Force on a Current-Carrying Conductor (Cambridge (CIE) IGCSE Physics)

Force on a current-carrying conductor

• A current-carrying conductor produces its own magnetic field

  • When interacting with an external magnetic field, it therefore will experience a force

• A current-carrying conductor will only experience a force if the current through it is perpendicular to the direction of the magnetic field lines

  • A simple situation would be a copper rod placed within a uniform magnetic field

  • When current is passed through the copper rod, it experiences a force which makes it move

A copper rod moves within a magnetic field when current is passed through it

• Two ways to reverse the direction of the force (and therefore, the copper rod) are by:

  • reversing the direction of the current

  • reversing the direction of the magnetic field

Flemings left-hand rule

• The direction of the force (aka the thrust) on a current-carrying wire depends on the direction of

  • the current

  • the magnetic field

• The direction of the force (or thrust) can be worked out by using Fleming's left-hand rule:

  • the thumb points in the direction of the force, or thrust, on the conductor

  • the first finger points in the direction of the magnetic field

  • the second finger points in the direction of current flow (from positive to negative)

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

• All three will be perpendicular to each other in Fleming's left-hand rule questions

  • This means that sometimes the force could be into and out of the page (in 3D)

Charged particles in a magnetic field

Extended tier only

• When a current-carrying wire is placed in a magnetic field, it will experience a force if the wire is perpendicular

  • This is because the magnetic field exerts a force on each individual electron flowing through the wire

• Therefore, when a charged particle passes through a magnetic field, the field can exert a force on the particle, causing it to deflect

  • The force is always at 90 degrees to both the direction of travel and the magnetic field lines

  • The direction can be worked out by using Fleming's left-hand rule

• In the case of an electron in a magnetic field

  • the second finger (current) points in the opposite direction to the direction of motion

  • this is because conventional current flows in the opposite direction to electron flow

When a charged particle (such as an electron) enters a magnetic field, it is deflected by the field

• If the particle is travelling perpendicular to the field lines:

  • it will experience the maximum force

• If the particle is travelling parallel to the field lines:

  • it will experience no force

• If the particle is travelling at an angle to the field lines:

  • it will experience a small force