Electromagnetic Effects: Question 1
Syllabus 4.5.3
A long straight wire passes vertically through a small hole in a flat horizontal sheet of card, with the current in the wire flowing vertically upward. You look down at the card from directly above, so the current is flowing straight out of the card towards you. Using the right-hand grip rule, what pattern do the magnetic field lines form on the card around the wire, and in which direction do they point, as seen from your viewpoint above?
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Worked solution
Step 1: Recall the shape of the field around a straight current-carrying wire
A current-carrying wire is surrounded by a magnetic field made up of concentric circles centred on the wire, lying in a plane at right angles to it. The field gets weaker further from the wire (the circles get more widely spaced), but it never radiates outward like an electric field and never runs parallel to the wire. That pattern only appears inside a solenoid.
This immediately rules out options C and D.
Step 2: Apply the right-hand grip rule to find the direction
The right-hand grip rule: point the thumb of your right hand in the direction of the (conventional) current; your curled fingers then show the direction the field lines circle around the wire.
Here the wire passes vertically through the card and the current flows upward, so looking down from your viewpoint above the card, the current is coming straight out of the card towards you. Point your right thumb towards yourself (out of the card, towards your eye). Curling your right-hand fingers around the wire in this position traces circles that go anticlockwise, as seen from above.
Step 3: Match to the correct option
An anticlockwise circular field, as seen from above with the current coming out of the card towards the viewer, matches option B.
Why the other options are wrong
| Option | Description | Error |
|---|---|---|
| A | Clockwise circles | Grip rule applied with the wrong hand (or thumb pointed the wrong way) |
| C | Radial lines outward | Confuses the magnetic field with an electric field pattern |
| D | Lines parallel to the wire | Confuses this wire’s field with the field inside a solenoid |
Final answers
- The field is a set of concentric circles, directed anticlockwise as seen by the viewer, option B.