The Motor Effect
Master The Motor Effect for GCSE Physics with this free worksheet and full mark scheme — Higher-tier exam-style questions with worked answers for AQA, Edexcel and OCR. A current-carrying conductor in a magnetic field experiences a force given by F = BIL when the current is at right angles to the field.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
This is a Higher tier only topic, so there's no Foundation paper — only the Higher worksheet and mark scheme below.
Topic overview
When a conductor carrying a current is placed in a magnetic field, the magnet and the conductor exert a force on each other. This is the motor effect. This is a Higher-only topic.
The force is greatest when the conductor is at right angles to the magnetic field, and it is zero when the conductor is parallel to the field.
The equation is force = magnetic flux density × current × length, F = BIl. Magnetic flux density is measured in tesla, current in amperes, and length in metres. The length is the length of conductor actually within the magnetic field, not the total length of the wire.
Revision notes
The motor effect
A conductor carrying a current in a magnetic field experiences a force, and exerts an equal and opposite force on the magnet.
The force is greatest when the conductor is at right angles to the field, and zero when it is parallel to the field.
The equation
Force = magnetic flux density × current × length, F = BIl.
Magnetic flux density in tesla, current in amperes, length in metres, force in newtons. The equation applies when the conductor is at right angles to the field.
Which length to use
The length in the equation is the length of the conductor that is actually within the magnetic field.
If a wire extends beyond the magnets, only the part inside the field contributes to the force. Using the total wire length is a common error.
Key points
- A current-carrying conductor in a field feels a force.
- The force is greatest at right angles to the field.
- The force is zero when parallel to the field.
- Force = magnetic flux density × current × length.
- Magnetic flux density is measured in tesla.
- Use only the length within the field.
Worked examples
Example 1
A wire of length 0.2 m carries a current of 3 A in a magnetic field of flux density 0.5 T. Calculate the force on it. [3 marks]
Model answer
Force = magnetic flux density × current × length = 0.5 × 3 × 0.2 (1 mark). 0.5 × 3 = 1.5 (1 mark). 1.5 × 0.2 = 0.3 N (1 mark).
Example 2
State the orientation at which the force on a current-carrying conductor in a magnetic field is zero. [1 mark]
Model answer
When the conductor is parallel to the magnetic field (1 mark).
Example 3
State the unit of magnetic flux density. [1 mark]
Model answer
The tesla (1 mark).
Common mistakes
Using the total wire length.
Only the length inside the magnetic field contributes to the force.
Saying the force is greatest when parallel to the field.
It is greatest at right angles and zero when parallel.
Giving flux density in newtons.
It is measured in tesla.
Forgetting the equation applies at right angles.
It holds when the conductor is perpendicular to the field.
Exam tips
- Use only the length of conductor within the field.
- State tesla as the unit of magnetic flux density.
- Remember the force is zero when parallel to the field.
- Remember this is a Higher-only topic.
Key terms
- Motor effect
- The force on a current-carrying conductor in a magnetic field.
- Magnetic flux density
- A measure of field strength, in tesla.
- Tesla
- The unit of magnetic flux density.
- Conductor
- A wire carrying the current.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.