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

• 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
  • The direction of the magnetic field

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• 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 appear to be acting either into or out of the page

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

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

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

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• 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 a electron in a magnetic field the second finger points in the opposite direction to the direction of motion
  • Conventional current is said to flow opposite to the direction of flow of electrons
  • The finger represents current
  • An alternative is to use the right hand to work out directions for charged particles

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