Force on a Current-Carrying Conductor
How? Interaction between magnetic fields
Magnetic field from magnet interacts with the magnetic field due to current in the wire. A combined magnetic field is produced which is stronger at region A and weaker at region B. A force then acts on the wire from the stronger field to the weaker field.
Force between Two Current-Carrying Wires
Fleming’s Left Hand Rule
From Fleming's Left Hand Rule, we can clearly see that the direction of force depends on
If the direction of current is parallel to direction of magnetic field, there will be NO force produced. The magnitude of force is maximum when the direction of current is perpendicular to the direction of magnetic field.
(b) The rod moves in when the switch is closed.
By Fleming’s Left Hand Rule, the direction of force (indicated by Thumb) is perpendicular to both the direction of magnetic field from North pole to South pole (indicated by Index Finger) and the direction of current (indicated by Middle Finger).
When answering this kind of question, always remember to state the fingers!
When the switch is closed, current flowing through the rod and producing magnetic field around it. This magnetic field interacts with the magnetic field from the magnet, which result in a combined magnetic field which is stronger outside and weaker inside. A force then acts on the rod from a stronger magnetic field to a weaker one, and hence the rod moves in.
Both answers are acceptable, but using Fleming's Left Hand Rule is easier to explain this.
(c) (i) When the current is increased, the rod moves faster.
(ii) When the current is reversed, the rod moves out of the magnet.
Force on a Moving Charge
In Current Electricity, we have learnt that current is defined as the rate of flow of charge.
The direction of movement of positively charged particle is same as the direction of current while the direction of movement of negatively charged particle is same as electron flow, which is opposite to the direction of current.
Force on a Current-Carrying Coil