IGCSE Physics / Electric motors and electromagnetic induction
Fleming's left-hand rule
Edexcel International GCSE Physics (4PH1), chapter 21: Electric motors and electromagnetic induction, specification point 6.13. Free lesson.
Read the revision notes · IGCSE Physics past papers
Fleming’s left-hand rule lets you work out the direction of the force on a current-carrying wire in a magnetic field. It is useful for predicting which way the wire moves, including when diagrams use dots and crosses.
How to hold your hand
Hold your left hand with your thumb and first two fingers all at right angles. Each points in the direction of one quantity, so line up the field and current first. Your thumb then shows the force on the wire, which is also its direction of motion. First finger: magnetic field, from north to south. Second finger: conventional current, from positive to negative. Thumb: motion, in the direction of the force.
Finding the force on a wire
For a wire in a uniform field, use your first two fingers to match the field and current directions. Keep them in place and read the force from your thumb.
Reading dots and crosses
A wire shown end on is viewed along its length. A cross represents the tail of an arrow moving away from you, so the current goes into the page. A dot represents the arrowhead coming towards you, so the current comes out of the page. With the field from left to right, a current into the page gives a force down the page. In the same field, a current out of the page gives a force up the page. Reversing one quantity reverses the force. Reversing both quantities leaves the force in its original direction.
Avoid using the wrong hand
Use your left hand for the force on a current in a magnetic field. The right hand gives the opposite direction, so using it reverses your answer. For wire A, the left-hand rule gives a force down the page.
Check what happens when both directions reverse
If both the field and current reverse, the force stays the same as before. The two reversals cancel out, which you can check using your left hand.