WASSCE SHS
A wire carrying a current in a magnetic field can feel a force. That force is the motor effect. Fleming's left hand rule gives its direction.
The force is greatest when the current is at right angles to the field, and it is zero when the current is parallel to the field. The direction of the force is at right angles to both the current and the field. Fleming's left hand rule keeps the three directions apart. The first finger points in the direction of the field, from north to south. The second finger points in the direction of the conventional current, from positive to negative. The thumb then points in the direction of the force, which is the motion of the wire.
An electric motor uses this effect. A coil carrying a current sits in a magnetic field. Opposite sides of the coil feel forces in opposite directions, so the coil turns. A split ring commutator reverses the current in the coil every half turn, so the forces keep turning it the same way. Without the commutator the coil would stop after half a turn, or swing and settle. Increasing the current, or using a stronger magnet, or using more turns, increases the turning effect.
Left hand for the motor, the force on a current. The right hand rule is for the induced current in a generator. Swapping the hands is a whole mark lost. Name which finger does which job if the question asks for the rule, not only "use Fleming". The commutator reverses the current. It does not reverse the magnetic field. Say which one changes.
These are practice questions in the style of the paper, with a model answer and a mark scheme. They are not a promise of what your paper will ask.
3 marks
State Fleming's left hand rule. (3 marks)
The first finger points in the direction of the magnetic field. The second finger points in the direction of the current. The thumb then points in the direction of the force on the wire.
4 marks
(a) Why does a simple direct current motor need a split ring commutator? (2 marks)
(b) State two ways to increase the turning effect on the coil. (2 marks)
(a) It reverses the current in the coil every half turn, so the forces continue to turn the coil in the same direction.
(b) Increase the current, and use a stronger magnetic field.
2 marks
A wire is parallel to a magnetic field and carries a current. State the size of the force on it, and why. (2 marks)
The force is zero. The motor effect needs a component of the current at right angles to the field, and here there is none.
More questions on this topic are inside GLearn, marked as you answer, with the reason written out. The first ones need no account.
Practise Physics freeFleming's left hand rule. The first finger is the field, the second finger is the current, and the thumb is the force. The right hand rule is for the current induced in a generator. Do not swap them.
In a direct current motor the split ring commutator reverses the current in the coil every half turn. The forces then keep turning the coil the same way. It does not reverse the magnet. All WASSCE Physics past questions