The Motor Effect

A wire carrying a current will experience a force in the presence of a magnetic field. This is known as the motor effect.

A current-carrying wire produces its own magnetic field, as does a permanent magnet. This means that if we place the wire (or coil) between the north and south poles of two permanent magnets, then the two magnetic fields will interact.

The interaction between the two magnetic fields will result in a force on the wire, pushing it out of the field. This force will be at a right angle to both the direction of the wire carrying current and the direction of the magnetic field.

However, to experience the full force, the wire has to be at exactly 90 degrees (right angle) to the magnetic field. This means that if the wire is at a different angle, it will experience less force. If the wire is going in the same direction as the field, then the wire will experience no force.

To find the direction of the force, we need to know:

  • The direction of the magnetic field
  • The direction of the current in the wire

To understand how the two factors affect the force, we can use a concept called Fleming’s left-hand rule.

Using your left hand, this rule involves:

  • Point your index finger forward – This represents the direction of the magnetic field, pointing from the north to the south pole.
  • Extend your middle finger to the side – This represents the direction of the current.
  • Point your thumb upward – The direction your thumb points indicates the direction of the force felt by the wire.

Calculating the Strength of Force

To calculate the magnitude of the force acting on a current-carrying wire placed at a right angle to the direction of a magnetic field, we use the following equation:

  • F = force in newtons (N)
  • B = magnetic flux density in tesla (T)
  • I = Current in amperes (A)
  • L = Length in metres (m)

Magnetic flux density is a measure of the strength of the magnetic field.


4 A of current flows through a 20 cm length of wire. The wire is placed at a right angle in a 0.6 T magnetic field. Calculate the force acting on the wire.

Step 1: Identify the equation:

F = B × I × L

Step 2: List the known quantities:

B = 0.6 T

I = 4 A

L = 20 cm

Step 3: Convert the length to metres:

L = 20 cm

= 0.2 m (since 20 ÷ 100 = 0.2)

Step 4: Substitute the values into the equation:

F = 0.6 × 4 × 0.2

= 0.48 N

You’ve used 0 of your 10 free revision notes for the month

Sign up to get unlimited access to revision notes, quizzes, audio lessons and more

Sign up