The force on a given length of wire in a magnetic field increases when:
For any given combination of current and magnetic field strength, the force is greatest when the direction of the current is 90° to the direction of the magnetic field. There is no motor effect force if the current and magnetic field are parallel to each other.
The direction of a motor effect force can be found using Fleming's left hand rule.
Hold your thumb, forefinger and second finger at right angles to each other:
In which direction will this wire feel a force?
With forefinger (magnetic field) pointing left to right, and second finger (current) pointing down, your left thumb (force) will point towards you. This is the direction in which the force acts.
Note that the direction of the force can be changed by changing either the direction of the current or the field.
To calculate the force on a wire carrying a current at right angles to a magnetic field, use the equation:
force = magnetic flux density × current × length
This is when:
2 A flows through a 50 cm wire. Calculate the force acting on the wire when it is placed at right angles in a 0.4 T magnetic field.
50 cm = 50 ÷ 100 = 0.5 m
= 0.4 × 2 × 0.5
force = 0.4 N
A 5.0 cm wire carries a current of 0.75 A. Calculate the force acting on the wire when it is placed at right angles in a 0.60 T magnetic field.
5.0 cm = 5 ÷ 100 = 0.05 m
= 0.60 × 0.75 × 0.05
force = 0.0225 N