Magnetic Force on a Current Carrying Conductor (WJEC GCSE Physics)

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

Magnetic Fields Interacting

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

Force on a current-carrying wire in a magnetic field

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 external magnetic field and wire are parallel there will be no interaction between the two magnetic fields and therefore no force produced

Fleming's Left Hand Rule

• The direction of the force (aka the thrust) on a current carrying wire depends on the direction of the current and the direction of the magnetic field
• All three will be perpendicular to each other
  • This means that sometimes the force could be into and out of the page (in 3D)
• The direction of the force (or thrust) can be worked out by using Fleming's left-hand rule:

Fleming's left-hand rule

Fleming's left-hand rule can be used to determine directions of the force, magnetic field and current

Use Fleming’s left-hand rule to show that if the current-carrying wire is placed into the magnetic field between the poles of the magnet, as shown below, there will be a downwards force acting on the wire.

Current into the plane of the page

Align your fingers with the diagram to use Fleming's left-hand rule

Step 1: Determine the direction of the magnetic field

• • Start by pointing your First Finger in the direction of the (magnetic) Field.

Step 2: Determine the direction of the current

• • Now rotate your hand around the first finger so that the seCond finger points in the direction of the Current

Step 3: Determine the direction of the force

• • The THumb will now be pointing in the direction of the THrust (the force)
  • Therefore, this will be the direction in which the wire will move

Different orientation of the left-hand rule

You may have to move your hand around for different questions - this is fine as long as you remember which finger represents which quantity

Higher Tier

• The size of the force acting on a current-carrying wire in a magnetic field can be calculated using the equation:

• 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-carrier perpendicular to a magnetic field

Force on this current carrying conductor is directed into the page