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

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

F = B~I~l

This is when:

  • F is force in newtons (N)
  • B is magnetic flux density (magnetic field strength) in tesla (T)
  • I is current in amperes – also referred to as amps – (A)
  • 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

F = B~I~l

F = 0.4 \times 2 \times 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.

50~cm = 50 \div 100 = 0.050~m

F = B~I~l

F = 0.60 \times 0.75 \times 0.050

force = 0.0225~N