Electromagnetic Induction In electromagnetism we learnt that a current carrying conductor placed inside a magnetic field produces a force (motion). Conversely, if a conductor is moved perpendicularly through a magnetic field, an e.m.f is induced in that conductor causing a current to flow in the conductor.
If the wire is moved downward as shown, the galvanometer gives a reading. When it is moved upward it again gives a reading but in the opposite direction. Moving the wire in the magnetic field as shown induces an e.m.f which causes a current to flow in the wire. The direction of current can be found by using Fleming’s Right Hand Rule.
Faraday’s Law It states that “the e.m.f induced in a conductor is directly proportional to the rate of cutting of the magnetic field lines (flux)." REMEMBER: the conductor must move perpendicular to the magnetic field lines to produce a current. Moving it parallel to the field does not produce any current in it. In other words the faster you move the conductor inside a magnetic field, the more current is produced in it.
D.C. Generator The D.C. generator is just like a D.C. motor. The only difference is that instead of having a battery it has a galvanometer. In D.C. motor the battery provides a current and the coil moves. In D.C. generator we move the coil and current is produced in it due to electromagnetic induction. The galvanometer gives a reading. In D.C the current in the outer circuit flows in one direction only.
Outer circuit
A.C Generator
In an A.C. generator, the current in the external circuit reverses for every 1800 turn that the coil makes.
Difference between D.C and A.C outputs. D.C.
A.C.
Induction of e.m.f. with a magnet If a magnet is pushed into a coil of wire, such as the one below, attached to a sensitive ammeter, the needle is seen to move. This means the magnet pushes the free electrons around the circuit as a current.
However, once the magnet's motion stops, the needle returns to zero. There are three types of demonstrations with the magnet and the coil
Here the magnet is being pushed into the coil. The ammeter shows current induced in a positive direction.
Now the magnet is stationary inside the coil. There is no current being produced in the coil, shown by the zero reading on the ammeter.
Finally the magnet is being pulled out. The ammeter shows current being induced in the opposite direction to before.
The direction of current induced in the coil can be found by using Lenz’s law and the solenoid rule.
Lenz’s Law Lenz’s law states that “the current in the coil flows in such a direction as to oppose the motion producing it.” When the North Pole of the magnet is moved into the coil, a North Pole is created according to Lenz’s law as shown. The direction of current can then be found by using the right hand solenoid rule.
Imagine a magnet falling through an iron ring. According to Lenz’s Law, the ring will oppose this by
Direction of induced current
producing a North Pole at its upper end as shown. Now we can find the direction of current induced in it using the solenoid rule.
Direction of induced current
similarly, when it goes through the ring, the ring will oppose the moving away South Pole by attracting it with a N–pole.
Mutual Induction When switch is closed current flows through coil A and a magnetic field is crated around the coil. This induces a current in coil B. This is known as mutual induction. As current in A is D.C. it flows in one direction only. This means that the current in B is induced temporarily for a moment. It is like moving a magnet into the coil and then leaving it there not moving it. The galvanometer moves to the right and then back to its original position. A
B
To make current flow once again the switch is opened. This time the galvanometer moves to the left and then back again to its original position. To keep the current flowing in coil B we can replace the D.C. supply with A.C. This keeps the direction of current changing in coil A producing a changing magnetic field which in turn induces continuous flow of current in coil B. This effect is used in transformers.
NOTE: The amount of current induced in the coil in the A.C. Generator or the induction coils can be increased by: 1. 2. 3. 4. 5. 6.
Increasing the size of force on the conductor Moving it faster i.e. cutting the magnetic field lines more rapidly Using stronger magnets Using an iron core on which the wire is coiled Increasing the number of turns on the coil Increasing the number of coils aligned at different angles to the axis of turn.
Transformer The transformer converts current of one voltage to current of another voltage. A simple transformer consists of two coils wrapped around an iron core. Transformers rely on the property of mutual induction: the change in current in one coil induces an e.m.f. in another coil. The coil with the applied current is called the primary coil, and the coil with the induced e.m.f. is called the secondary coil.
This is a step-up transformer. There are more turns on the secondary coil so a low A.C. voltage in the primary coil will produce a high output voltage in the secondary coil.
This is a step-down transformer. There are less turns on the secondary coil so less voltage is produced in it.
The Transformer Formula �� �� �� = = �� �� �� Where, Np = number of turns on the primary coil Ns = number of turns on the secondary coil Vp = primary voltage Vs = secondary voltage Ip = current in the primary coil Is = current in the secondary coil.
Transmission of Electricity through overhead cables The voltage produced in a power station is stepped up using a transformer. This is then sent as a current to your homes where it is stepped down again using a transformer. Purpose of stepping up the voltage in the power station is to reduce energy loses to the surrounding when electric current is transported through long distances from the power station to your homes in overhead cables. Using high voltage means low current in the cables. A low current will produce less heating effect on its way and therefore less energy will be lost as heat to the surroundings. REMEMBER:
đ?‘ˇđ?’?đ?’˜đ?’†đ?’“, đ?‘ˇ = đ?‘˝đ?‘° and đ?‘° =
� �
so current and voltage are inversely proportional in this case.
Comparing overhead electric cables and underground electric cables OVERHEAD ELECTRIC CABLES 1. Subject to physical stress by weather and wind 2. Not affected by underground excavations 3. Come in way of tall moving objects 4. Takes less time to repair 5. Less expensive
UNDERGROUND ELECTRIC CABLES Unaffected by weather and wind Can be damaged by excavations Do not cause disturbance to tall moving objects Takes more time to repair More expensive
So the advantages and disadvantages of both overhead and underground electric power lines/cables are clear.
By Shafaq Hafeez shafaq@physics.com.pk