4.5.4 Force on a current-carrying conductor
- Syllabus
- 0625–2026–2027
- Topic
- 4.5.4
- Level
- —
Learning objectives
4.5.4.1An experiment to show that a force• 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 field4.5.4.2Relative directions of force, magnetic• Recall/use the relative directions of force, magnetic field and current4.5.4.3Direction of the force on beams of• Determine the direction of the force on beams of charged particles in a magnetic field
Show the force on a current-carrying conductor
Place a light metal rod on two conducting rails so that the rod lies between the poles of a strong magnet and can roll or swing freely. Connect the rails to a low-voltage d.c. supply through a switch.
| Test | Observation | Conclusion |
|---|---|---|
| close the switch with the rod in the field | the rod moves sideways | a current-carrying conductor in a magnetic field experiences a force |
| reverse the supply connections only | the rod moves in the opposite direction | reversing current reverses the force |
| restore the current, then exchange N and S poles | the rod moves in the opposite direction | reversing field reverses the force |
| reverse both current and field | motion is in the original direction | two reversals cancel |
Keep the rod position, current magnitude and magnet spacing unchanged when comparing directions. With the switch open there is no current and no motor-effect force; outside the field the effect should disappear or become much smaller.
If the current repeatedly reverses while the field stays fixed, the force repeatedly reverses and the conductor vibrates.
The force is caused by the interaction between the conductor's current-produced field and the external magnetic field. The conductor itself does not need to be made from magnetic material.
Use Fleming's left-hand rule for force
Hold the left-hand thumb, first finger and second finger mutually perpendicular. First finger points along the magnetic field from N to S, second finger points along conventional current, and thumb gives the force or motion.
| Left-hand digit | Direction |
|---|---|
| first finger | magnetic field, N to S |
| second finger | conventional current, from positive to negative |
| thumb | force / motion of the conductor |
Read the field direction first, then the conventional current. Orient the two fingers before reading the thumb. The force is perpendicular to both field and current.
| Change | Force direction |
|---|---|
| reverse current only | reverses |
| reverse field only | reverses |
| reverse both | unchanged |
| increase current or field strength without reversing | same direction, larger force |
Use the left hand for the motor effect: current is supplied and a force results. Fleming's right-hand rule is for generator action, where motion produces an induced current.
Find the force on a charged-particle beam
A moving charged particle behaves like a current in a magnetic field and can experience a force perpendicular to both its motion and the field.
| Particle | Direction to use as conventional current |
|---|---|
| positive charge, such as an α-particle | same as the particle's motion |
| negative charge, such as an electron or β⁻-particle | opposite to the particle's motion |
| neutron | no conventional current; no magnetic force from its charge |
Identify the field direction from N to S or from dot/cross symbols. Convert the beam motion to conventional-current direction, then use Fleming's left-hand rule. For a negative particle, this gives the force opposite to that on a positive particle moving the same way.
When the beam crosses the field at right angles, the force deflects it sideways and continually changes its direction, producing a curved path while it remains in the field.
A charged particle moving parallel to the magnetic field is not deflected because there is no perpendicular component of motion. Do not put electron motion directly into the left-hand current finger without reversing it.