Electromagnetic Effects: Question 6
Syllabus 4.5.4
In an experiment, a straight wire carrying a current is placed between the poles of a magnet, and the wire experiences a force (the motor effect) that pushes it vertically upward.
Without changing anything else, a student then reverses both the direction of the current in the wire and the direction of the magnetic field (by turning the magnet around so its poles are swapped).
What happens to the direction of the force on the wire?
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Worked solution
Step 1: Recall the effect of reversing current or field alone
The motor effect can be investigated by reversing the current or the field one at a time. Reversing just the current (keeping the field the same) reverses the direction of the force. Reversing just the field (keeping the current the same) also reverses the direction of the force.
Step 2: Combine the two reversals
Here, both the current and the field are reversed at the same time.
- Reversing the current alone would flip the force direction once (upward downward).
- Reversing the field as well flips it a second time (downward upward again).
Two reversals cancel each other out, so the force ends up pointing in its original direction.
Step 3: Match to the correct option
The force still acts vertically upward, exactly as before both reversals were made, this matches option B.
Why the other options are wrong
| Option | Description | Error |
|---|---|---|
| A | Force reverses (downward) | Only accounts for one of the two reversals, not both |
| C | Force disappears | Confuses “cancelling out to the original direction” with “cancelling out to nothing” |
| D | Force acts sideways | The force stays along the same line of action; reversing current and field does not rotate its line of action |
Final answers
- The force still acts vertically upward, reversing both the current and the field returns the force to its original direction, option B.