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15.5 The Generator Effect

GCSE Physics · AQA · Chapter 15: Electromagnetism

A free GCSE Physics lesson for AQA (8463) from OBA School: higher tier, physics only. Inducing a potential difference, the factors that affect its size and direction, and why the induced current opposes the change. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • (Higher tier) describe the generator effect and when a potential difference is induced
  • (Higher tier) recall the factors that affect the size of the induced potential difference and current
  • (Higher tier) recall the factors that affect the direction of the induced potential difference and current
  • (Higher tier) explain that the induced current produces a magnetic field that opposes the change that caused it
  • (Higher tier) apply the generator effect to new situations

Key words

  • Induced p.d.: A potential difference produced across a conductor because of relative movement between it and a magnetic field, or a changing field.
  • Electro- magnetic induction: Producing a potential difference in a conductor using a magnetic field.
  • Generator effect: An induced p.d. driving a current round a complete circuit.
  • Centre-zero meter: A sensitive ammeter (or voltmeter) whose pointer can move either way, showing the direction of the current.

Inducing a potential difference

A potential difference (p.d.) is induced across a conductor when:

  • the conductor moves relative to a magnetic field so that it cuts across the field lines, or
  • the magnetic field around the conductor changes (gets stronger, weaker or reverses).

If the conductor is part of a complete circuit, the induced p.d. drives a current round it. This is the generator effect. It is the reverse of the motor effect: here motion produces a current; in the motor effect a current produces motion.

Key fact: There must be relative movement or a changing field. A wire held still in a steady field, or moved along the field lines, has no p.d. induced.

A magnet and a coil

Connect a coil to a centre-zero meter and move a bar magnet in and out of it. A coil works better than a single wire because the p.d. induced in each turn adds up.

  • push the N-pole into the coil; What the meter shows a brief deflection one way
  • hold the magnet still inside the coil; What the meter shows zero (no change in the field)
  • pull the N-pole back out; What the meter shows a brief deflection the opposite way
  • push the S-pole in; What the meter shows a deflection opposite to pushing the N-pole in
  • move the coil instead of the magnet; What the meter shows the same effects: only relative movement matters

What affects the size and direction?

  • the movement is faster; The direction reverses if… the direction of movement is reversed (in instead of out)
  • the magnet is stronger; The direction reverses if… the magnetic field is reversed (the other pole is used)
  • the coil has more turns (or a larger area); The direction reverses if… (reversing both leaves the direction unchanged)

Worked example: predicting the meter

Pushing the N-pole of a magnet into a coil makes the pointer move 4 divisions to the right.
The S-pole of a magnet twice as strong is now pulled out of the coil at the same speed.
S-pole instead of N-pole: direction reverses. Pulling out instead of pushing in: reverses again. So the deflection is to the right. The stronger magnet doubles the size.

Answer: about 8 divisions to the right

The induced current opposes the change

The induced current produces its own magnetic field. This field always opposes the change that produced it:

  • As a N-pole is pushed in, the near end of the coil becomes a N-pole, which repels the magnet.
  • As the N-pole is pulled out, the near end becomes a S-pole, which attracts the magnet back.
  • So you must always do work against a force to move the magnet. That work is the energy transferred to the electrical circuit. If the circuit is broken, no current flows and there is no opposing force.

Exam tip: Use the coil-end rule from 15.2 to find the induced pole: looking at an end, anticlockwise current = N-pole, clockwise current = S-pole.

A changing field with no movement

Place two coils side by side, one connected to a battery and switch, the other to a meter. When the switch is closed or opened, the field of the first coil grows or collapses, so a p.d. is induced in the second coil and the meter flicks. While the current is steady, the field does not change and the meter reads zero. With an alternating current the field changes all the time, so an alternating p.d. is induced continuously. This is how a transformer works (15.7).

Common mistakes

  • Saying a p.d. is induced when the magnet is inside the coil. It is only induced while the field through the coil is changing.
  • Mixing up the motor effect and the generator effect. Generator: movement → p.d. Motor: current → force.
  • Saying the induced current helps the magnet move. Its field always opposes the change.
  • Saying “a current is induced” in an open circuit. A p.d. is induced, but there is no current unless the circuit is complete.

Revision questions

Try these, then check your answers below each one.

Question 1. (Higher tier) In which situation is a potential difference induced across the ends of a coil? (Choose from: A magnet is being pushed into the coil, A magnet is resting inside the coil, The coil is resting near a magnet, A magnet is held still beside the coil.)

Answer: A magnet is being pushed into the coil

Working: A p.d. is only induced while there is relative movement (or the field through the coil is changing).

Question 2. (Higher tier) True or false? A steady magnet held still inside a coil induces a steady potential difference.

Answer: False

Working: False. No movement means no change in the field, so no p.d. is induced.

Question 3. (Higher tier) The N-pole of a magnet is pushed towards end A of a coil in a complete circuit. What does end A become while the magnet is moving? (Choose from: A N-pole, which repels the magnet, A S-pole, which attracts the magnet, A N-pole, which attracts the magnet, Not a pole at all.)

Answer: A N-pole, which repels the magnet

Working: To oppose the approaching N-pole, end A becomes a N-pole and repels it.

Related lessons

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This lesson is section 15.5 (AGCPH1505) of our free GCSE Physics course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Physics course.

 

 

 

 

 

 

 

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