The motor effect – Higher

A wire carrying a current creates a magnetic field. This can interact with another magnetic field, causing a force that pushes the wire at right angles. This is called the motor effect.

Wire carries current towards screen. Magnetic field travels north to south across screen, left to right. Resultant force on wire travels down screen, top to bottom.There is a force at right angles to the current and the magnetic field

Fleming's left hand rule - Higher

The force on a given length of wire in a magnetic field increases when:

  • the current in the wire increases
  • the strength of the magnetic field increases
  • the length of conductor in the field is increased

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.

Left hand, thumb, forefinger, second finger at 90 degrees with arrows. Thumb shows direction of movement. Forefinger shows field (north to south). Second finger shows current positive to negative.

Hold your thumb, forefinger and second finger at right angles to each other:

  • the forefinger is lined up with magnetic field lines pointing from north to south
  • the second finger is lined up with the current pointing from positive to negative
  • the thumb shows the direction of the motor effect force on the conductor carrying the current

In which direction will this wire feel a force?

A bar magnet in an angular 'U' sort of shape. The N and S poles are marked. There is a wire running through and round the magnet, with arrows indicating direction of current.

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.

Calculating the motor effect force

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

\text{F} = \text{BIl}

This is when:

  • \text{F} is force in newtons (N)
  • \text{B} is magnetic flux density (magnetic field strength) in tesla (T)
  • \text{I} is current in amperes – also referred to as amps – (A)
  • \text{l} is length in metres (m)

Example

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

\text{F} = \text{BIl}

= 0.4 × 2 × 0.5

force = 0.4 N

Question

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

\text{F} = \text{BIl}

= 0.60 × 0.75 × 0.05

force = 0.0225 N