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

Spec point — Describe an experiment to show that a force acts on a current-carrying conductor in a magnetic field, including the effect of reversing: (a) the current (b) the direction of the field

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

Exam Hint This phenomenon is sometimes referred to as 'the motor effect'. The direction of the force is determined by Fleming’s left-hand rule.

Fleming's left-hand rule

Spec point — 4.5.2d · Recall and use the relative directions of force, magnetic field and current

Fleming’s left-hand rule

• The direction of the force (also called 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)

Worked Example A current-carrying wire is placed into the magnetic field between the poles of the magnet, as shown in the diagram.

Use Fleming’s left-hand rule to show that there will be a downward force acting on the wire.

[3]

Answer:

Step 1: Determine the direction of the magnetic field

• Start by pointing your First Finger in the direction of the (magnetic) Field [1 mark]

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 [1 mark]

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 [1 mark]

Exam Hint Remember that the magnetic field is always in the direction from North to South and current is always in the direction of a positive terminal to a negative terminal.

Charged particles in a magnetic field

Spec point — 4.5.3 · Determine the direction of the force on beams of 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 to the magnetic field

  • 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

Exam Hint Remember that the direction of current is the direction of positive charge. Therefore, if a particle has a negative charge (such as an electron), then the second finger (current) must point in the opposite direction to its direction of travel.

The left-hand rule can be applied to any charged particles, but in the IGCSE exam questions are likely to stick to electrons.

Video of the motor effect

Spec point — nan