7. Electromagnetic Induction

Question 1

Faraday’s Law of Electromagnetic Induction

Answer 1

Faraday’s Law of electromagnetic induction states that the magnitude of the induced e.m.f in a circuit is directly proportional to the rate of change of magnetic flux in the circuit.

Question 2

How to increase the magnitude of the induced e.m.f across the coil during electromagnetic induction?

Answer 2

The magnitude of the induced e.m.f. increases if the:

1. Strength of the magnet is increased
2. Speed of the relative motion between the coil and the magnet is increased
3. Number of turns of the coil is increased.

Question 3

Lenz’s Law

Answer 3

Lenz’s Law states that the direction of the induced e.m.f. and hence the induced current in a closed circuit, is always such that its magnetic effect opposes the motion or change producing it.

Lenz’s Law is a good example of the principle of conservation of energy. It shows how the energy from the work done against the opposing force experienced by the moving magnet is transformed into electrical energy.

Question 4

In electromagnetic induction, why is there an induced e.m.f. in the solenoid?

Answer 4

There is a change in magnetic flux in the solenoid.

Question 5

In electromagnetic induction, what determines the magnitude of the induced e.m.f. in the solenoid?

Answer 5

The rate of change of magnetic flux in the solenoid.

Question 6

When there is already an induced e.m.f. in the coil, when will there be an induced current?

Answer 6

When there is a closed circuit.

Question 7

What determines the direction of the induced current in the solenoid?

Answer 7

The direction of the induced current is such that the magnetic field produced opposes the change in magnetic flux or motion of magnet producing it.

Question 8

How to predict the direction of current when in a moving conductor in a magnetic field?

Answer 8

1. Using Fleming’s right-hand rule,
2. The right hand is held out with the index finger, middle finger, and thumb perpendicular to each other.
3. The thumb points in the direction direction of motion of the conductor.
4. The index finger points in the direction of the magnetic field from North to South.
5. The middle finger will point in the direction of conventional current from (A) to (B).

Question 9

How does an a.c. generator work?

Answer 9

1. When the generator is rotated by turning the handle, there is a change in magnetic flux in the rotating coil, which induces an e.m.f across the coil.
2. The carbon brush and slip rings provide connection to an external circuit.
3. The induced e.m.f will induce a current to flow in the external circuit.

Question 10

Function of Slip Rings

Answer 10

Slip rings provide the electric contact with the carbon brushes so that electric current can flow continuously in the a.c. generator.

Question 11

What is the induced e.m.f. across the coil at different points in time?

Vertical & Horizontal

Answer 11

When the coil is vertical, the rate of change of magnetic flux in the coil is the greatest since the movement of the coil is perpendicular to the magnetic field lines.
Hence the magnitude of the induced e.m.f. is also the greatest.

When the coil horizontal, the rate of change of magnetic flux in the coil is zero since the movement of the coil is parallel to the magnetic field lines.
Hence the magnitude of the induced e.m.f. is zero.

Question 12

How to increase the voltage output of an a.c. generator?

Answer 12

1. Doubling the number of turns of the coil doubles the maximum output voltage.
2. Doubling the frequency of rotation doubles the maximum output voltage and also doubles the frequency.
3. Using stronger magnets increases the magnetidue of the induced e.m.f.
4. Winding the coil around a soft iron core increases the magnetic flux in the coil.

Question 13

Describe what happens in a transformer when the switch is closed in the primary coil

Answer 13

1. When the switch is closed, there is a potential difference across the primary coil, P.
2. The current through P slowly increases over time.
3. A proportional magnetic flux that increases over time is produced.
4. The iron ring links the magnetic flux from coil P to coil S.
5. As there is a change in magnetic flux in S, an e.m.f. is induced across S.
6. Since S is part of a closed circuit, a current is induced in S.

Question 14

Purpose of the soft iron core in a transformer

Answer 14

The soft iron core links the magnetic flux from the primary coil to the secondary coil.

Question 15

Equations for transformers

Answer 15

Voltage ratio is equal to turns ratio:

• V_S/V_P = N_S/N_P

For an ideal transformer, input power = output power

• V_P = V_S
• V_P × I_P = V_S × I_S
• V_S/V_P = I_P/I_S

Question 16

Step-up Transformers & Step-down Transformers

Answer 16

Step-up transformer:

• More turns in secondary coil than primary coil
• Increases voltage, decreases current

Step-down transformer

• Less turns in secondary coil than primary coil
• Decreases voltage, increases current

Question 17

Why transfomers are not ideal in reality & how to improve

Answer 17

There is power loss due to heat generated by eddy currents in the soft iron core.

Improve efficiency by using a laminated soft iron core to reduce the flow of eddy currents.
Lamination cuts off the path taken by an eddy current, keeping these induced currents small.

Question 18

Why are a.c. generators used in power stations to transmit electricity?

Answer 18

The power loss during transmission is in a.c. is less when compared to d.c. as a.c. has the advantage of stepping up and stepping down the voltage when required.

Question 19

How to reduce power loss from heating in the transmission cables

Answer 19

1. Use thick cables to decrease the resistance and heat loss to surroundings is reduced. However, thicker cables increase the cable and construction cost.
2. Reduce the current transmitted in the cables by using a step-up transformer to step-up the transmission voltage while maintaining the power that is transmitted.