Motors and induced pd extra: GCSE Physics
Motor effect, F = BIl, generator effect, induced pd factors, microphones and loudspeakers
The short version
F = BIl and Fleming’s left hand for motors. A changing field induces a pd (Faraday). Transformers need a.c. because the field must change.
Key idea
F in N; B in tesla (T); I in A; L is length in the field in m. Current at 90° to B
Grade 2–3: A wire with a current in a magnet gets a push. That push can spin a motor.
Grade 5–6: I can use F = BIL and I know FBI on the left hand.
Grade 9: I can reverse one quantity and predict the new force, and I can say why a split ring is needed for d.c. rotation.
Go a bit deeper
More turns on the coil increases total force / moment. Soft-iron core concentrates the field
Grade 2–3: Stronger magnet, bigger current, more wire in the magnet - bigger push.
Grade 5–6: I can reverse the battery and say the motor turns the other way.
Grade 9: I treat more turns as more L, I mention a radial field, and I do not confuse force with a guaranteed higher steady rpm.
Worked example
Worked example
Grade 2–3: A wire with a current in a magnet gets a push. That push can spin a motor. Grade 5–6: I can use F = BIL and I know FBI on the left hand. Grade 9: I can reverse one quantity and predict the new force, and I can say why a split ring is needed for d.c. rotation.
Lessons in this topic
- The motor effect
- Factors that change the motor force
- The generator effect
- Alternators and dynamos
- Loudspeakers and microphones
- Induction, transformers and exam links
In the app each lesson is video (when we have a real YouTube id) then intro, learn and summary steps.
What the examiner wants
Apply F = BIl, state factors affecting induced pd, explain a.c. generators and loudspeakers/microphones, and link to electromagnetic induction.
Try these questions
Cover the answers and have a go first. Worked solutions are underneath.
The motor-effect force (90°) is
Fleming’s left hand: thumb is
If the wire is parallel to the magnetic field, F is
A d.c. motor uses a split-ring commutator to
Common mistakes
- Do not use the right hand for the motor effect (that is the generator / dynamo rule). L is the length in the field, not the whole reel of cable on the bench.
- Turning the coil faster is not a factor in F = BIL for a given I and B - that is the generator story. Do not use the right-hand rule here.
- A steady field and a stationary coil give no induced pd. Do not use the left-hand motor rule to find the induced current direction.
- Do not mix slip rings with the split-ring commutator. An alternator does not give "smooth battery d.c.".
Answers and working
Frequently asked questions
Is GCSE Motors and induced pd extra on Combined Science and Triple?
Motors and induced pd extra is Triple-only Physics. Combined Science students can skip it.
Which exam board is this Motors and induced pd extra page for?
The ideas are board-agnostic for AQA, Edexcel, OCR, WJEC and CCEA. Paper lengths and question numbers differ — check your specification.
Is Motors and induced pd extra Foundation or Higher?
The core ideas appear on both tiers. Higher papers add extra application and some HT-only detail, marked in lessons with a Higher badge.
Learn, try, get feedback, fix the mistake
Open the stepped lesson, then the quiz with hints. Required-practical and calculation questions use the same method you will need in the paper.
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