Induced potential (HT only) — AQA GCSE Physics
Test yourself on Induced potential (HT only) with AQA GCSE practice questions.
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Induced potential (HT only) explained
There are two ways to get a potential difference for nothing: move a conductor across a magnetic field, or hold the conductor still and change the field around it.
Read the full explanation
Both amount to the same thing, the conductor cutting magnetic field lines. Push a wire down between the poles of a magnet and a potential difference appears across its ends; hold it still and the reading drops to zero, because nothing is being cut. Move the wire along the field lines rather than across them and again nothing is induced. A coil carrying an alternating current does the second job with nothing moving at all: its field grows, collapses and reverses, and any conductor sitting in that changing field has a potential difference induced across it. The word induced matters, because nothing is connected to the conductor.
If the conductor is part of a complete circuit, a current is induced in the conductor.
An induced potential difference is a push; a current only flows if there is somewhere for the charge to go. Connect the ends of the moving conductor into a complete loop, through a sensitive ammeter, a lamp or a resistor, and the induced potential difference drives a current round it. Break the loop and the potential difference is still there, and a voltmeter will still read it, but the ammeter reads zero. That distinction earns marks, because questions ask what is induced and the right answer depends on whether the circuit is closed. It also explains a familiar observation: a dynamo is easy to spin with nothing plugged into it and much harder once a lamp is connected, because only then is there an induced current to oppose the movement.
This is called the generator effect.
The name is worth learning precisely, because the two effects are mirror images and the examiner is watching which one you pick. The motor effect starts with a current and produces movement. The generator effect starts with movement, or with a changing field, and produces a potential difference. Anything that works one way round usually works the other: spin a motor by hand and it generates, and run a loudspeaker backwards and you have a microphone. Sort a question by what is supplied: supply a current in a field and a force is produced, which is the motor effect; supply movement or a changing field and a potential difference is induced, which is the generator effect. Quote the wrong effect and the rest of an otherwise correct answer usually earns nothing.
An induced current generates a magnetic field that opposes the original change, either the movement of the conductor or the change in magnetic field.
An induced current is not free. It makes a magnetic field of its own, and that field always opposes whatever caused it, so pushing a north pole into a coil makes the near end of the coil behave as a north pole and push back. You must do work against that opposition, and it is the work you do that ends up as electrical energy in the circuit. The neatest exam version is a dynamo with a lamp. With the lamp connected there is an induced current, that current produces a magnetic field, and that field opposes the movement of the coil, so the handle is hard to turn. Disconnect the lamp and the circuit is broken, so there is no current, no field produced by the coil and no opposing force, and the dynamo spins easily. Energy is conserved either way.
Students should be able to recall the factors that affect the size of the induced potential difference/induced current.
Three things set the size of what you induce, and all three are the same idea in different clothes: how fast field lines are being cut. Move the magnet or the coil faster and the induced potential difference rises. Use a stronger magnet, so there are more field lines to cut in the same space, and it rises. Wind more turns onto the coil, so each field line is cut once per turn, and it rises again; a larger coil area does the same. Stop the movement and it falls to zero, which is why a generator has to keep turning. The induced current follows the induced potential difference, so anything that increases one increases the other, provided the resistance of the circuit stays the same.
Students should be able to recall the factors that affect the direction of the induced potential difference/induced current.
Only two things set which way the induced potential difference points, and both of them are reversals. Push a magnet into a coil and the galvanometer needle swings one way; pull it out and it swings the other, because the movement has reversed. Turn the magnet round so that the south pole goes in first and the needle swings the other way again, because the field direction has reversed. Make both changes at once and the two reversals cancel, so the deflection is back where it started. That is why a coil spinning steadily in a fixed field gives a potential difference that reverses twice per turn: each side swaps from moving up through the field to moving down through it.
Students should be able to apply the principles of the generator effect in a given context.
Unfamiliar contexts are the same physics in new clothes, so answer them with the same four moves: find the conductor, find the magnetic field, say what is changing, then say what is induced. A shake torch has a magnet sliding through a coil, so the field through the coil changes and a potential difference is induced that lights the lamp. An electric guitar pickup has a steel string vibrating above a magnet and a coil, so the field through the coil changes at the frequency of the string. A roller coaster brake drags metal fins between magnets, and the induced currents produce fields that oppose the motion and slow the car. That last one is the opposition rule doing useful work, and none of these devices needs a battery.
Your focus
- Describe two different arrangements that induce a potential difference in a conductor.
- Explain why a wire moved along the field lines induces nothing, using the idea of cutting field lines.
- Predict the reading on a voltmeter connected to a wire held stationary in a steady magnetic field.
Show all 21 objectives
- Distinguish between an induced potential difference and an induced current, stating the condition needed for each.
- Predict the ammeter and voltmeter readings for a conductor moved in a field with the circuit first open and then closed.
- Describe a circuit that would show that an induced current exists.
- Name the generator effect and state the two inputs that can produce it.
- Distinguish the generator effect from the motor effect by what is supplied and what is produced.
- Give one everyday device that relies on each of the two effects.
- Explain why the magnetic field of an induced current must oppose the change that caused it.
- Predict the change in the effort needed to turn a generator when its load is connected or disconnected.
- Relate the work done against the opposing force to the electrical energy the generator delivers.
- List three changes that increase the size of an induced potential difference.
- Explain each of those factors in terms of the rate at which field lines are cut.
- Design an experiment with a magnet and a coil that tests one factor while controlling the others.
- State the two changes that reverse the direction of an induced potential difference.
- Explain why the potential difference induced in a coil rotating steadily in a fixed field reverses twice in every revolution.
- Predict the direction of the galvanometer deflection when a magnet is pushed into a coil, pulled out, and then turned round and pushed in again.
- Identify the conductor, the field and the change in an unfamiliar device that uses induction.
- Explain how a shake torch lights its lamp without a battery.
- Apply the opposition rule to explain a braking effect produced by induced currents.
Induced potential (HT only) exam tips
Marking Points
- one mark for the conductor and the magnetic field moving relative to one another, or for the field around the conductor changing
- one mark for the conductor cutting magnetic field lines
- one mark for a potential difference being induced across the ends of the conductor
- one mark for stating that with no relative movement and no change of field, nothing is induced
- one mark for the conductor being part of a complete circuit
- one mark for the induced potential difference driving an induced current round that circuit
- one mark for stating that an open circuit gives a potential difference but no current
- one mark for a suitable way of detecting the current, such as a sensitive ammeter or a lamp
- one mark for naming the generator effect, where movement or a changing field produces a potential difference
- one mark for contrasting it with the motor effect, where a current in a field produces a force
- one mark for identifying a device that uses the generator effect, such as an alternator, a dynamo or a microphone
- one mark for a potential difference being induced across the conductor, and a current only where the circuit is complete
- one mark for the induced current producing a magnetic field of its own
- one mark for that field opposing the change that produced it, whether a movement or a change in field
- one mark for the resulting force opposing the motion of the conductor
- one mark for linking the extra work done against that force to the electrical energy generated
- one mark for increasing the speed of the relative movement, or the rate at which the field changes
- one mark for using a stronger magnet or a stronger magnetic field
- one mark for increasing the number of turns on the coil
- one mark for stating that the induced current rises with the induced potential difference for a fixed circuit resistance
- one mark for reversing the direction of the relative motion reversing the induced potential difference
- one mark for reversing the direction of the magnetic field, by turning the magnet round, reversing it as well
- one mark for stating that making both reversals together leaves the direction unchanged
- one mark for the induced potential difference reversing twice per revolution, because each side of a rotating coil changes from moving up through the field to moving down through it
- one mark for identifying the conductor and the magnetic field in the unfamiliar arrangement
- one mark for stating what is changing, either the relative position or the strength of the field
- one mark for a potential difference, and where the circuit is complete a current, being induced
- one mark for applying the opposition rule where the question asks about a force or a braking effect
Examiner Tips
- 💡Look for the word change; if nothing about the field or the position changes, nothing is induced.
- 💡State clearly which is moving, the conductor or the magnet, because only the relative motion matters.
- 💡Use the phrase a potential difference is induced across the ends, which is the wording the mark scheme prints.
- 💡Check the diagram for a break in the circuit before deciding whether to write current or potential difference.
- 💡If the question says a lamp lights, you can safely write about an induced current.
- 💡Name the meter you would use; a sensitive ammeter or a galvanometer scores where a meter may not.
- 💡Ask what goes in: a current in means the motor effect, movement in means the generator effect.
- 💡Learn one example of each effect, so that a one mark give an example question is free.
- 💡Keep the word induced for the generator effect; it is the signal the examiner looks for.
- 💡A question phrased explain why it is harder to turn is always this idea; name the current, its field and the opposing force.
- 💡Use conservation of energy as a check: a generator must be hard to turn, or you would be making energy for free.
- 💡Keep to the order current, field, force, opposition, because each step tends to carry a mark.
- 💡Give each factor a direction of change: write increase the speed, not just the speed.
- 💡If a graph is supplied, read off two points and quote the numbers when you describe the effect.
- 💡In a described experiment, change one variable at a time and say what you kept the same.
- 💡Sketch the two reversals separately and then combine them, so you do not lose track of a double reversal.
- 💡Describe what the galvanometer needle does, left or right, rather than calling the reading positive or negative.
- 💡In a spinning coil question, tie each reversal to the moment a side crosses the top or the bottom of its circle.
- 💡Underline the moving part in the stem; that is almost always where the induction happens.
- 💡Write your answer using the nouns the question uses, so the fins rather than the coil.
- 💡Two marks usually means cause then effect, so do not give two versions of the effect.
Common Mistakes
- saying a current is induced when the circuit is open, since only a potential difference appears until the circuit is complete
- moving the wire along the field lines and still expecting a reading, when no field lines are being cut
- describing a steady magnetic field near a stationary coil as inducing a potential difference
- writing that the magnet gives the wire charge, rather than that a potential difference is induced
- using induced current and induced potential difference as though they meant the same thing
- claiming a current flows when the ends of the conductor are not joined
- reading a diagram with a break in the circuit and still answering that a current is induced
- saying the magnet supplies the current, when the source is the induced potential difference
- calling it the motor effect because a magnet and a coil are involved
- saying the generator effect creates energy, rather than transferring it from the kinetic store of whatever turns the coil
- naming the effect but never saying what is induced
- pairing the left hand rule with a generator question
- saying the induced field helps the motion along, which would make energy out of nothing
- explaining that a disconnected dynamo spins easily by friction, rather than by there being no induced current
- saying no potential difference is induced when the lamp is removed, when only the current stops
- forgetting that the opposition applies to a changing field as well as to a moving conductor
- writing a bigger magnet when what matters is a stronger field
- suggesting that reversing the direction of movement makes the induced potential difference larger, when it only reverses it
- quoting the number of turns on a transformer primary in an answer about a magnet moved into a coil
- saying that holding the magnet inside the coil keeps a steady reading, when a stationary magnet induces nothing
- believing that a faster movement can change the direction, when speed only changes the size
- assuming that holding the magnet still inside the coil keeps the needle deflected to one side
- using the left hand rule, which predicts a force and not an induced current
- saying the needle deflects the same way on the way in and on the way out
- answering with a memorised dynamo description that never mentions the apparatus in the question
- saying the device creates electricity rather than naming the induced potential difference
- missing that a vibrating steel object near a magnet changes the field, so no cause for the induction is identified
- leaving out the need for a complete circuit before claiming that a current flows