What does an electric current produce in the space around its wire?
A straight wire runs up through a hole in a flat board. Six small compasses sit on the board around the wire. The current is switched on, and every needle swings at once.
You know why the needles move: the current makes a magnetic field in the space around the wire. This lesson asks what shape that field has.
Each compass needle lines up with the magnetic field where the needle sits. Look at where the six needles come to rest. The needles settle along a ring around the wire.
Every compass tells the same story. The field near a straight wire does not point toward the wire, and it does not run parallel to the wire. It circles the wire.
What shape are the field lines around a straight current-carrying wire?
Back in the Fields topic, you saw that field lines drawn closer together show a stronger field. The standard drawing of a wire's field uses that convention.
Near the wire, the circles sit close together, so the field there is strong. Farther out, each circle sits farther from its neighbor, so the field there is weaker. So the drawing records two facts at once: the field's circular shape, and its fading strength with distance.
In a field drawing of a straight current-carrying wire, the circles are drawn farther apart farther from the wire. What does that spacing show?
A compass sits 2 cm from a straight current-carrying wire. A second compass sits 8 cm out. Which compass sits where the drawn circles are closer together?
The circles sit closest together nearest the wire.
2 cm is nearer the wire than 8 cm.
The compass 2 cm from the wire sits where the circles are closer together.
In a field drawing of a straight wire, one circle sits 1 cm out and the next sits 3 cm out. Is the field stronger at 1 cm or at 3 cm?
Field lines drawn closer together show a stronger field.
The circles sit closer together at 1 cm than at 3 cm.
The field is stronger 1 cm from the wire.
Where is a straight wire's magnetic field strongest?
Which statement describes the magnetic field around a straight current-carrying wire?
A small compass is carried slowly around a vertical current-carrying cable, always following the field. Which path does it trace?
A student draws a wire's field as evenly spaced circles at every distance. What should the correct pattern show instead?