Electromagnetic Induction (EdExcel)
1. Potential difference
Definition
The potential difference is the energy transferred per unit charge as it moves between two points in an electric circuit.
It is measured in volts (V) and represents the work done to move a charge between two points.
Formula:
V = W/Q
V = potential difference (volts, V)
W = work done (joules, J)
Q = charge (coulombs, C)
Explanation of Formula
The formula shows that potential difference is the amount of work (energy) done per unit charge. If you move a charge through a component like a resistor, the energy used to move the charge through the resistor is the work done, and the potential difference across the resistor tells you how much energy is used per coulomb of charge.
Energy in Circuits
Potential difference also determines how much energy is available to be used by components like light bulbs or motors. A higher potential difference means more energy can be transferred.
Relationship with Current
When a potential difference is applied across a conductor, it causes a flow of charge, which is the electric current. The larger the potential difference, the greater the current (assuming resistance remains constant). This relationship is described by Ohm’s Law:
V = I * RV = potential difference (volts)
I = current (amperes, A)
R = resistance (ohms, Ω)
Inducing a potential difference
A potential difference is created in a conductor when it moves through a magnetic field, either by moving a coil in the field or moving a magnet into a coil. This is called electromagnetic induction or the generator effect.
If the conductor is part of a complete circuit, the potential difference causes an induced current, which creates a magnetic field that opposes the original change (e.g., repelling the magnet).
Factors affecting induced potential
The induced potential difference increases if:
The speed of movement is faster.
The magnetic field strength is stronger.
The number of turns in the coil is higher.
2. Alternators
Definition: An alternating current (AC) generator is a device that produces a potential difference by converting mechanical energy into electrical energy.
A simple AC generator consists of a coil of wire rotating within a magnetic field.
This process is used in power stations for large-scale electricity generation to supply homes and factories.
As the coil rotates, one side moves up through the magnetic field, and a potential difference is induced in one direction.
As the coil continues to rotate and that side moves down, the potential difference reverses direction.
This continuous reversal of direction creates an alternating current (AC), meaning the current constantly changes direction, unlike direct current (DC), which flows in one direction only.
Alternator output on a graph
The output of an alternator can be represented on a voltage–time graph, with potential difference (voltage) on the vertical axis and time on the horizontal axis.
The graph shows an alternating sine curve. The maximum potential difference or current can be increased by:
increasing the rate of rotation
increasing the strength of the magnetic field
increasing the number of turns on the coil
The diagram shows four different positions of the coil in an alternator, and the corresponding voltage produced.
A – The coil is at 0°. The coil is moving parallel to the direction of the magnetic field, so no potential difference is induced.
B – The coil is at 90°. The coil is moving at 90° to the direction of the magnetic field, so the induced potential difference is at its maximum.
C – The coil is at 180°. The coil is moving parallel to the direction of the magnetic field, so no potential difference is induced.
D – The coil is at 270°. The coil is moving at 90° to the direction of the magnetic field, so the induced potential difference is at its maximum. Here, the induced potential difference travels in the opposite direction to what it did at B.
A – The coil is at 360°, ie it is back at its starting point, having done a full rotation. The coil is moving parallel to the direction of the magnetic field, so no potential difference is induced.
3. Dynamos
Definition: A direct current (DC) generator is another device that produces a potential difference. A simple DC generator consists of a coil of wire rotating within a magnetic field.
However, unlike an alternating current (AC) generator, it uses a split ring commutator instead of two slip rings.
A common example of a DC generator is the dynamo, which is used in bike lights to power the lamps while the wheels are turning.
In a dynamo, the split ring commutator changes the connections of the coil every half turn.
This reversal of connections happens just as the induced potential difference is about to change direction.
As a result, the current flowing to the external circuit always flows in the same direction, providing direct current (DC).
Dynamo output on a graph
The output of a dynamo can be shown on a voltage–time graph.
The graph shows a sine curve that stays in the same direction all the time.
The maximum potential difference or current can be increased by:
increasing the rate of rotation
increasing the strength of the magnetic field
increasing the number of turns on the coil
The diagram shows four different positions of the coil in a dynamo, and the corresponding voltage produced.
A – The coil is at 0°. The coil is moving parallel to the direction of the magnetic field, so no potential difference is induced.
B – The coil is at 90°. The coil is moving at 90° to the direction of the magnetic field, so the induced potential difference is at its maximum.
C – The coil is at 180°. The coil is moving parallel to the direction of the magnetic field, so no potential difference is induced.
D – The coil is at 270°. The coil is moving at 90° to the direction of the magnetic field, so the induced potential difference is at its maximum. Here, the induced potential difference travels in the same direction as at B.
A – The coil is at 360°, ie it is back at its starting point, having done a full rotation. The coil is moving parallel to the direction of the magnetic field, so no potential difference is induced.
4. Microphones
A microphone is a device that converts sound waves into electrical signals.
Microphones use the generator effect to induce a changing current based on the pressure variations of sound waves.
Moving-coil microphones
In a moving-coil microphone:
Pressure variations in sound waves cause a flexible diaphragm to vibrate.
These vibrations cause the coil attached to the diaphragm to vibrate as well.
The coil moves relative to a permanent magnet, inducing a potential difference in the coil.
Since the coil is part of a complete circuit, the induced potential difference causes a current to flow through the circuit.
The changing size and direction of the induced current match the vibrations of the coil.
The electrical signals generated correspond to the pressure variations in the sound waves.
5. Loudspeakers and headphones - Higher
Headphones, which contain small loudspeakers, use the reverse effect to microphones, known as the motor effect.
In these devices, variations in an electric current cause changes in the magnetic field produced by an electromagnet.
This leads to the movement of a cone, creating pressure variations in the air, which form sound waves.
Alternating current supplied to the loudspeaker creates sound waves in the following way:
A current in the coil generates a magnetic field.
This magnetic field interacts with the permanent magnet, creating a force that pushes the cone outwards.
The current is then reversed, causing the magnetic field to also reverse direction.
The reversed magnetic field creates a force that pulls the cone back in.
By repeatedly alternating the current direction, the cone vibrates in and out.
These cone vibrations cause pressure variations in the air, which are sound waves.
To ensure the loudspeaker cone vibrates correctly, the alternating current must vary in the same way as the desired sound.udspeaker creates sound waves in the following way:
A current in the coil generates a magnetic field.
This magnetic field interacts with the permanent magnet, creating a force that pushes the cone outwards.
The current is then reversed, causing the magnetic field to also reverse direction.
The reversed magnetic field creates a force that pulls the cone back in.
By repeatedly alternating the current direction, the cone vibrates in and out.
These cone vibrations cause pressure variations in the air, which are sound waves.
To ensure the loudspeaker cone vibrates correctly, the alternating current must vary in the same way as the desired sound.