Force on a current-carrying conductor: Fleming's left-hand rule
A current-carrying conductor in a magnetic field feels a force. Its direction comes from Fleming's left-hand rule, and it is largest when current and field are at right angles.
Activity: the moving aluminium rod
This shows that the force depends on the directions of both the current and the field.
- 1Hang a small aluminium rod horizontally from a stand, using two connecting wires.
- 2Place a strong horseshoe magnet so the rod lies between its poles, with the field pointing upwards (N pole below the rod, S pole above it).
- 3Connect the rod in series with a battery, a plug key and a rheostat.
- 4Close the key. Observation: the rod is pushed (displaced) to one side.
- 5Reverse the current. Observation: the rod moves the opposite way.
- 6Turn the magnet over so that the field points downwards. Observation: the rod again moves the opposite way.
Why the rod moves
Oersted showed that a current exerts a force on a nearby magnet, the compass needle. The French scientist André-Marie Ampère suggested that the magnet must also exert an equal and opposite force on the current-carrying conductor. The current in the rod makes its own magnetic field. This field and the magnet's field act on each other, so the rod feels a force. The force is largest when the current is at right angles to the field. It is zero when the current is parallel to the field. This force is used in loudspeakers and in electrical measuring instruments.
Fleming's left-hand rule
- Stretch the thumb, forefinger and middle finger of your left hand so that they are at right angles to one another.
- Point the forefinger along the magnetic field (from N to S).
- Point the middle finger along the current.
- The thumb then points in the direction of the force, that is, the motion of the conductor.
- Memory: Forefinger = Field, Centre finger = Current, thuMb = Motion.
Worked example: using the rule
A horizontal wire carries a current from south to north. A magnetic field acts vertically upwards. In which direction is the force on the wire?
- Left hand: point the forefinger vertically upwards, along the field.
- Point the middle finger towards the north, along the current.
- Keep the three fingers at right angles. The thumb now points towards the east.
The force on the wire is towards the east.
How to draw the aluminium rod activity
- Draw a horseshoe magnet with its arms one above the other: label the lower pole N and the upper pole S.
- Draw a short horizontal rod AB between the poles, hung from a stand by two connecting wires.
- Join the two wires to a battery, a plug key and a rheostat in series.
- Draw an upward arrow between the poles labelled magnetic field, and an arrow along the rod labelled current.
- Draw a bold arrow labelled force at right angles to both, showing which way the rod moves. Label every part.
Tiny currents in our nerve cells make weak magnetic fields, and the heart and brain have measurable ones. Hospitals use MRI (magnetic resonance imaging) scans, which rely on magnetism, to see inside the body. The speaker in your phone moves because of this force.
2 marks: “State Fleming's left-hand rule.” Stretch the thumb, forefinger and middle finger of the left hand at right angles to each other. If the forefinger points along the field and the middle finger along the current, the thumb gives the direction of the force.
3 marks: “In the aluminium rod activity, what happens to the rod when (a) the current is reversed, (b) the poles of the magnet are swapped, and (c) the rod is placed parallel to the field?” (a) It moves the opposite way. (b) It moves the opposite way. (c) There is no force, so it does not move.
1 marks: “When is the force on a current-carrying conductor in a magnetic field the largest?” When the current is at right angles to the magnetic field.
Field, Current, Motion: Fore, Centre, thuMb of the left hand. Left hand for force.