The Motor Effect

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

A current-carrying wire will produce its own magnetic field and so 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 there 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:

  • Pointing your index finger forwards – This represents the direction of the magnetic field, so pointed from the north to the south pole.
  • Your middle finger sticking out to the side – This represents the direction of the current.
  • Pointing your thumb upwards – The direction of your thumb will be the direction of the force felt by the wire.

Calculating the Strength of Force

To calculate the size of the force acting on a current carrying wire, at a right angle to the direction of a magnetic field, we use the 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: Write down 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

20 ÷ 100 = 0.2 m

Step 4: Substitute the values into the equation

= 0.6 × 4 × 0.2

= 0.48 N