Magnetic fields and the force on a current Cambridge International AS & A Level Physics revision
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In plain words
Hold a wire carrying a current between the poles of a magnet and it gets a sideways shove. That force is what turns every electric motor. And how big it is tells us how strong the magnetic field is.
4 things to know
- A magnetic field is a region in which a magnet, a current-carrying conductor or a moving charge experiences a force. Magnetic fields are made by permanent magnets and by moving charges. Field lines run from north to south outside a magnet.
- The force on a conductor of length L carrying a current I in a field of flux density B is F = BIL sin θ, where θ is the angle between the current and the field. It is greatest when they are at right angles and zero when they are parallel.
- Magnetic flux density B is the force per unit current per unit length on a wire at right angles to the field. Unit: the tesla (T).
- The direction of the force comes from Fleming's left-hand rule: first finger for the field, second finger for the current, thumb for the force.
Worked example
A wire 0.20 m long carries a current of 3.0 A at right angles to a magnetic field of flux density 0.050 T. Find the force on it.
- F = BIL sin θ, and sin 90° = 1.
- = 0.050 × 3.0 × 0.20.
- = 0.030 N.
Tips and tricks
- The force is at right angles to both the current and the field.
- Use your left hand, and point your second finger the way conventional current flows, from positive to negative.
It lands in your notebook with its questions as flashcards.
Magnetic fields and the force on a current: 5 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
What is the unit of magnetic flux density?
One tesla is one newton per ampere per metre.
A current-carrying wire lies parallel to a magnetic field. What is the force on it?
sin 0° = 0.
In Fleming's left-hand rule, what does the thumb show?
First finger: field. Second finger: current.
The current in a wire in a magnetic field is doubled. What happens to the force on it?
F is proportional to I.
A wire 0.10 m long carries 2.0 A at right angles to a field of flux density 0.50 T. What is the force on it?
0.50 × 2.0 × 0.10.
Quiz
5 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
4 questions, 9 marks. Write your answers on paper, then check them.
Magnetic fields and the force on a current
Cambridge International AS & A Level Physics 9702 · 9 marks · papermunch.org
Name ______________________________ Date ______________
A wire 0.15 m long carries 4.0 A at 30° to a field of flux density 0.20 T. Find the force on it.[2]
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0.060 N. 0.20 × 4.0 × 0.15 × sin 30°.
A wire 0.050 m long at right angles to a magnetic field feels a force of 0.012 N when it carries 2.0 A. Find the flux density.[2]
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0.12 T.
Define magnetic flux density.[2]
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The force per unit current per unit length acting on a wire placed at right angles to the magnetic field.
Describe how a top-pan balance can be used to measure the flux density between the poles of a magnet.[3]
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Stand the magnet on the balance and clamp a wire so that a known length L lies between the poles, at right angles to the field. Pass a current I through the wire and record the change in the balance reading; multiply it by g to get the force. Repeat for several currents. A graph of force against current is a straight line with gradient BL.
Answers: Magnetic fields and the force on a current
- 1. 0.060 N. 0.20 × 4.0 × 0.15 × sin 30°.
- 2. 0.12 T.
- 3. The force per unit current per unit length acting on a wire placed at right angles to the magnetic field.
- 4. Stand the magnet on the balance and clamp a wire so that a known length L lies between the poles, at right angles to the field. Pass a current I through the wire and record the change in the balance reading; multiply it by g to get the force. Repeat for several currents. A graph of force against current is a straight line with gradient BL.