Recall the factors that affect the size of the induced potential difference/induced current.
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GCSE Physics Revision
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GCSE Physics revision
Induced potential (HT only)
Induced potential, transformers and the National Grid (physics only) (HT only)
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AQA student objectives
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Apply the scientific explanation of induced potential to a relevant example.
Analyse a new situation involving induced potential and explain the scientific reasoning.
Revision summary
Key knowledge
Read on screen, then print for Cornell-style active revision.
What is Electromagnetism?
- Electromagnetism is the phenomenon whereby an electric current flowing through a wire produces its own magnetic field around it.
- The magnetic field lines around a straight current-carrying wire are represented as concentric circles, closest together near the wire where the field is strongest.
The Right-Hand Rule
- The right-hand rule helps determine the direction of the magnetic field: curl your right hand into a fist with your thumb pointing in the direction of the current, and your fingers show the direction of the magnetic field.
- When current flows upwards through a wire, the magnetic field travels anticlockwise around it; reversing the current reverses the direction of the magnetic field.
Magnetic Field Around a Flat Circular Coil
- When a wire is formed into a flat circular coil, the concentric circular field lines from each side of the coil interact and stretch into ellipses.
- The combined magnetic fields of both sides of the coil merge to produce a single magnetic field that passes straight through the centre of the coil.
Solenoids and Their Magnetic Fields
- A solenoid is formed by winding many turns of wire closely together in a long coil, producing a strong and uniform magnetic field inside.
- The magnetic field outside a solenoid resembles that of a bar magnet, with field lines emerging from the north pole and entering the south pole.
Properties of Electromagnets
- An electromagnet is a solenoid that acts as a magnet only while current flows through it
- — switching off the power source causes the magnetic field to disappear instantly.
- The direction of the magnetic field, and therefore the positions of the north and south poles, can be reversed by reversing the direction of the current through the solenoid.
Increasing the Strength of an Electromagnet
- Increasing the current flowing through the solenoid increases the strength of the magnetic field produced.
- Increasing the number of turns of wire within the same length of solenoid increases the magnetic field strength, as more densely packed coils produce a stronger field.
- Decreasing the length of the coil while keeping the number of turns the same also increases the density of the coils and therefore the field strength.
- Adding a soft iron core inside the solenoid causes the iron to become an induced magnet when the current is on, massively increasing the overall magnetic field strength.
Why Iron is Used as a Core
- Iron is a soft magnetic material, meaning it becomes magnetised (an induced magnet) when placed inside an active solenoid, greatly amplifying the electromagnet's strength.
- Because iron is a soft magnetic material, it loses its magnetism as soon as the current is switched off, which is essential for applications where the electromagnet needs to be turned on and off.
Direction of the Induced Potential Difference
- Each time the direction of movement is reversed, the direction of the induced potential difference also reverses, causing the positive and negative terminals to swap.
- Moving a magnet into and out of a coil induces a potential difference, and reversing the direction of the magnet's movement reverses the direction of the current.
- Swapping the poles of the magnet (e.g. turning it around) also reverses the direction of the induced current.
Conditions Required for Induction
- A potential difference is only induced when there is relative movement between the wire and the magnetic field — if the wire is stationary and the field is not changing, no induction occurs.
- Moving the magnets up and down whilst keeping the wire still also induces a potential difference, because the wire still experiences a change in magnetic field.
- Moving the wire back and forth parallel to the field (without cutting through field lines) does not induce a potential difference, as the wire experiences no change in magnetic field.
From Potential Difference to Current
- An induced potential difference alone cannot drive a current unless the wire forms a complete (closed) circuit.
- When the two ends of the wire are connected to complete the circuit, the induced potential difference causes electrons to flow, generating an induced current.
Factors Affecting the Size of the Induced Potential Difference
- Increasing the strength of the magnetic field (e.g. using stronger magnets) increases the size of the induced potential difference.
- Increasing the speed at which the wire or magnets move causes the magnetic field to change more rapidly, resulting in a larger induced potential difference.
- Increasing the number of turns on the coil increases the induced potential difference, as more wire is cutting through the magnetic field lines.
The Coil and Single Magnet Model
- Moving a single magnet into and out of a coil of wire induces a potential difference in the coil due to the changing magnetic field experienced by the coil.
- Because the coil forms a complete circuit, the induced potential difference drives a current around the circuit.
- The direction of the induced current depends on both the direction of the magnet's movement and which pole of the magnet is facing the coil.