Fleming's right-hand rule

When a straight conductor is placed at right angles to a magnetic field and moved at right angles to both the field and itself, an emf is induced across its ends. If the conductor is part of a closed circuit, this electromotive force makes a current flow in it. The direction of the induced current is found with Fleming's right-hand rule.

Hold the thumb, first finger and middle finger of the right hand at right angles to one another. Point the thumb in the direction the conductor moves and the first finger in the direction of the magnetic field it cuts; the middle finger then shows the direction of the current flowing in the conductor.

Figure to be added

Figure 13.19 — Fleming's right-hand rule: a green rod moved upwards between N and S poles; the thumb points up (motion), the first finger along the field, the middle finger along the induced current.

Left hand — motors

Given the current and the field, find the force.

Thumb = force, first finger = field, middle finger = current

Right hand — generators

Given the motion and the field, find the induced current.

Thumb = motion, first finger = field, middle finger = current

Why a different hand

In a motor, the current comes first and the movement follows. In a generator, the movement comes first and the current follows. The induced current, flowing in the moving wire, is itself pushed on by the field — and by the left-hand rule that push is always backwards, against the motion. That is why it takes effort to turn a generator, and why the energy of the current has to come from whatever turns it. The two hands give exactly this result: try the same field and motion with both hands and the currents come out opposite. Keeping the hands straight is the whole trick of these questions.

Right hand for the induced current

Right hand: thumb — motion, first finger — field, middle finger — induced current.