Aim
To study the force that acts on a current-carrying conductor placed in a magnetic field and how it depends on the direction of the current and the field.
Variables
- Manipulated: Direction of the current (and of the magnetic field)
- Responding: Direction in which the conductor moves
- Constant: The magnetic field strength and the conductor used
Apparatus & materials
- U-shaped magnet (or two slab magnets on a yoke)
- Stiff bare copper wire hung as a swing
- Low-voltage power supply
- Rheostat
- Switch
- Connecting wires
- Retort stand
Procedure
- Hang the stiff copper wire so it can swing freely between the poles of the U-shaped magnet.
- Connect the wire in series with the battery, rheostat and switch.
- Switch on the current and observe the direction in which the wire moves.
- Reverse the direction of the current and observe the direction of movement again.
- Reverse the magnetic poles instead, and observe the direction of movement once more.
- Record the direction of the current, the direction of the field and the resulting direction of movement.
Tabulating results
Record the observations in a table showing the direction of the current, the direction of the magnetic field, and the resulting direction of movement of the wire for each case.
The graph
No graph is needed because the results are qualitative. The observations are compared with Fleming's left-hand rule.
Analysis
A current-carrying conductor in a magnetic field experiences a force produced by the interaction (catapult) of the two fields. Its direction is given by Fleming's left-hand rule; reversing the current or the field reverses the force.
Precautions
- Switch the current on only in short bursts to avoid overheating the wire.
- Make sure the wire hangs freely so it can move.
- Ensure all connections are firm so the current flows steadily.
Electromagnetism · Graph skills
Sample results and what they show
This is a qualitative experiment, so you record directions rather than numbers. Here is an example set of observations (label them as example results):
- Current downward, field from N to S (left to right): wire kicks outward, away from the magnet gap.
- Current reversed (upward), same field: wire kicks inward.
- Current downward again, magnet poles swapped: wire kicks inward.
- Both current and field reversed: wire kicks outward again.
The pattern is the key finding. Reversing either the current or the magnetic field on its own reverses the direction of the force, while reversing both together brings the force back to its original direction.
This shows the wire feels a real force whenever a current flows across a magnetic field, and that the force direction is set jointly by the current direction and the field direction.
Reading the results and finding the answer
No graph is drawn here because the results are directions, not measured values. Instead you check each observation against Fleming's left-hand rule.
Hold the thumb and first two fingers of the left hand at right angles: the First finger points along the Field (N to S), the seCond finger along the Current, and the thuMb then points in the direction of the resulting Motion or force.
Take the first row as a worked check. The field points left to right and the current points downward.
Setting the left hand so the first finger points right and the second finger points down, the thumb points outward from the magnet gap, which matches the observed kick.
Doing the same for the reversed rows shows why one reversal flips the motion and two reversals restore it, confirming the rule for every case.
Marks examiners look for
Precautions that keep the result clear: switch the current on only in short bursts so the wire does not overheat, make sure the wire hangs freely so it can actually move, and keep every connection firm so the current is steady.
For the Paper-3 science-process skills on a qualitative task:
- Record the current direction, the field direction and the movement direction for every case, not just one.
- Change only one thing at a time (current, or field) so the cause of each reversal is clear.
- State the inference clearly: the conductor experiences a force, and its direction obeys Fleming's left-hand rule.
- Link the effect to the interaction of the two magnetic fields (the catapult field).
- Keep the field strength and the wire the same throughout as your controlled variables.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)