Force on a current-carrying conductor
Section: Electricity & Magnetism | Syllabus: Cambridge IGCSE Physics (0625)
The Motor Effect
When a current-carrying conductor is placed in a magnetic field, it experiences a force. This is called the motor effect. If the wire is free to move, it does so.
Demonstrating the Motor Effect
Experiment: Wire Between Magnets
- Loosely support a flexible wire in the strong magnetic field of a C-shaped magnet, connected to a low-voltage, high-current supply
- Close the switch so current flows through the wire
- Observe: the wire jumps upward (it experiences a force)
- Reverse the current direction → the wire moves downward instead
- Reverse the magnet poles → the wire again moves downward
- The force increases if the strength of the field increases and if the current increases
A current-carrying wire in a magnetic field experiences a force, moving it out of the field region
Why the Wire Moves
The wire's own current produces a circular magnetic field around it, which combines with the magnet's field between the poles. On one side of the wire the two fields point the same way, so the field lines bunch together there; on the other side they oppose, so the field lines are more spread out. Like stretched elastic trying to straighten out, the bunched field lines push the wire towards the side with fewer lines, producing the force.
The force acts from the side with more (bunched) field lines towards the side with fewer field lines
Fleming's Left-Hand Rule
The direction of the force (thrust) on the wire can be found by Fleming's left-hand rule, also called the motor rule.
Hold the thumb and first two fingers of the left hand at right angles to find the direction of the force
- First finger = Field (N to S)
- seCond finger = Current (+ to −)
- Thumb = Thrust (force)
Factors Affecting the Force
The force on a current-carrying conductor increases with:
- Stronger magnetic field
- Larger current
- Longer length of wire in the field
Effect of Angle
If the wire is not at right angles to the field, the force is smaller, and it is zero if the wire is parallel to the field.
Force on Charged Particles
Moving charged particles in a magnetic field also experience a force.
- A beam of electrons passing through a magnetic field at right angles is deflected into a circular path
- The direction follows the left-hand rule (but remember electrons are negative, so their "conventional current" is opposite to their direction of travel)
- Positive particles: force direction from the left-hand rule directly
- Negative particles (e.g. electrons): force direction opposite to the left-hand rule prediction
A charged particle moving at right angles to a magnetic field follows a circular path, since the force is always perpendicular to its motion
Applications
- Cathode ray tubes: deflect electron beams in old TVs and oscilloscopes
- Particle accelerators: bend particle paths
- Mass spectrometers: separate particles by mass
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