Magnetic field around a current
How does the field around a wire change when its current changes?
Change the direction and magnitude of current in a straight wire, using a compass to test the surrounding field’s direction and relative strength.
Learning goals
- Use a test compass to determine field direction at a point around the wire.
- Predict that reversing current reverses the field direction.
- Relate relative field strength to current magnitude and distance from the wire.
Before you begin
- Know that current is directed charge flow.
- Read arrows, clockwise/counter-clockwise direction, and relative strength.
Interactive science experiment
- 1Current
- 2Field lines
- 3Reverse
- 4Magnitude
Preparing the interactive model…
Qualitative teaching model: values and diagrams compare trends; they are not precision measurements.
Observe, then explain
Work through four steps. Each button moves the interactive model to the matching observation state.
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A current-carrying wire produces a magnetic field. Around a straight wire, field direction is tangent to circles centred on the wire, not radially outward.
Your task: Turn on the current and trace one complete circular field line.
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The north-seeking end of a small compass aligns with the local field. Around the wire its direction turns, always tangent to the local circle.
Your task: Compare compass directions above, below, left, and right of the wire.
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When current reverses, the field direction at every point reverses too; the circular pattern remains centred on the same wire.
Your task: Predict how the compass will turn, then use Reverse direction to check.
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At the same distance, a larger current gives a stronger field. For the same current, the field is weaker farther from the wire. Current direction sets field direction; magnitude does not.
Your task: Increase the current and compare strength at one point, then compare a near point with a far point.
Build an explanation
Right-hand grip rule
Point the right thumb along conventional current; curled fingers show the field direction. Reverse the current and both thumb and curl reverse.
Field lines are representations
Circular field lines show direction and spacing compares relative strength; there are no physical rings in space. Lines do not cross because the resultant field has one direction at a point.
Check your understanding
Printable worksheet
Complete the worksheet first. Answers and explanations are on a separate page.
Physics
Magnetic field around a current
How does the field around a wire change when its current changes?
- What shape best represents field lines around a long straight wire?
- What happens to the field when current in the wire reverses?
- At the same distance from the wire, what usually happens when current increases?
Answer page
Magnetic field around a current
- Concentric circles centred on the wire
In a section perpendicular to the wire, the field is tangent to circles centred on the wire.
- Direction reverses; the circular pattern remains
The right-hand grip rule reverses with current, so every circular arrow and compass direction reverses.
- The field strengthens; magnitude alone does not reverse direction
The field around a long straight wire strengthens with current; its direction is set by current direction.