Exercise 13.1
Exercise 13.1: use Fleming's left-hand rule to find the direction of the force on a conductor carrying a current in each of four magnetic fields.
Figure to be added
How to work each one
- The field runs from N to S across the gap: point the first finger that way.
- ⊙ is current out of the page, towards you; ⊗ is into the page: point the middle finger that way.
- Read the force from the thumb.
| Case | Field | Current | Force |
|---|---|---|---|
| (i) | left to right | out of the page | upwards |
| (ii) | left to right | into the page | downwards |
| (iii) | right to left | out of the page | downwards |
| (iv) | right to left | into the page | upwards |
Why the answers pair up
Compare (i) and (ii): the field is the same and the current is reversed, so the force reverses — up becomes down. Compare (i) and (iii): the current is the same and the field is reversed, so again the force reverses. In (iv) both are reversed, and two reversals cancel, so the force is the same as in (i). You can use this as a check on any set of left-hand-rule answers: change one thing and the force flips; change two and it comes back.
The book writes the poles with the Sinhala letters උ (උත්තර, north) and ද (දක්ෂිණ, south); they are N and S here.
One reversal flips the force, two cancel
With N on the left and current out of the page, the force is upwards; reversing either the current or the field sends it downwards.