The induced e.m.f. always opposes the change that caused it
- The direction of an induced e.m.f. is always such that its effects oppose the change producing it.
- Push the N pole of a magnet into a coil and the induced current makes the near face of the coil an N pole, which pushes back against the magnet.
- Pull the magnet out and the current reverses, so the near face becomes an S pole that pulls back and resists the magnet leaving.
- Either way you have to do work against that opposing force, and that work is the source of the electrical energy produced.
- If the induced effects helped the motion instead, the coil would speed the magnet up and produce energy from nothing, which would break conservation of energy.
- That is why a bicycle dynamo makes the pedals harder to turn, and why a power station generator needs a turbine driving it continuously.
Fleming's right-hand rule gives the direction for a moving conductor
- For a straight conductor moving through a field, hold the right hand with the thumb, first finger and second finger at right angles to one another.
- The thumb points along the motion of the conductor.
- The first finger points along the field, from N to S.
- The second finger then gives the direction of the induced conventional current.
- Use the right hand for induction, where movement produces a current, and keep the left hand for the motor effect, where a current produces movement.
- The rule only applies when the conductor cuts across the field lines, since moving it along them induces nothing at all.
Working out the direction of the current in a coil
- Identify which pole of the magnet is approaching the coil or moving away from it.
- Decide what the coil must do to oppose that change: repel an approaching pole, or attract one that is leaving.
- That tells you which face of the coil has to become an N pole and which an S pole.
- Then use the fact that the current runs anticlockwise when seen from an N-pole face and clockwise when seen from an S-pole face.
- Finish with the energy check: the force on the magnet should always make the movement harder, never easier.
Common Mistake
- Use the right hand for induction; reaching for the left hand out of habit reverses every answer.
- The opposition is to the change, not to the magnet, so pulling a magnet out produces an attraction rather than a repulsion.
- The second finger gives the conventional current, from positive to negative, and not the direction the electrons move.
- Anticlockwise and clockwise depend on which side you are looking from, so state the viewing direction when you answer.
Exam technique
- Actually hold your right hand up in the exam and line the fingers up with the diagram, because doing it in your head is where the errors come from.
- For an "explain the direction" question, the mark is for saying the induced effect opposes the change, so write that sentence before anything else.
- Back it up with energy: work has to be done against the opposing force, and that work becomes the electrical energy.
- Mark the field direction and the motion on the diagram before applying the rule, since a wrongly labelled field flips the whole answer.
Self review
- State the rule that fixes the direction of an induced e.m.f.
- An N pole is pushed into a coil. Which pole appears on the near face, and why?
- Name the three quantities given by the thumb, first finger and second finger in the right-hand rule.
- Which hand is used for induction, and which for the force on a current-carrying wire?
- Explain, using energy, why the induced effects must oppose the movement rather than help it.
