مراجعة الفيزيا لغات

Question 1) A- Mention only one function for each of the following : 1) The large resistance connected in series with the coil of the sensitive moving coil galvanometer.

2) The transformer.

3) Electric or magnetic fields in cathode ray tube.

4) The two springs in the moving coil galvanometer.

B) Find the value of the intensity of the electrical currents in all electrical resistance and voltage difference between both ends of each battery .

C) The electric circuit shown in figure consists of R1= 6Ί , R2 =3Ί , R3=2Ί A and a battery whose internal resistance is one ohm. If the current passing through R1 is 1 ampere, calculate: 1. The reading of the ammeter (A) 2. The reading of the voltmeter (V).

3. The e.m.f of the battery.

Question 2) Write down the scientific term that expresses each of the following statements :A First 1. Whenever an e.m.f is induced, the induced current must be in a direction such as to oppose the change producing it.

2. The incidence of high energy photon into a free electron, the photon frequency decreases, changes its momentum, and the electron velocity increases and changes direction .

3. The work done to transfer an electric charge of 4 coulombs between two points an electric circuit = 20 joule.

4. A very high resistance joined in series with the moving coil galvanometer, to convert it to voltmeter .

B) First : 1). Mention one application for: (a) Mutual induction between two coils. (b) Eddy currents

Second : Write down only one equivalent unit for each of the following mention the physical quantity measured by each of them: 1) Volt .Second

2) Volt .sec/Amp

4.The The torque acting on a coil carrying a current in a uniform magnetic field becomes maximum when the plane of the coil is:( perpendicular to - parallel to - inclined by an angle 30° to) the direction of the magnetic field.

2 5.Cathode rays have energy equal. ( h υ - 1 / 2 m v – m v - 2 m v ) .

6.If the frequency of the emitted photons from a glowing body increases, their number will….( increase , decrease , remain constant).

7. Two currents I, 2I pass through two parallel straight wires in opposite direction . When moving the wire Y further away from the wire X, the magnetic flux density at the point C …… (increase- decrease-does not change ) 8. At the instant when the coil of an AC dynamo is parallel to the direction of the magnetic flux, the value of the magnetic flux through the coil (Ø) and the induced electromotive force (E) would be: A B

(Ø) Maximum Zero

(E) Zero Maximum

C

Maximum

Maximum

D Zero Zero 9. At the instant of generating the maximum emf in the dynamo coil, the angle between the coil plane and the direction of the magnetic flux.( 0 – 45 – 90 )

Question 4) Give reason for each of the following: 1.

irregularity of the scale in The ammeter

2.

It is preferred to use the ultraviolet to incident on the cathode of photoelectric cell.

3.

The core in the transformer is made of thin insulated sheets of a siliconic soft iron.

4.

A movable rectangular coil carrying a direct current does not move when placed in a magnetic

field.

5.

In the hot wire ammeter the metallic wire is made of iridium platinum alloy.

6.

Doubling the radius of a copper wire leads to decreasing its resistance to one quarter.

Question 5) What is meant by each of the following?

1.

Work function of a surface.

2.

The electric conductivity of a certain material equals 4 ohmֿٰ​ٰ.meter.

3.

The effective value of AC.

4.

Eddy current .

6) Question : First.

Compare Between : 1. The magnetic flux density at the centre of a circular coil and that at a point inside a solenoid on its axis from the point of view of its relation with the radius of turns.

2. . Connecting a number n of identical electric bulbs to a battery of negligible internal resistance. (series –parallel ) in The current intensity.

3. Ampere’s right hand rule and Fleming’s left hand rule with respect to the application of each.

4. The primary coil and the secondary coil in step up transformer with respect to their number of turns.

5. Hot Wire Ammeter -Moving coil Ammeter(The reason of the pointer deflection) 6. Lenz's rule and Fleming's right hand rule (concerning their uses).

Second What is the scientific principle upon which the operation of the device is based: (a) The electric motor? (b) The electric generator?

Question 7) :.

1. A rectangular coil of dimensions 20 cm x 10 cm and of 200 turns placed in a regular field of magnetic flux density of 0.4 Tesla. A current of 3 Amp is passing through the coil.

Calculate the torque acting on the coil in the following two cases: First : When the coil plane subtends an angle 60° to the direction of the field.

Second : When the coil plane is perpendicular to the direction of the field.

2. An electric power 400 kilowatts transmitted from power station to factory through electric line its resistance 5 Ohms given that the potential difference at the power station = 20000 volts ,

calculate :

1. Current intensity in the electric line .

2.The transmitted power.

3. Transmitting efficiency.

3. A series circuit consists of a condenser of reactance 30Ω, a resistance of 44Ω and a coil of 90Ω inductive reactance and 36Ω resistance. The circuit is supplied from 60 Hz sinusoidal source with 200 V. Calculate : 1- The circuit current.

2- The potential difference across each element of the circuit.

Question 8) :first .

1-A resistance of 2000 Ί is connected to the terminals of an ohmmeter. The ohmmeter pointer deflects to half-scale. What is the value of the resistance that if connected to the ohmmeter terminals causes its pointer to deflect to quarter of the current scale?

2- Explain why a rectangular coil placed perpendicular to the magnetic field direction is not affected by any torque when a current passes through it, although there are forces affecting its sides.

3- ( Write down two characteristics of the magnetic flux produced as an electric current passes through a straight wire.

Second.

1 -Mention one factor that can increase: (a) Self-inductance of a coil. (b) The maximum electromotive force in the dynamo coil.

Model Answer Question 1): 1-convert to voltmeter.

2- Raising or lowering E.M.F.

3- To control electron beam to sweep the screen point by point generating the picture 4- To control the rotational motion of the coil and also serve as current leads to the coil.

B)

I2+I3-I1=0  

In the pass DCBAD In the pass ABEFA

24 = 6I3 - 3I: 16 = 3I2 - 21

(I1 = 6A , I2 = A , I3 =

by using calculator

C) 1- The Reading of the ammeter A:V across R1= IR = 1×6 = 6V 2- I tolal (through R3) = 1+2 = 3 A 3- VB = IR total + Ir = 3(

V across R3 = 3×2 = 6V ×

Question 2): first A 1- Lenz’s law.

2-Compton effect.

3-Potential difference = 5 volt.

4-Multiplier resistance (Rm).

First B 1-transforner 2-electric furnse Second 1-weber 2-henary

+ )+3×1 = 15V

A)

Question 3): 1-decrease 2-6 volt 3-10 volt 4-parallel to.

2

5-1/2 mv

6-constant.

7-increase.

8-C. 9-0.

Question 4): 1-due to the weakness of spiral spring elasticity and the weakness of the magnet with time.

2- Because the ultraviolet has high frequency .

3- To minimize the energy lost in the iron core in a form of eddy currents which produce heat inside the core.

4- The magnetic dipole moment is parallel to the field and the couple is zero. ::: OR: in this case θ=0 and the couple = 0 .

5- Because the resistance R is inversely proportional with ('radius)2 or R = pe L/A Or R Îą 1/A Or R Îą 1/r2.

Question 5: 1- The least energy of a photon needed to free (liberate) an electron from the metallic solace.

2- The resistivity of this material is 1/4 ohm . m.

3- The effective value of the A.C current ( Ieff ) is the value of the direct current which generates the same

rate of thermal effect in a resistance as that generated by the considered AC current

( Ieff = 0.707 Imax

e.m.feff = 0.707 e.m.fmax ).

4- Eddy Currents: Are induced currents in closed path produced in the metal cores inside the coils

of motors, generators, transformers due to the change of magnetic flux passing through these cores by i) the variable currents passing in these coils or; ii) by moving the solid conductor in a suitable magnetic field.

Question6 :first: 1-At the centre of a circular coil B =  

I .e B α ,

Or the magnetic flux density is inversely proportional with the radius , In case of solenoid B = μN l / L i . e ( B ) does not depend on the radius.

2-a-series :nR---------I=constant b-parallel : R/n------I=I1+I2+I3 3-

Ampere's right hand rule : to determine the direction of the magnet:: flux lines due to a current in a straight wire.

Fleming left hand rule : to determine the direction of the force on the current carrying wire placed in a magnetic field 4- The number of turns of primary coil less than number of turns of secondary coil.

5hot wire ammeter :thermal expansion moving coil ammeter:torque 6Len's rule: To define the direction of the induced current generated by induction in a coil.

Fleming's R. H. R: indicate the direction of the induced current in a wire moves in a magnetic field

Second: 1-torque 2-electric induction Question 7): 1τ =B I A N *sin (30)= . ×

×

×

×

×

×

τ =zero 2Power at station = 400 K.W = 4 x 105 watts P=V×I

:: I=(

× )/(

× ) =20A

P = I2 × R

:: P= 20 × 20 × 5 = 2000W

= .

.

The transmitting power. = Original power − Lost power =( 4×

Efficiency % = 3Z=√

+(

−

.

× ×

)2

×

)-(2×

= 99.5% :: Z=

+ (

−

)2 = 100Ω

I= / = 200/100 = 2A VR= I×R = 2×44=88V ZL = (

)+(

)2 = 96.93Ω

:: VC = I×XC = 2×30=60V :: VL = I × ZL = 2 × 96.93 = 193.86V

Question 8): first : 1-4000Ω 2-due to inertia 3-circular and obey ampere right hand role for direction determination .

Second : 1-iron core. a-number of turns.

)=3.98×105 watts