A current-carrying wire in a magnetic field feels a force at right angles to both the current and the field. Fleming’s left-hand rule gives the direction.
This lesson is part of SPM Physics electromagnetism. Its reverse effect is in explaining electromagnetic induction.
How does the left-hand rule work?
Use your left hand. Point the first finger along the field (north to south), the second finger along the conventional current, and the thumb shows the force.
| Finger | Quantity | Direction rule |
|---|---|---|
| First | magnetic field | north pole to south pole |
| Second | current | positive to negative |
| Thumb | force | result of the rule |
Worked example 1: current out of the page
A straight wire carries current out of the page. The field between two magnets points from left to right.
Point the first finger to the right and the second finger out of the page towards you. The thumb points up the page, so the wire is pushed upward.
Worked example 2: change one quantity
Keep the same field, but the current is now into the page. Reversing the current reverses the force, so the wire is pushed down the page.
If the field is also reversed, the force goes back to upward. Reversing two quantities cancels.
The mistake that loses marks
A common slip is to use the direction of electron flow for the second finger. The force then comes out the wrong way round, and every step after it is wrong.
Mark the conventional current arrow on the diagram first. Then apply the rule to that arrow, not to the electrons.
Check yourself
A horizontal wire carries current to the right. The magnetic field points out of the page towards you. In which direction is the force on the wire?
Answer
First finger points out of the page (field). Second finger points to the right (current).
The thumb points down the page. Check: reversing the current would give upward, so downward is consistent with the rule.
What to study next
The reverse effect, making a current from motion in a field, is in explaining electromagnetic induction. See both in machines in comparing motors and generators.
Test direction questions in the practice set. The mistake log helps track the ones you flip. For a teacher to check your hand rule with you, see online one-to-one Physics tuition or the one-hour trial class (from RM50).