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

Key Points 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

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.

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.
• Each direction is 90° to each other
• Use this to work out the unknown factor out of the three (usually the direction of the force felt)
• Remember current is conventional current, which moves in the opposite direction to the electrons

Formula

• $F$ = force in newtons (N)

• $B$ = magnetic flux density in tesla (T)

• $I$= Current in amperes (A)

• $L$ = Length in metres (m) Where Magnetic Flux Density is measured in Tesla

• And it is the number of flux lines per metre squared