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GCSE level Physics exam revision notes:
Electromagnetism
Motor effect of electric current:
11.1 and 11.2
The motor effect - the interaction of a electric current carrying wire and the
external field of a
magnet - predicting the direction of the motor effect
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11.1
Explaining the motor effect - the interaction of a current carrying wire and a magnetic
field
This page will help you answer questions
such as ... Why does a current carrying wire experience a force when placed
in a magnetic field? What is Fleming's left-hand rule?
When a conductor e.g. a wire, carrying an
electric current, is placed between the poles of a magnet the magnetic field
around the conducting wire interacts with the magnetic field it is placed in.
We are dealing with the
interaction of two magnetic fields, each with their own north and
south poles because a magnetic field is always produced around a wire
carrying an electric current..
This will cause the magnet and the
conductor to exert a non-contact force on each other.
The force will cause the wire to move
and this phenomena is called the motor effect.
Here you are dealing with two magnetic
fields (from the wire and magnet) each with its north and south pole, hence
the interaction (as you get with any two
magnetic poles)
The resulting magnetic field is
stronger in one area and weaker in another, so there is a resultant
force.
To get the maximum full force effect the wire should
be at 90o to the direction of the magnetic field flux.
If the wire is parallel to the magnetic field, it won't
experience a force at all.
So from 0o to 90o you get a steady
increase in the force exerted on the wire.
The diagram above illustrates the
'kicking wire' experiment and Fleming's left-hand rule which allows you
to predict the direction of wire's motion - the direction of the
resultant force or thrust.
The force always acts at right angles to the magnetic
field of the magnet AND the direction of the current in the wire see
more on Fleming's left-hand rule below.
The magnitude of the force increases with ..
(i) increase in current flow, which increases the
strength of the magnetic field around the wire,
(this could be increased by increasing
p.d. (V) or a thicker wire creating a smaller resistance (R in
Ω) for the same p.d.
(ii) the strength of the magnetic field
of the
permanent magnet - a 'stronger 'magnetic field around the magnet,
(iii) the length of wire
exposed to the magnetic filed, greater length or area where the two
magnetic fields interact.
INDEX physics notes: motor
effect of an electric current and applications
11.2 Predicting the
direction of maximum force - the direction motion in the motor effect
You can predict the direction of the force-motion
effect from
Fleming's left-hand rule (illustrated below with doc b's handsome left
hand!).
doc b's left hand! handy!
Diagram of Fleming's left-hand rule prediction
Imagine a set of x,y, z axes at 90o to each
other, represented by the thumb, first finger and second finger of your left
hand.
The thuMb represents the direction the
force acts
-
direction of motion (phonetically emphasise M).
The
First Finger represents the direction of the
magnetic field N => S (phonetically emphasise the F).
The SeCond finger represents the direction of the
convention current (phonetically emphasise the 'hard' C).
Fleming's left-hand rule and the motor effect are 'combined'
in the diagram above.
Imagine a current carrying wire at the most favourable angle
of 90o to a magnetic field created by permanent magnets.
If you do this in the lab you will see
the wire kick to
the right with respect the magnetic N=>S pole alignment.
The direction of force creating the motion can be
predicted from Fleming's left-hand rule (top right).
However!, you have to twist your hand around,
physically (or in your head) to fit in with a given diagram situation.
The result of twisting your hand around is shown in the
bottom right of the diagram - check it out!
Note the 'change in direction' rules ....
(i) if you reverse the direction of the current (e.g.
↑
to
↓), you
reverse the direction of the force
and the wire kicks the other way,
and (ii) if you reverse the direction of the
magnetic field (e.g. N=>S to S<=N), you also reverse the
direction of the force and the wire kicks the other way.
The motor effect has all sorts of applications e.g. electric
motors, loudspeakers, generators, microphones, so see
See 11.4
Motor effect of an electric
current - d.c. electric motor
and 12.
Generator effect, applications e.g. generators
generating electricity and microphone
INDEX physics notes: motor
effect of electric current and applications
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Key points about electromagnetism
-
motor effect of electric current - Flemings left-hand rule
Information
sources for Doc Brown's key points: IGCSE-GCSE physics are based on
textbooks & syllabus-specifications for students taking the UK AQA, Edexcel,
OCR 21st Century Science, OCR Gateway science suite, WJEC, CCEA and CIE GCSE
physics 9-1 level science examinations
Here's a structured and exam-board-aligned
summary of The Motor Effect and Fleming’s Left-Hand
Rule, tailored for IGCSE/GCSE Physics students across WJEC, CCEA,
CIE, AQA, Edexcel, and OCR.
The Motor Effect
What is the motor effect?
- When a current-carrying conductor
(e.g. a wire) is placed in a magnetic field, it experiences
a force.
- This phenomenon is called the
motor effect.
- It’s the principle behind electric
motors and loudspeakers.
Key Conditions to produce the motor effect
- The wire must be perpendicular
to the magnetic field for maximum force.
- If the wire is parallel,
no force is experienced.
- The force is always perpendicular
to both the magnetic field and the current.
Fleming’s Left-Hand Rule
Mnemonic
Use your left hand:
- Thumb
= Force (Motion)
- First finger
= Magnetic Field (North to South)
- Second finger
= Current (Positive to Negative)
All three fingers must be mutually
perpendicular to each other.
Application
- Used to predict the direction
of the force on a conductor.
- Essential for understanding how
electric motors rotate.
Practical Applications
Typical Exam Board Syllabus Content
| Key Points |
| Motor effect, Fleming’s rule, electric
motors (HT only) |
| Force on a wire, equation ( F = BIL ),
Fleming’s rule |
| Magnetic fields, motor effect,
Fleming’s rule |
| Magnetic fields, force on conductors,
Fleming’s rule |
| Motor effect, Fleming’s rule,
practical applications |
| Magnetic effects of current, force on
a wire, Fleming’s rule |
Student tips on the motor effect
- Visualise
the left-hand rule with diagrams—practice with real questions.
- Experiment:
Use simulations or classroom demos to see the motor effect in action.
- Remember:
Force is zero when the wire is parallel to the field.
- Practice
calculations using ( F = BIL ) with correct units.
- Link
this topic to electromagnetic induction and generators (right-hand rule).
Keywords, phrases and learning objectives
on the motor effect of an electric
current
Be able to explain the motor effect from the interaction of electric
current carrying wire and an external magnetic field
Be able to explain the cause of movement in
demonstrations like the kicking
magnet experiment.
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