara Town Lahore. 8/14/2018

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PHYSICAL QUANTITIES AND MEASUREMENT

1.2 What is difference between base and derived quantities? Give 3 examples in each case. [L.B. 2015,16,17] Base Quantities: “Base quantities are the quantities on the basis of which other quantities are expressed.� Examples; Length, mass and time are base quantities. 1.3 Pick out the base units in the following; ampere, meter, Kelvin, coulomb and watt.

Derived Quantities: “The quantities that are expressed in terms of base quantities are called derived quantities.� Examples; Volume, speed and force are derived quantities. joule, Newton, kilogram, hertz, mole,

Base Units: Kilogram, mole, ampere, meter and Kelvin. Derived Units: Joule, Newton, hertz, coulomb and watt. 1.4

Find the base quantities involved in the following derived quantities:

a) speed

b) volume

c) Force

d) work

a) Speed: Speed is derived from length and time. b) Volume: Volume is derived from length. c) Force: Speed is derived from mass, length and time. d) Work: Speed is derived from mass, length and time. [L.B. 2014,15,17,18]

1.5 Estimate your age in seconds. Age in Seconds:

= = 15 Ă— 1 = 15 Ă— 365 = 15 Ă— 365 Ă— 1

= 15 Ă— 365 Ă— 24 â„Ž = 15 Ă— 365 Ă— 24 Ă— 1 â„Ž

= 15 Ă— 365 Ă— 24 Ă— 60 = 15 Ă— 365 Ă— 24 Ă— 60 Ă— 1 = 15 Ă— 365 Ă— 24 Ă— 60 Ă— 60 = 15 Ă— 31536000 = = 473040000

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[L.B. 2016]

1.6 What role SI units have played in development of science?

Importance of SI: System international has fulfilled the need of a commonly acceptable worldwide system of units for science and technology. 1.7 What is meant by Vernier constant?

[L.B. 2014,15,17**]

Vernier Constant: “The minimum measurement that can be taken by vernier callipers is called least count of vernier callipers or vernier constant.” Value:

Vernier constant = 0.1 mm = 0.01 cm

1.8 What do you understand by zero error of a measuring instrument?

[L.B. 2014]

Zero Error: “The extent of error in any measuring instrument is called zero error of that instrument.” Types: Zero error may be Positive or Negative. 1.9 Why is the use of zero error necessary in a measuring instrument? Use of zero error: Use of zero error ensures the precision and accuracy of measurement taken by the measuring instrument. 1.10 What is a stop watch? What is the least count of mechanical stop watch you have used in laboratories? Stop Watch: “A stop watch is used to measure time interval of an event.” Least Count of Mechanical Stop Watch = 0.1 s Least Count of Digital Stop Watch = 0.01 s 1.11 Why do you need to measure extremely small interval of times? Measuring Interval of Time: Different physical quantities like displacement, velocity and acceleration, power and momentum are related to time. So we need to measure extremely small intervals of time to get more precise readings. 1.12 What is meant by significant figures of a measurement? Significant Figures: “All the accurately known digits and first doubtful digit in an expression are called significant figures.” 1.13 How is precision related to significant figures in a measuring quantity? Precision & Sig. Figs.: Number of significant figures reflects the precision of a measured value of a physical quantity.

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IMPORTANT SHORT QUESTIONS FROM BISE LAHORE 01 Define physics.

[L.B. 2014,15]

Physics: “Physics is the branch of science in which we study matter, energy and their interactions.” Mechanics, optics, sound and electromagnetism are few branches of physics. 02 Define plasma physics, geo physics and nuclear physics.

[L.B. 2016,17,18]

Plasma Physics: “Plasma physics is the study of production and properties of ionic state of matter – fourth state of matter.” Nuclear Physics: Nuclear Physics is the study of properties and behavior of nuclei and particles within the nuclei.” Geo Physics: “Geo physics is the study of internal structure of earth.” 03 How many divisions are present on vernier scale?

[L.B. 2016]

No. of Divisions: 10 divisions are present on vernier scale of vernier callipers. 04 What is meant by prefixes? Give example.

[L.B. 2016]

Prefixes: “Prefixes are the words or letters added before SI units. These are multiples of 10.” Examples: Common examples of prefixes are kilo (k = 103), milli (m = 10-3), centi (c = 10-2) etc. 05 Write 2 rules to find significant figures in a measurement. Rules:

[L.B. 2016]

1. Digits other than zero are significant. 2. Zero between significant digits is also significant.

06 Write the names of 2 length measuring instruments.

[L.B. 2017]

Instruments: Two common length measuring instruments are metre rule and measuring tape. 07 What do you mean by scientific notation? Give one example.

[L.B. 2018]

Scientific notation: “A way to express a given number as a number between 1 and 10 multiplied by 10 having an appropriate power is called scientific notation or standard form.”

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KINEMATICS

2.2 Explain translatory motion and give examples of various types of translator motion. [L.B. 2016,17] Translatory Motion: “In translational motion, a body moves along a line without any rotation. The line may be straight or curved.” Types: There are 3 types of translatory motion. 1. Linear motion: A car moving in a straight line and objects falling downward have linear motion. 2. Circular motion: A bicycle moving on a circular track possesses circular motion. Motion of moon around earth is circular motion. 3. Random motion: The motion of insects is random motion. The motion of dust particles in air is also random motion. 2.3 Differentiate between the following; (1) Rest and Motion Motion: “A body is said to be in motion, if it Rest: “A body is said to be at rest, if it does not change its position with respect to its changes its position with respect to its surroundings.” surroundings.” Example: A passenger sitting inside a moving bus is at rest with respect to other passengers but to another observer outside the bus passenger inside the bus is in motion. (2) Circular motion and rotatory motion Circular Motion: “The motion of an object in circular path is called circular motion.” Example: Motion of earth around sun. (3) Distance and displacement

Rotatory Motion: “The spinning motion of an object about its axis is called its rotatory motion.” Example: Motion of earth about its own axis. [L.B. 2016,17]

Distance: “Length of a path between two points is called distance between those points.” Quantity: Distance is a scalar quantity.

Displacement: “Displacement is the shortest distance between two points which has magnitude and direction.” Quantity: Displacement is a vector quantity.

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[L.B. 2014,15]

(4) Speed and velocity

Speed:: “The distance covered by an object in Velocity:: “The rate of displacement of a unit time is called its speed.� body is called velocity.� Quantity: Speed is a scalar quantity. Quantity: Velocity is a vector quantity. (5) Linear and random motion [L.B. 2014] Linear Motion: “Straight line motion of a body is called linear motion.� Example: A car moving in a straight line and objects falling downward have linear motion. (6) Scalars and vectors

Random Motion: “The The disordered or irregular motion of an object is called random motion.� Example: The motion of insects is random motion. The motion of dust particles in air is also random motion. [L.B. 2014,15**,17**] 2014

Scalars: “A scalar quantity is described Vectors: “A A vector quantity is described completely by its magnitude only only.� completely by it magnitude and direction.� direction Example: Distance, time, speed etc. Example:: Force, displacement, velocity etc. 2.4 Define the terms speed, velocity and acceleration. [L.B. 2015**,17] Speed: “The The distance covered by an object in unit time is called its speed.� Velocity: “The The rate of displacement of a body is called velocity.� Acceleration: “Acceleration is defined as the rate of change of velocity of a body.� 2.5 Can a body moving at constant speed have acceleration?

[L.B. 2014] 2014

Body with Constant Speed: Yes Yes! The body moving with constant speed may have acceleration if it is moving in circular pa path. 2.6 How riders in Ferris wheel possess translator motion but not rotator motion? Motion of Riders:: The motion of riders is; Translatory because they move in circular path without rotation. Not rotatory because the axis of rotation is not passing thro through ugh the wheel. 2.7 Sketch a distance-time time graph for a body starting from rest. How will you determine the speed of a body from this graph? Graph: Distance-time time graph is shown. Its slope gives the speed of object. Consider points A & B on the graph. = ! ! #$ =

20 " " = = 10 %& 2

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2.8 What would be the shape of a velocity velocity-time graph of a body moving with variable speed? [L.B. 2014,15] v-t Graph: Velocity-time time graph of a body moving with variable speed is shown. 2.9 Which of the following can be obtained from speed speed-time graph of a body; (1) Initial speed (2) Final speed (3) Distance covered in time t (4) Acceleration of motion. Speed-Time Graph: All of the above quantities can be find by vv-tt graph if the body is moving in straight line. 2.10 How can vector quantities be represented graphically?

[L.B. 2014]

Graphical Representation: To represent a vector graphically: Draw a straight line (length of line is magnitude of vector). Put an arrowhead at on end (to signify direction). 2.11 Why vector quantities cannot be added and subtracted like scalar quantities? Vector Addition:: Vectors cannot be added like scalars due to their direction. direction During vector addition, direction is also added. 2.12 How are vector quantities important to us in our daily life?

[L.B. 2016]

Importance: Vector quantities es are important to us in Locating exact position of objects. Indicating direction of motion of object. Drawing maps. 2.14 Sketch a velocity-time time graph for the motion of the body. From the graph explaining each step, calculate total distance covered by tthe body. Velocity-Time graph: (2nd Equation of motion.) IMPORTANT SHORT QUESTIONS FROM BISE LAHORE 01 Convert speed of 20 ms-11 into kmh-1. ' = 20 %& = 20 ×

[L.B. 2016] 3600 × 10( %& 10( × 3600

72000 ) 20 × 3600 10( ' = × = × 1000 ℎ 3600 10( ' = 72 *+,%&

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[L.B. 2016]

02 Define uniform speed.

Uniform Speed: “A body has uniform speed if it covers equal distance in equal intervals of time however short the interval may be.” 03 What is meant by gravitational acceleration?

[L.B. 2016]

Gravitational Acceleration: “The acceleration of freely falling objects is called gravitational acceleration.” Value: The value of gravitational acceleration g on the surface of earth is 10 ms-2. [L.B. 2016,17]

04 Define uniform acceleration.

Uniform Acceleration: “A body has uniform acceleration if it has equal changes in velocity in equal intervals of time however short the interval may be.” [L.B. 2017]

05 Define vibratory motion.

Vibratory Motion: “To and fro motion of a body about its mean position is called vibratory motion.” Example: Motion of pendulum and motion of child sitting on sea saw is vibratory motion. [L.B. 2018]

06 Define acceleration and write its unit.

Acceleration: “Acceleration is defined as the rate of change of velocity of a body.” Unit: SI unit of acceleration is ms-2.

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DYNAMICS

3.2 Define the following terms; (1) Inertia [L.B. 2015,16] Inertia: “Inertia of a body is its property due to which it resists any change in its state of rest or motion.” Dependence: Inertia of a body depends upon its mass.

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[L.B. 2014,15]

(2) Momentum

Momentum: “Momentum Momentum of a body is the quantity of motion it possesses due to its i mass and velocity.” -./ = 0 ./

Formula: Unit: SI unit of moment is kgms-1.

[L.B. 2016]

(3) Force

Force: “A A force moves or tend tends to move, stops or tends to stop the motion of a body. The force can also change the direction of motion of a body. body.” Unit:: Unit of force is Newton ((N). (4) Force of friction Friction: “The force that opposes the motion of a moving object is called friction.” Friction depends upon nature of surface, contact area and pressing force. [L.B. 2015,16,17]

(5) Centripetal force.

Centripetal Force: “Centripetal Centripetal force is a force that keeps a body to move in circle.” Formula:

12 =

345 6

3.3 What is the difference between; (1) Mass and weight Mass: “Mass is the amount of matter in any substance.” Quantity: Mass is a scalar quantity quantity.

[L.B. 2014,15**] Weight: “Weight is the force of gravitation acting on an object.” Quantity: Weight is a vector quantity.

(2) Action and reaction Action and Reaction: If “The force that book applies on the table downward is referred as action.” Then “The upward force by the table on book will be referred as reaction.” (3) Sliding and rolling friction.. Rolling Friction: “The force Sliding Friction: “The forcee of friction faced rce of friction faced by a surface when it slides over other other.” by a surface when it rolls over other.” other Note: Rolling friction is 100 times less than sliding friction.

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3.4 What is the law of inertia? OR State Newton’s 1st law of motion.

[L.B. 2014,16,18]

Law of Inertia: Newton’s first law of motion is also called as law of inertia. Statement: “A body continues its state of rest or uniform motion in a straight line provided no net force acts on it.” 3.5 Why is it dangerous to travel on the roof of a bus? Travel on the Roof: The passengers traveling on the roof of bus are always in a danger to fall in front of bus. Reason: When brakes are applied, the passengers continue their straight line motion due to inertia. 3.6 Why does a passengers move outward when a bus takes a turn? Passengers Move Outward: When a bus takes a sharp turn, passengers continue their straight line motion due to inertia. So they feel moving outward from the bus. 3.7 How can you relate a force with the change of momentum of a body? Force and Momentum: “When a force acts on a body, it produces acceleration in the body and will be equal to the rate of change of momentum of the body.” Mathematical Form:

1=

78 %79 :

3.8 What will be the tension in the rope that is pulled from its ends by two opposite forces 100 N each? Tension: The tension in the string is equal to the force acting on either side of string. So the tension will be 100 N. 3.9 Action and reaction are always equal and opposite. Then how does a body move? Action and Reaction: Action and reaction act on different bodies. Forces of action and reaction produce acceleration in different bodies and cannot nullify the effect of each other. 3.10 A horse pulls the cart. If action and reaction are equal and opposite the how does the cart move? Horse-Cart: Action and reaction act on different bodies. Forces of horse and cart produce acceleration in different bodies and cannot nullify the effect of each other. Hence the cart moves.

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3.11 What is the law of conservation of momentum? Law of Conservation of Momentum Momentum: “The momentum of an isolated system of two or more than two interacting bodies remains constant constant.� 3.12 Why is the law of conservation of momentum important? Importance: Law of conservation of momentum is important in ord order er to understand The movement of system of gun bullet bullet. The rocket and jet engine. Almost all isolated systems. 3.13 When a gun is fired, it recoils. Why?

[L.B. 2018]

Gun Recoils: The gun recoils to conserve the momentum of system. Before firing, both the gun and bullet are at rest, so the total momentum of system is zero. The bullet gets momentum when fired, so the gun recoils in opposite direction. 3.14 Describe two conditions in which force of friction is needed.

[L.B. 2017]

Need of Friction: To walk or to run friction is needed to push the ground backward. Friction is highly desirable when climbing up a hill hill. 3.15 How does oiling ng the parts of a machine lower friction? Oiling: Due to oiling parts of machine become smooth the movement of one surface over other becomes easy. Hence friction is lessened. 3.16 Describe ways to reduce friction.

[L.B. 2014]

Reducing Friction:: The friction can be reduced by

Making sliding surfaces smooth Making fast moving objects streamlined shape Lubricating the sliding surfaces Using ball bearings or roller bearings bearings.

3.17 Why rolling friction is less than slid sliding friction? Rolling Friction: During rollingg motion contact area is minimum and rolling surface rolls without rupturing the cold welds which makes rolling friction about 100 times less than sliding friction. 3.18 What do you know about the following;

[L.B. 2014,18]

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(1) Tension in a string Tension: “The force acting along a stretched string is called as tension.” (2) Limiting Force of Friction:: ““The maximum amount of friction that can be generated between two static surfaces in contact with each other is referred as limiting force of friction.” (3) Braking Force: “A A sufficient amount of friction must be generated between tyres of vehicle and surface of track which can stop the vehicle, this friction is called braking br force.” (4) Skidding of Vehicle: “If brakes are applied too strongly, the tyres of vehicle will lock up (stop turning) and will slip on the track. This slipping is called skidding of vehicle.” (5) Seatbelts: Seatbelts secure the person sitting in fast ast moving vehicle from harmful collision. When brakes are applied the person inside the vehicle continues his motion due to inertia and may collide with windscreen or any other thing. (6) Banking of Roads:: “Banking of road means that outer edge of road is raised.” raised Advantage: Banking causes a component of vehicle’s weight to provide necessary centripetal force while taking a turn. Thus banking of road prevents skidding of vehicle and thus make makes the driving safe. (7) Cream separator: Cream separator is a high speed spinner used to control the fat contents of various products. Principle of Working:: Cream separator works on the same principle of centrifuge machine. The high speed spinning bowl causes the heavier contents of milk to move outward in the bowll and lighter contents move in centre where it is collected through pipe. 3.19 What would happen if all friction suddenly disappears? Friction Disappears:: If all friction disappears we will not able to move on earth, it will be difficult to stop any moving ving object, birds will not be able to fly etc. 3.20 Why the spinner of washing machine is made to spin at high speed? Spinner: When spinner is rotated at high speed, the water droplets from wet clothes are forced out through holes of container due to lack of centripetal force. IMPORTANT SHORT QUESTIONS FROM BISE LAHORE 01 What is meant by centrifugal force? [L.B. 2014] Centrifugal Force: “The The opposite reaction of centripetal force is called centrifugal force.” Direction:: The direction of centrifugal fforce is outward from the centre of circle.

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02 The weight of a body is 147 N. What is its mass?

[L.B. 2015]

; ℎ = ; = = 147 < ∴ = 3 =

> 147 = 10 ?

= = 14.7 ) [L.B. 2017]

04 Define the SI unit of force. Unit of Force: SI unit of force is kgms-2 which is equal to Newton (N).

Newton: “One newton (1N) is the force that can produce an acceleration of 1 ms-2 in a body of mass 1 kg.” 1 < = 1 ) × 1 %A 05 State Newton’s 3rd law of motion.

[L.B. 2017]

Newton’s 3rd Law of Motion: “To every action there is an equal but opposite reaction.” Example: When the fuel of rocket burns, hot gases escape out from tail with high speed. The reaction of these gases on the rocket causes it to move opposite to gases rushing out of its tail.

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TURNING EFFECT OF FORCE

4.2 Define the following; (1) Resultant vector Resultant Vector: “Such a vector, which shows the combined effect of two or more vectors is called resultant vector.” NOTE: Two or more vectors are added to find resultant vector by head to tail rule. (2) Torque Torque: “The turning effect of a force is called torque or moment of force.” Dependence: Torque depends on force B ∝ 1 and Torque also depends on moment arm B∝D Formula:

B=1×D

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[L.B. 2014,15,17]

(3) Centre of mass

Centre of Mass: “Centre Centre of mass of a system is such a point where an applied force causes the system to move without rotation rotation.” [L.B. 2014**,16**]

(4) Centre of gravity.

Centre of Gravity: “A point where the whole weight of the body appears to act vertically downward is called centre of gravity of a body.” 4.3 Differentiate the following; [L.B. 2014,15**,17]

(1) Like and unlike forces Like Parallel Forces: “Like Like parallel forces are the forces that are parallel to each other and have same direction.” Example:: Weights of apples in a bag are vertically downward. These are like parallel forces. (2) Torque and couple

Unlike Parallel Forces:: “Unlike parallel paralle forces are the forces that are parallel but have opposite direction to each other.” Example: Weight of apple,, suspended by string, and tension in string vertically upward are unlike parallel forces. [L.B. 2016]

Torque: ““The turning effect of a force is called torque or moment of force.”

Couple: “A couple is formed med by two unlike parallel forces of the same magnitude but not along the same line. Couple is a combination of unlike parallel forces which produces torque.

Torque is produced by a single force or by a combination of forces. (3) Stable and neutral equilibrium.

Neutral Equilibrium:: “A body is said to be in neutral equilibrium if after a slight tilt it remains in its new w position.” position Position of C.G.: In neutral equilibrium centre of gravity remains in the same position irrespective to its new position. 4.4 How head to tail rule helps to find the resultant of forces? Stable Equilibrium: “A A body is said to be in stable equilibrium rium if after a slight tilt it returns to its previous position.” Position of C.G.: In stable equilibrium centre of gravity is at lowest position position.

Head to Tail Rule: Head to tail rule is graphical m method ethod to find resultant of 2 or more than 2 vectors. ./ and .F ./ as 1st vector Description: Consider any of the vectors E ./ and .F ./ graphically such and the other as 2nd. Draw the vectors E that tail of the 2nd vector coincides with the head of 1st vector. ../ Similarly draw the next vector (if any). Now draw the vector G (resultant vector) such that its tail is at the tail of 1st vector while its head is at the head of last vector.

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4.6 When a body is said to be in equilibrium? [L.B. 2015,16] According to 1st condition: “A body is said to be in equilibrium if resultant of all forces acting on it is zero.” According to 2nd condition: “A body is said to be in equilibrium if resultant of all torques acting on it is zero.” So, a body is in equilibrium if it satisfies either both or any one condition for equilibrium. 4.7 Explain the first condition for equilibrium. First Condition: “A body is said to satisfy 1st condition for equilibrium if resultant of all forces acting on it is zero.” H1 = I Example: A book lying on the table or a picture hanging on the wall satisfy the 1st condition for equilibrium. 4.8 Why there is a need of second condition for equilibrium if a body satisfies first condition for equilibrium? Need of 2nd Condition: Only the 1st condition for equilibrium does not ensure that a body is in equilibrium as in the figure (a) resultant of F1 and F2 is zero (1st condition is satisfied). When location of F1 and F2 is shifted as in figure (b), the body has tendency to rotate. So F 2 body is not in equilibrium. 4.9 What is second condition for equilibrium?

[L.B. 2016,18]

Second Condition: “A body is said to satisfy 2nd condition for equilibrium if resultant of all torques acting on it is zero.” HB = I Example: A ladder leaning at a wall satisfies the 2nd condition for equilibrium. 4.10 Give an example of a moving body which is in equilibrium. Example: Any object moving in straight line with uniform velocity is in equilibrium. A paratrooper coming down with terminal velocity (constant velocity) also satisfies 1st condition for equilibrium. 4.11 Think of a body which is at rest but not in equilibrium.

[L.B. 2016]

Body at Rest: In ideal situation, two forces act on a body thrown vertically upward. One is the applied force vertically upward and other is weight vertically downward. At peak point, body is at rest and applied force is zero but weight acts on body. Hence body is not in equilibrium in presence of single force.

F1

F2

F1

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4.12 Why a body cannot be in equilibrium due to single force acting on it?

[L.B. 2015]

Single Force: Because single force cannot make resultant force to be zero. Hence the body is not in equilibrium. 4.13 Whyy the height of vehicles is kept as low as possible? Height of Vehicles:: The height of vehicles is kept low to make centre off gravity as lower as possible which makes the equilibrium of vehicle more stable. 4.13 Explain what is meant by stable, unstable and neutral equilibrium. Give an example in each case. Stable Equilibrium:: “A body is said to be in stable equilibrium if after a slight tilt it returns to its previous position.� Example: When we slightly tilt a book lying on table by lifting it from opposite side, it returns to previous position. This state of book is stable equilibrium. Unstable Equilibrium:: “A body is said to be in unstable equilibrium if after a slight tilt it does not return to its previous position.� Example: A pencil balanced on its tip is in unstable equilibrium. Neutral Equilibrium:: “A body is said to be in neutral equilibrium if after a slight tilt it remains in its new position position.� Example: A ball is in neutral equilibrium because it always remains in new displaced position. IMPORTANT SHORT QUESTIONS FROM BISE LAHORE 01 Define perpendicular components components.

[L.B. 2014]

Perpendicular Components:: ““The The components of a force which are mutually perpendicular to each h other are called perpendicular components.� 02 In a right angled triangle length of base is 4 cm and its perpendicular is 3 cm. Find length of hypotenuse. [L.B. 2014] $ = 4 L - ! = 3 L K =MM By Pythagorean Theorem

JJK . NA = J$ NA O J- . NA JK JK . NA = J4NA O J3. NA = 16 O P QJK . NA = R25 = 5 K = 5

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03 Define moment arm, rigid body and axis of rotation?

[L.B. 2014,15,16,17]

Moment Arm: “The perpendicular distance between axis of rotation and the line of action of force is called moment arm.� Rigid Body: “If the distance between particles of a body remains unchanged, the body will be called as rigid body.� 04 Write the mathematical form of conditions of equilibrium. 1st Condition for Equilibrium:

H1 = I

2nd Condition for Equilibrium:

HB = I

[L.B. 2017]

[L.B. 2017**]

05 What is meant by resolution of forces?

Resolution of Force: “Splitting up of a force in 2 mutually perpendicular components is called resolution of force.� [L.B. 2018]

06 Define resolution of forces.

Resolution of Forces: “Splitting up of a force into perpendicular components is called resolution of forces.� The perpendicular components are 1S = 1TUVW, 1X = 1 YZ[\

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GRAVITATION

[L.B. 2015**,16**,17]

5.2 What is meant by the force of gravitation?

Force of Gravitation: “The force due to which every body of the universe attracts every other body.� &

Formula: ] ^ ! _ � & ] ^ ! _ � abcdefgh i ] ^ ! _ = 1 =

j 3 35 k5

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5.3 Do you attract the earth or the earth attracts you? Which one is attracting with a larger force? You or the earth. By Gravitation Law: “Everybody in the universe attracts every other body with a force which is directly proportional to the product of their masses and inversely proportional to the square of distance between their centres.” Thus, we and earth all attract each other with a mutual (same) force. [L.B. 2018]

5.4 What is a field force?

Field Force: “Field force is such a force which one body can apply on other weather the body is in contact with it or not.” Example: Gravitational force is an example of field force. [L.B. 2017]

5.5 Why earlier scientists could not guess about gravitational force?

Guess about Gravitational Force: Due to very small value of G, the gravitational force of attraction between objects around us is very small. That is why earlier scientists could not guess it. 5.6 How can you say that gravitational force is a field force? Gravitational Field Force: The velocity of a body, thrown up, goes on decreasing while on returns its velocity goes on increasing. The earth’s gravitational force produces acceleration in the body weather the body is in contact with earth or not. So earth’s gravitational force is a field force. 5.7 Explain, what is meant by gravitational field strength?

[L.B. 2014,15,18]

Gravitational Field Strength: “In the gravitational field of earth, the gravitational force per unit mass is called the gravitational field strength of the earth.” Value: On the surface of earth value of gravitational field strength is 10 N kg-1. 5.8 Why is law of gravitation important to us? Importance: Gravitation law is important for us because we can use it; To calculate gravitational force of attraction between objects To calculate the mass of Earth and other solar planets To understand the orbital motion of natural and artificial satellites.

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[L.B. 2014,17**,18]

5.9 Explain the law of gravitation.

Gravitation Law: “Everybody in the universe attracts every other body with a force which is directly proportional to the product of their masses and inversely proportional to the square of distance between their centres.� Mathematical Form:

] ^ ! _ = 1 =

j 3 35 k5

5.10 How can the mass of earth be determined? Mass of Earth: Mass of earth can be calculated by gravitation law. lm = Here,

?n5 = 6.0 Ă— 10Ao ) j

lm = p â„Ž, ? = ^ ! ! , n = p â„Ž, j = ] ^

5.11 Can you determine the mass of moon? If yes then what do you need to know? Mass of Moon: Yes! Mass of moon can be determined by the following formula; ?3 Jn3 N5 l3 = = 7.35 Ă— 10AA ) j Here, lm = , ? = ^ ! ! , n3 = , j = ] ^

5.12 Why does the value of g vary from place to place? Value of g: As by formula,

?q =

[L.B. 2016]

rst JuvqNi &

?q � ∴ gh is inversely proportional to JuvqNi Jw O ℎNA So value of gh varies with height and is different on different places. 5.13 Explain how value of g varies with altitude? Value of g: As by formula,

rs

?q = JuvqNt i &

?q � JuvqNi ∴ gh is inversely proportional to Jw O ℎNA At a height equal to one earth radius above earth surface g becomes one fourth of its value and so on.

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5.14 What are artificial satellites? Artificial Satellites: “Scientists Scientists have sent many objects into space. ace. Some of these objects revolve around earth.. These are called artificial satellites.� Use:: Most of the artificial satellites orbiting the earth are used for communication purposes. 5.15 How Newton’s law of gravitation helps in understanding the motion motio of satellites? Understanding the Motion otion of Satellites Satellites: A satellite requires centripetal force keeps it to move around the earth. The gravitational force of attraction between earth and satellite provides the necessary centripetal force. So Newton’s law of gravitation is important to understand the motion of satellites. 5.16 On what factors the orbital speed of a satellite depends? Factors: By formula;

[L.B. 2016]

z{ = Q q Ă— |}

Here, xy = ! ^ ! , ?~ = ^ ! ! â„Ž â„Ž ~,, 6y =

So, orbital speed of satellite depends on q ( ^ ! ! â„Ž â„Ž ~) and 6y J = w â„Ž O #! ). 5.17 Why communication satellites are stationed at geostationary orbits? Satellites in Geostationary Orbits Orbits: Communication satellites are stationed at geostationaryy orbits so that signal transmission could face no problem. Explanation: Communication satellites take 24 hours to complete their one revolution around earth.. As earth also completes its one rotation about its own axis in 24 hours so they appear to be stationary. ionary. Hence they face no problem in signal transmission. IMPORTANT SHORT QUESTIONS FROM BISE LAHORE

01 Describe navigation system. [L.B. 2014] Navigation System: GPS (Global Positioning System) is a satellite navigation system. It helps us to find exact position of object anywhere on land, on sea and in air. GPS consists of 24 satellites revolving at speed of 3.87 kms-1 around the earth.

P a g e | 20

02 What is the height and speed of geostationary satellite with respect to earth?[L.B. 2014] Height of Geostationary Satellite: The height of geostationary satellite from centre of earth is 42300 km and 35900 km from surface of the earth. Speed of Geostationary Satellite: The speed of geostationary satellite with respect to earth is zero. rs

03 How the equation q = JuvqNt i changes when R is doubled? Change in gh: As by formula,

[L.B. 2015]

rs

?q = JuvqNt i ?q ∝

& JuvqNi

∴ gh is inversely proportional to

Jw O ℎNA

At a height equal to one earth radius above earth surface, g becomes one fourth of its value and so on. 04 What is the orbital speed of lower orbiting satellite?

[L.B. 2016]

Orbital Speed: The orbital speed of lower orbiting satellite is 8000 m/s and can be calculated by following formula z{ = Q q × |} . 05 Write the definition of satellite and give example.

[L.B. 2017]

Satellite: “An object that revolves around a planet is called a satellite.” Example: Moon revolves around earth. So moon is a natural satellite of earth. 06 What is meant by geostationary satellite?

[L.B. 2017]

Geostationary Satellite: “The satellite which remains stationary with respect to earth is called geostationary satellite.” These satellites take 24 hours to complete their one revolution around the earth. The earth also completes its one rotation about its axis in 24 hours. So these satellites appear to be stationary with respect to earth. 07 Why we cannot feel gravitational force around us?

[L.B. 2018]

Feeling Gravitational Force: Due to small value of G, the gravitational force of attraction between objects around us is very small and we do not feel it.

P a g e | 21

U

nit 06

WORK AND ENERGY

[L.B. 2014,15,17]

6.2 Define work. What is its SI unit?

Work: “Work is done when force acting on a body displaces it in the direction of the force.â€? Unit: The SI unit of work is Joule (J). Definition of Joule: “The amount of work done is one joule when a force of one newton displaces a body through one metre in the direction of force.â€? 6.3 When does a force do work? Explain. Work by Force: Work is said to be done when a force acts on a body and moves it in direction of force. = 1׀ This work done depends on the force acting, displacement covered by body and the angle between force and displacement. 6.4 Why do we need energy? Need of Energy: As “Energy is the ability of a body to do workâ€? So, we need energy to do some useful work in daily life. 6.5 Define energy give two types of mechanical energy. Energy: “A body possesses energy if it is capable to do work.â€? Types: Two types of mechanical energy are 1) Kinetic Energy 2) Potential Energy. [L.B. 2014,15**]

6.6 Define K.E. and derive its relation.

Kinetic Energy: “The energy possessed by a body due to its motion is called its kinetic  . p. = ) energy.â€? Formula:

&

 . p. = ^ A A

= ^ = ^ !

6.7 Define potential energy and derive its relation. Potential Energy: “The energy possessed by a body due to its position is called its potential = energy.� Formula:

] ^ ! - ! p = â„Ž

= ^ ! ! â„Ž = â„Ž â„Ž

P a g e | 22

6.8 Why are fossil fuel called non non-renewable form of energy? Fossil Fuels: Current supply of fossil fuels will soon be exhausted and ffossil ossil fuels took millions of years for their formation. That is why fossil fuels are called as non-renewable non sources of energy. 6.9 Which form of energy is preferred and why? Preferred Energy Form: Electric energy is preferred over other formss of energy and is widely used because it is easy to convert in forms and easy to supply to any desired place. 6.10 How is energy rgy converted from one form to another? Explain.

[L.B. 2014]

Energy Conversion:: Energy can be converted from one form to another by doing work. Explanation: For example, rub your hands together quickly. You will feel them warm. You have ve used your muscular energy in rubbing your hands. By the process of rubbing mechanical energy can be converted into heat. 6.11 Name any five devices that convert electrical energy into mechanical energy. Devices:: Following are the devices that convert electrical energy to mechanical energy. 1) Electric Fan 2) Electric Drill 3) Electric Motor 4) Washing Machine 5) Sinner 6.12 Name a device that convert converts mechanical energy to electrical energy.

[L.B. 2016]

Device: Electric Generator conver converts mechanical energy to electric energy.

Mechanical Energy

Electric Generator

Electric ectric Energy

6.13 What is meant by the efficiency of a system?

[L.B. 2014]

Efficiency of System: “Efficiency Efficiency of a system is the ratio of required form of energy obtained from the system as output to the total energy given to it as input.� p =

‚ !

- p p =

‚ Ă— 100ƒ 100 !

P a g e | 23

6.14 How can you find the efficiency of a system? Efficiency: Efficiency of a system can be calculated by dividing ‚ on ! .

p =

„h…†b„ha ‡{„ˆ {‡ {†d‰†d d{deŠ bf‰†d hfh„‹Œ

[L.B. 2014,15,16,17,18]

6.15 What is meant by the term power? Power: “Power is defined as the rate of doing work.� -=

Formula:

�{„Ž a{fh dbˆh

[L.B. 2014,16]

6.16 Define watt.

Watt: “The power of a body is one watt if it does work at the rate of one joule per second.� 1 � =

1 Â? 1

IMPORTANT SHORT QUESTIONS FROM FROM BISE LAHORE 01 Write Einstein’s equation and value of c.

[L.B. 2014,15]

Einstein’s Equation: Einstein’s equation ‘ = 325 shows the relation between mass and energy. Here,

2 = ! â„Ž = 3 Ă— 10Â’ %&

02 What is difference between sound energy and mechanical energy?

[L.B. 2014,15]

Sound Energy: “Sound energy is produced by a vibrating surface.�

Mechanical Energy: “The energy possessed by a body due to its motion or position is called mechanical energy.� 03 Differentiate between chemical and mechanical energy. [L.B. 2015]

Chemical Energy: “The energy stored in Mechanical Energy: “The energy possessed form of chemical bond in a substance is by a body due to its motion or position is called chemical energy.� called mechanical energy.� 04 Define mechanical energy and give example. [L.B. 2017] Mechanical Energy: “The energy possessed by a body due to its motion or position is called mechanical energy.� Example: The energy possessed by running water or energy stored in stretched bow. 05 Write 2 major disadvantages of fossil fuels.

[L.B. 2017]

Disadvantages of Fossil Fuels: 1) Fossil fuels causes air pollution. 2) Fossil Fuels are nonrenewable sources of energy.

P a g e | 24

06 A body of mass 50 kg is raised to a height of 3 m. What is its potential energy? [L.B. 2018] Potential Energy:

= 50 ) ,

ℎ ℎ = 3 ,

By definition,

7. . = 3?~

Substituting values,

-. p. = J50NJ10NJ3N

-. p. = M

-. p. = 1500 07 What is meant by nuclear energy?

[L.B. 2018]

Nuclear Energy: “Nuclear energy is the energy released in the form of nuclear radiations during nuclear reactions such as fission and fusion reaction.” Benefit: Heat energy released in nuclear reactors can be converted into electrical energy.

U

nit 07

PROPERTIES OF MATTER

7.2 How is kinetic molecular model of matter helpful in differentiating various states of matter? [L.B. 2014,16] Kinetic Molecular Model: Kinetic molecular model of matter is helpful in differentiating various states of matter as it describes all states as follows: All states of matter are made up of particles called molecules. The molecules remain in continuous motion. Molecules attract each other. 7.3 Does there exist a fourth state of matter? What is that?

[L.B. 2017]

Fourth State of Matter: Yes! Plasma is called fourth state of matter. At very high temperature, the collisions between atoms and molecules tears off their electrons. Atoms become positive ions. This ionic state of matter is called plasma. 7.4 What is meant by density? What is its SI unit? Density: “Density of a substance is defined as its mass per unit volume.” Unit: The SI unit of density is kilogram per cubic metre (kgm-3).

[L.B. 2017]

P a g e | 25

7.5 Can we use a hydrometer to measure the density of milk? Hydrometer: Yes!! A hydrometer can be used to measure the density of milk. The hydrometer must be calibrated as lactometer to measure density of milk. [L.B. B. 2015,16**,18]

7.6 Define the term pressure.

Pressure: “The force acting normally per unit area on the surface of a body is called pressure.” Formula:

76mYY 6m =

1y62m 6m

7.7 Show that atmosphere exerts pressure. Atmosphere Exerts Pressure:: Place a tin can, half filled with water, over flame. Wait till water begins to boil and steam expels the air out of the can. Now place the can can, over chilled running water, with closed cap. The steam in can condenses by running water and changes into water leaving an empty space be behind it. This lowers the pressure inside the can and atmospheric pressure outside will cause the can to collapse from aall directions. This shows that atmosphere exerts pressure. 7.8 It is easy to remove air from balloon but it is very difficult to remove air from a glass bottle. Why? Air Removal: Balloon is filled with compressed gas or air. The pressure of air inside the t balloon is higher than outside. That is why air rushes out from balloon easily while the pressure of air inside the bottle is same as outside. Hence it is difficult to remove air from bottle. [L.B. 2018]

7.9 What is a barometer? Barometer: “The instrument that measures the atmospheric pressure is called barometer.” Mercury barometer is a simple barometer that consist 1 m long glass tube filled with mercury and d closed at one end end. 7.10 Why is water not suitable to be used in barometer?

[L.B. 2014] 2014

Water in Barometer: The height of water column at sea level will be 13.6 times greater than that of mercury column i.e. 0.76 m x 13.6 = 10.34 m due to greater density of water.. Thus a glass tube more than 10 m long is required to make a water barometer barometer.. That is why water is not suitable to be used in barometer.

P a g e | 26

7.11 What makes a sucker pressed on a smooth wall sticks to it? Sucker: The air between sucke sucker and wall expels out on pressing it hence outside atmospheric pressure becomes higher than inside pressure and makes the sucker pressed on the wall. 7.12 Why does atmospheric pressure vary with height? Atmospheric Pressure Variation Variation: Atmospheric pressure depends on density of air. Density of air changes with height hence atmospheric pressure varies with height. 7.13 What does it mean when the atmospheric pressure at a place fall suddenly? Atmospheric Pressure Fall Suddenly Suddenly: Rapid fall in atmospheric pressure indicates a windy and showery condition in the nearby region. 7.14 What changes are expected in weather if the barometer reading shows a sudden increase? Changes in Weather: A sudden increase in reading shows increase in atmospheric pressure with ith a decline later on predicting an intense weather condition. 7.15 State Pascal’s law.

[L.B. 2015**,17]

Pascal’s Law: “Pressure applied at any point of a liquid enclosed in a container, is transmitted without ithout loss to all other parts of the liquid.” Applications: Pascal’s law finds its numerous applications in automobiles, hydraulic brake system, hydraulic jack and hydraulic press etc. 7.17 What is meant by elasticity?

[L.B. 2016,17**]

Elasticity: “The The property of a body to restore its original size and shape as deforming force ceases to act is called elasticity. elasticity.” 7.18 State Archimedes principle.

[L.B. 2016]

Archimedes Principle: “When When an object is totally or partially immersed in a liquid, an upthrust acts on it equal to the weight of the liquid it displaces. 7.19 What is upthrust? thrust? Explain the principle of floatation. Upthrust: “Upthrust Upthrust on a body immersed in liquid is eq equal ual to the weight of the liquid displaced.” Principle of Floatation: “A A floating object displaces a fluid having weight equal to the weight of the object.”

P a g e | 27

7.20 Explain how a submarin submarine floats on the water surface and dives down in water? Submarine Floats: Submarine floats over water when weight of water equal to its volume is greater than its weight. Submarine Dives:: Submarine dives into water when its tanks are filled with seawater aand its weight increases than upthrust. 7.21 Why does a piece of stone sink in water but a ship with a huge weight floats? [L.B. 2014,15] Piece of Stone:: Piece of stone sinks in water because the weight of displaced water is less than its own weight. Ship Floats:: Ship floats over water because the weight of displaced water is greater than its own weight. [L.B. 2017]

7.22 What is Hook’s law? What is meant by elastic limit? Hook’s Law: “The The strain produced in a body by the stress applied to it is directly proportional to the stress within the elastic limit of the body.� Y:6mYY = 2y[Y:•[: Y:6•Z[

7.23 Take a rubber band. Construct a balance of your own using rubber band. Check its accuracy by weighing various objects. IMPORTANT SHORT QUESTIONS FROM BISE LAHORE 01 The mass of 200 cm3 of stone is 5500 g. Find its density. Density:

= 500 ,

^ ! = 200 ( ,

[L.B. 2014,18] " = M

3•YY

By definition,

–m[YZ:X = xy—“3m

Substituting values,

" =

˜™™ A™™

" = 2.5 %( 02 Define stress. Stress: “The force acting on unit are area at the surface of a body is called stress.� Unit:: In SI the unit of stress is newton per square metre (( š 3%5 ).

[L.B. 2016]

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U

nit 08

THERMAL PROPERTIES OF MATTER

8.2 Why does heat flows from hot body to cold body? Heat Flow: When two bodies of different temperature come into contact with each other, the temperature of the hot body falls and it loses energy. This energy enters the cold body due to which its temperature rises. The transfer of energy continues till both bodies have the same temperature. 8.3 Define the terms heat and temperature.

[L.B. 2014**,16**]

Heat: “Heat is the energy that is transferred from one body to the other in thermal contact with each other as a result of the difference of temperature between them.” Unit: Unit of Heat energy is joule (J). Temperature: “Temperature of the body is the degree of hotness or coldness of the body.” Unit: SI unit of temperature is Kelvin (K). 8.4 What is meant by internal energy of a body?

[L.B. 2014]

Internal Energy: “The sum of kinetic energy and potential energy associated with the atoms, molecules and particles of a body is called its internal energy.” ! p = . . O7. . 8.5 How does heating affect the motion of molecules of a gas?

[L.B. 2015]

Effect of Heat: Gas molecules move randomly and when heat energy is given to these molecules their kinetic energy increases due to which their velocity is increased. 8.6 What is thermometer? Why mercury is preferred as a thermometric substance? Thermometer: “A device which is used to measure the temperature of a body is called thermometer.” Mercury as Thermometric Material: Mercury is preferred as thermometric material because it has all the thermometric properties like low freezing point i.e. -39oC, high boiling point i.e. 357oC and small specific heat capacity.

P a g e | 29

8.7 Explain the volumetric thermal expansion. Volumetric Thermal Expansion: “The volume of a solid changes with the change in temperature and is called as volumetric thermal expansion.â€? Â?â„Ž ^ ! = z − z{ = Â&#x;z{ âˆ†ÂĄ 8.8 Define specific heat. How would you find the specific heat of solid?[L.B. 2014,15,17,18] Specific Heat: “Specific heat of the substance is the amount of heat required to raise the temperature of 1 kg mass of that substance through 1 K.â€? Formula:

' K = 2 =

∆¢ 3âˆ†ÂŁ

[L.B. 2016]

8.9 Define and explain latent heat of fusion.

Latent Heat of Fusion: “Heat energy required to change unit mass of a substance from solid to liquid state at its melting point without any change in its temperature is called its latent heat of fusion.� Formula:

¤ K _ = ¼Œ =

ƢΠ3

8.10 Define and explain latent heat of vaporizations. Latent Heat of Vaporization: “Heat energy required to change unit mass of a liquid completely into gas at its boiling point without any change in its temperature is called its latent heat of vaporization.� Formula:

¤ K z § = ¼4 =

∆¢4 3

8.11 What is meant by evaporation? On what factors the evaporation of a liquid depends? Explain how cooling is produced by evaporation? [L.B. 2015] Evaporation: “Evaporation is the changing of a liquid into vapours from the surface of liquid without heating it.� Factors: Evaporation of a liquid depends on temperature, surface area, wind and nature of liquid. Evaporation Causes Cooling: During evaporation, fast moving molecules escape from the surface of liquid leaving lower K.E. molecules behind. This lowers the average kinetic energy of liquid molecules and temperature of liquid. Hence evaporation causes cooling.

P a g e | 30

IMPORTANT SHORT QUESTIONS FROM BISE LAHORE 01 What will be the temperature on Kelvin scale when it is 20 C° on Celsius scale? [L.B. 2014] ¡ = ¡J { N = 20 C°,

Temperature:

¡ = ¡J N =M

By Formula,

¡ = £J N = £Jªy N O 5«¬

Substituting values,

¡ = ¡J N = 20 O 273 ¡ = ¡J N = 2P3 [L.B. 2014]

02 Define coefficient of linear thermal expansion.

Coefficient of Linear Thermal Expansion: “Coefficient of linear thermal expansion of a substance is defined as the fractional increase in its length per Kelvin rise in temperature.” C­®¯°±²±®³´ ­¯ l±³®a| ´,®|+al ®xpa³s±­³ = º =

∆D Dy ∆£ [L.B. 2014,18]

03 Convert 100 F° into temperature on Celsius scale. Temperature:

¡ = ¡J { N =M

¡ = ¡J_N = 100 F°,

By Formula,

¡ = £Jªy N =

£J1y N%¬5

Substituting values,

¡ = ¡J { N =

& %(A

.¼

&.

=

½ &.

¡ = ¡J { N = 37.8 { 04 What is meant by thermal equilibrium?

[L.B. 2015]

Thermal Equilibrium: “The property of a system when all parts of the system have the same temperature along with its surrounding.” 05 Write 2 uses of bimetallic strip.

[L.B. 2015]

Uses: Following are uses of bimetallic strip; 1) Bimetallic strips are used in thermostats. 2) Bimetal thermometer is used to measure temperature in furnaces and oven. 06 What is meant by upper and lower fixed points of thermometer?

[L.B. 2015]

Upper Fixed Point: “The upper fixed point is the mark that shows the position of mercury in the thermometer when it is placed in steam from boiling water at standard pressure.” Lower Fixed Point: “The lower fixed point is the mark that shows the position of mercury in the thermometer when it is placed in ice”

P a g e | 31

[L.B. 2014]

07 Convert 300 K into temperature on Celsius scale. Temperature:

¡ = ¡J N = 300 K,

¡ = ¡J { N =M

By Formula,

¡ = £Jªy N = £J N − 5«¬

Substituting values,

¡ = ¡J { N = 300 − 273 { ¡ = ¡J { N = 27 {

08 Write the names of types of latent heat.

[L.B. 2015]

Types of Latent Heat: Two types of latent heat are 1) Latent Heat of Fusion and 2) Latent Heat of vaporization. 09 Write two applications of thermal expansion.

[L.B. 2016]

Applications: 1) Thermal expansion is used in thermometer for temperature measurements. 2) Fixation of iron rims on wooden wheels. 3) Use of bimetallic strip 10 Define thermal expansion.

[L.B. 2016]

Thermal Expansion: “Most of the substances expand on heating, this phenomenon is called thermal expansion.” Types: Its 2 types are linear thermal expansion and volumetric thermal expansion. 11 Which scales are used to measure temperature?

[L.B. 2017]

Scales: Three scales are used to measure temperature. 1) Celsius Scale or Centigrade Scale 2) Fahrenheit Scale 3) Kelvin Scale. 12 What is meant by melting point and freezing point?

[L.B. 2017]

Melting Point: “The temperature at which a solid starts melting is called its melting point.” Freezing Point: “The temperature at which a substance changes from liquid to solid is called its freezing point.” 13 Gaps are left in railway tracks. Why?

[L.B. 2017]

Railway Track Gaps: Gaps are left in railway tracks to compensate thermal expansion during hot season.

P a g e | 32

U

nit 09

TRANSFER OF HEAT

9.2 Why are metals good conductors of heat? Metals: Metals have free electrons which remain in continuous motion and can transfer heat energy at a very fast rate from hot part to cold part. So metals are conductors due to free electrons. 9.3 Explain why: a. A metal feels colder to touch than wood kept in a cold place? Metal Feels Colder: Metal is good conductor while wood is insulator. Metal transfers the heat energy from your hand on touching touching, due to which it feels colder. b. Land breeze blows from land to sea? Land Breeze: Att night, the land cools faster than the sea. Therefore, air above the he sea is warmer, rises up and the cold air from the land begins to move towards the sea. It is called land breeze. c. Double walled glass vessel is used in thermos flask? Double Walled Glass Vessel:: Double walled glass vessel is used to reduce heat transfer as; Vacuum between Double walled glass vessel vessels reduces conduction of heat. Outer glass vessel is silvered from inner side and the other from outside which reduces heat transfer by radiation. d. Deserts soon get hot during the day and soon get cold after suns sunset? et? Deserts: Deserts soon get hot during the day and soon get cold after sunset due to less specific heat capacity of sand i.e. 835.0 J kg-1 K-1. 9.4 Why conduction of heat does not take place in gases? Conduction in Gases: Conduction can take place due e to vibrations of molecules packed close or due to free electrons. Gases do not have closely packed molecules or free electrons. That is why gases are not good conductors of heat.

P a g e | 33

[L.B. 2016**]

9.5 What measures do you suggest to conserve energy in houses?

Conserve Energy in Houses: Following measures may be taken to conserve energy; Hot water tanks are insulated by plastic or foam lagging. Wall cavities are filled with plastic foam or wool. 9.6 Why does transfer of heat in fluids tak take place by convection? Transfer of Heat in Fluids: Molecules of fluids are far apart due to weak intermolecular forces. On heating these hese molecules move from hotter place to colder place and hence transfer heat by convection. [L.B. 2018]

9.7 What is meant by convecti convection current?

Convection Current: “On On heating, molecules of fluid start flow from hot part to cold part which is called convection current.” Example: Rise of smoke from chimneys is aan n example of convection current. 9.9 How does the heat reach us from the sun?

[L.B. 2016]

Heat from Sun: The heat energy reaches us neither by conduction nor by convection, because the space between Sun un and Earth’s atmosphere is empty. It is through radiation that heat reaches us from the sun. 9.11 What is greenhouse house effect?

[L.B. 2015,17**]

Greenhouse Effect: “Layer of carbon dioxide and water vapours traps infrared radiations emitting out from earth surface and does not allow them to escape escape, hence behaves like greenhouse. This phenomenon is called greenhouse effect.” Greenhouse effect causes rise in average earth temperature called global warming. 9.12 Explain the impact of greenhouse effect in global warming? Impact: During the recent years, the percentage of CO2 has been increased considerably. This has caused an increase in average temperature of earth which is called global glob warming. This has serious implications for the global climate.

P a g e | 34

IMPORTANT SHORT QUESTIONS FROM BISE LAHORE 01 Write the names of modes of transfer of heat.

[L.B. 2014]

Modes of Heat Transfer: 1) Conduc Conduction 2) Convection 3) Radiation, are three modes of heat transfer. 02 Why dark colored clothes are prohibited to wear in summer?

[L.B. 2014,17]

Dark Colored Clothes: Dark colored clothes being good absorber of heat radiations can cause us to feel hot. That is why dark colored clothes are prohibited to wear in summer. sum 03 Differentiate between land breeze and sea breeze. Land Breeze: “Land breeze blows from land to sea during night.”

[L.B. 2014]

Sea Breeze: “Sea breeze blows from sea to land during day time.”

[L.B. 2014]

04 Define thermal conductivity.

Thermal Conductivity: “The The rate of flow of heat across the opposite faces of a metre cube of a substance maintained at a temperature difference of one kelvin is called the thermal conductivity of that substance.” À=

¢ Á × : J£ − £5 N

05 Describe the effect of length on thermal conductivity.

[L.B. 2015**]

Effect of Length: Thermal conductivity is a material property and is independent of change in length, but rate of heat flow through a sample changes inversely with length .

P a g e | 35

06 On which factors radiations depend?

[L.B. 2015]

Radiations Dependence: The rate of emission of radiation depends on 1) Colour & Texture of surface 2) Surface Temperature 3) Surface Area 07 Define convection.

[L.B. 2016**,17]

Convection: “Transfer of heat by actual movement of molecules from hot place to a cold place is known as convection.” 08 Describe the effect of cross sectional area on thermal conductivity.

[L.B. 2017]

Effect of Area: Thermal conductivity is a material property and is independent of change in cross sectional area, but rate of heat flow through a sample changes directly with cross sectional area. 09 What causes a glider to remain in air?

[L.B. 2017]

Glider: “Gliders ride over thermals (upward moving hot air currents due to convection). The upward movement of air currents in thermals helps them to stay in air for a long period. 10 Write two names of expert thermal climbers.

[L.B. 2018]

Expert Thermal Climbers: Eagles, hawks and vultures are expert thermal climbers.

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