10.2.5Magnetism and the Motor Effect
Calculating Magnetic Force — Edexcel GCSE Combined Science 1SC0 - Physics Revision Notes & Topical Questions
5 min read 8 parts Lesson 10 of 10
Part 1 of 6The equation (HT only)
F = B × I × L
- F = B × I × L This gives the size of the force on a straight wire carrying a current while at right angles to a magnetic field - the motor effect.
Symbols and units
- Symbols and units F is the force in newtons (N).
- B is the magnetic flux density in tesla (T).
- I is the current in amperes (A).
- L is the length of wire inside the field in metres (m).
The tesla
- The tesla 1 T = 1 N/A m: a field that exerts 1 N on 1 m of wire carrying 1 A.
Part 2 of 6How each quantity affects the force
| Quantity | If doubled, force... |
|---|---|
| Magnetic flux density B | doubles |
| Current I | doubles |
| Length of wire in the field L | doubles |
| Wire parallel to the field | is zero; the equation applies to a wire at 90° to the field |
Part 3 of 6Worked example
1
A 5 cm wire at 90° to a field carries 1.5 A and feels a force of 0.06 N. Find B.
2
Convert the length: 5 cm = 0.05 m.
3
Rearrange F = B × I × L to B = F ÷ (I × L).
4
Substitute: B = 0.06 ÷ (1.5 × 0.05) = 0.06 ÷ 0.075.
5
B = 0.8 T.
Part 4 of 6Rearrangements
| To find | Use |
|---|---|
| Force | F = B × I × L |
| Flux density | B = F ÷ (I × L) |
| Current | I = F ÷ (B × L) |
| Length | L = F ÷ (B × I) |
Part 5 of 6Second worked example: finding the force
1Given
- Given A wire of length 0.20 m at 90° to a field of 0.50 T carries a current of 3.0 A.
2Equation
- Equation F = B × I × L.
3Substitute
- Substitute F = 0.50 × 3.0 × 0.20.
4Answer
- Answer F = 0.30 N, acting at right angles to both the field and the current.
Part 6 of 6Using the equation in context
Applying F = B × I × L (HT only)
Electric motors
- Electric motors The coil in a motor has wire inside a magnetic field;
- a larger current or stronger magnets increase the force and so the turning effect.
Direction
- Direction Fleming's left-hand rule gives the direction: first finger for field, second finger for current, thumb for the force (motion).
Unit checking
- Unit checking Check that T × A × m gives newtons;
- this helps you spot when a length has not been converted to metres.
- Always check that the wire is at right angles to the field before using the equation, because the force falls as the wire turns towards parallel.
- Write the unit on every line of working; the final answer for force should be in newtons (N).
Key terms
| Term | Meaning |
|---|---|
| Magnetic flux density | A measure of the strength of a magnetic field; the unit is the tesla. |
| Tesla (T) | The unit of magnetic flux density, equal to 1 N/A m. |
| Motor effect | The force on a current-carrying conductor in a magnetic field. |
| Perpendicular | At right angles (90°) to a given direction. |
Avoid these mistakes
1
Using the length in centimetres - the length must be in metres.
2
Using the whole length of the wire instead of only the part inside the magnetic field.
3
Applying F = BIL for a wire parallel to the field, where there is no force.
4
Multiplying instead of dividing when rearranging for B.
5
Using the number of turns of a coil as the length without multiplying by the length of wire in the field for each turn.
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