The Motor Effect | Physics for Grade 10 PDF Download

About The Motor Effect

• The motor effect occurs when:
  A wire with current flowing through it is placed in a magnetic field and experiences a force
• This effect is a result of two interacting magnetic fields
  • One is produced around the wire due to the current flowing through it
  • The second is the magnetic field into which the wire is placed, for example, between two magnets
• As a result of the interactions of the two magnetic fields, the wire will experience a force

The magnetic field between opposite poles of magnets interact with the magnetic field produced around a current-carrying wire

The motor effect is a result of two magnetic fields interacting to produce a force on the wire

Factors Affecting Force

• The size of the force exerted by the magnetic fields can be increased by:
  • Increasing the amount of current flowing through the wire
    • This will increase the magnetic field around the wire
• Using stronger magnets
  • This will increase the magnetic field between the poles of the magnet
• Placing the wire at 90^o to the direction of the magnetic field lines between the poles of the magnet
  • This will result in the maximum interaction between the two magnetic fields
• Note: If the two magnetic fields are parallel there will be no interaction between the two magnetic fields and therefore no force produced

Exam Tip

It is key to remember that the magnetic force on the magnetic and conductor is maximum when their field lines are perpendicular to each other.  The force is zero when they are parallel.

Calculating Magnetic Force on a Current-Carrying Conductor

• The size of the force acting on a current-carrying wire in a magnetic field can be calculated using the equation:
  F = BIL
• Where:
  • F = force acting on current-carrying wire in Newtons (N)
  • B = magnetic flux density (which is the strength of the magnetic field) in Tesla (T)
  • I = current flowing through the conductor in Amps (A)
  • L = length of the conductor that is in the magnetic field in metres (m)

Force on a current carrying conductor (directed into the page)

Solved Example

Exam Tip

For the maximum force, F, then B and I must be perpendicular to each other. If the question states that the wire and field are parallel then F = 0!
Make sure that the units for F is Newtons, B is Tesla, I is Amps and L is metres. Make sure to convert any units if you're given otherwise (e.g. cm to m in the worked example.)