What shape are the field lines around a straight current-carrying wire?
The field around a straight current-carrying wire is a set of circles centered on the wire. Each circle has a direction: the field points one way around the wire, not the other. Which way does it run?
Watch the compasses answer. Point a straight wire toward you, and switch its current on, flowing toward you. Compasses around the wire settle counterclockwise, as you look at the wire's end. Reverse the current, and every needle turns around: the field now runs clockwise.
Your right hand predicts this. Grip the wire with your right hand, with your thumb pointing the way the current flows. Your fingers curl around the wire the way the field circles run. This trick is called the right-hand grip rule.
A vertical wire carries current straight up. Viewed from above, which way does the field circle the wire?
Set the thumb:
Grip the wire with your right hand, thumb pointing up, with the current.
Read the curl:
Looking down from above, your fingers curl counterclockwise.
Viewed from above, the field runs counterclockwise around the wire.
Viewed end-on, a wire's field circles run clockwise. Is the current flowing toward you or away from you?
Match the curl:
Curl your right-hand fingers clockwise, the way the field runs.
Read the thumb:
Your thumb points away from you. The thumb gives the current's direction.
The current flows away from you.
Here are six fast reads. Keep your right hand on the desk and use it every time.
A wire's current flows straight up. Viewed from above, which way does the field circle?
A wire's current flows straight down. Viewed from above, which way does the field circle?
Viewed from above, a wire's field circles counterclockwise. Which way does the current flow?
A wire points at you, current flowing toward you. Which way does the field circle?
Viewed end-on, a wire's field circles clockwise. Does the current flow toward you or away?
A wire's current flows away from you. Which way does the field circle?
A horizontal wire runs north-south, with the current flowing north. A compass sits directly above the wire, where the wire's field is far stronger than Earth's. Which way does the needle's north end point?
A horizontal wire runs east-west, with the current flowing east. A compass sits directly below the wire, where the wire's field is far stronger than Earth's. Which way does the needle's north end point?
A vertical wire carries current straight up. Beside it, where the wire's field is far stronger than Earth's, a compass needle points north. On which side of the wire does the compass sit?