← Electromagnetic induction

Predict the induced direction

You face a coil. Its current circulates counterclockwise. Which pole faces you?

A magnet's north pole is pulled away from a closed coil. Lenz's law says the induced current opposes the change. But which way, exactly, does the current flow? You can work it out in three steps.

Lenz's law gives the goal: the induced current flows in the direction whose magnetic effect opposes the change producing it.

Step 1 is to name the change. Here, a north pole is moving away from the coil's near face.

Step 2 is to choose the pole that opposes the change. To oppose a retreat, the coil must pull the magnet back. Unlike poles attract, so the near face must become a south pole.

Step 3 is to read off the current. The grip rule links a coil's pole to its current: fingers curl with the current, thumb points out of the north face. With a south face toward the magnet, the current flows clockwise as seen from the magnet's side.

The three steps, on the pulled-away north poleStepThis casename the changea north pole moves away from the near facechoose the opposing polesouth face, to attract the magnet backread off the currentclockwise, as seen from the magnet's side
The worked case: a retreating north pole meets a south face, and the grip rule gives the current.

An approach runs the same steps the other way. To oppose an approach, the coil must push the magnet back. Like poles repel, so the near face becomes the same pole as the one approaching.

An approach, by contrastStepAn approaching polename the changea pole moves toward the near facechoose the opposing polethe same pole, to repel the magnetread off the currentthe grip rule gives it from the face's pole
An approach is met by the same pole, which repels it.
An approaching pole is met by the same pole, to repel it. A retreating pole is met by the opposite pole, to attract it back.
The sorting lineThe changeThe near face becomesWhya pole approachesthe same polelike poles repel: to push it backa pole retreatsthe opposite poleunlike poles attract: to pull it back
An approach meets the same pole; a retreat meets the opposite pole.

A magnet's south pole approaches a closed coil's near face. Which pole does that face become, and which way does the induced current flow, as seen from the magnet's side?

Name the change: a south pole is approaching the near face.

Choose the opposing pole: to oppose an approach, the coil must repel the magnet. Like poles repel, so the near face becomes a south pole.

Read off the current: with a south face toward the magnet, the grip rule gives a clockwise current as seen from the magnet's side.

The near face becomes a south pole, and the induced current flows clockwise as seen from the magnet's side.

A north pole approaches a closed coil's near face. To oppose the approach, which pole must the face become?

A coil's near face must be a north pole. Seen from the magnet's side, which way does the induced current flow?

A magnet's south pole is pulled away from a closed coil's near face. Which pole does that face become?

Your turn

A bar magnet falls north-pole-first toward a closed coil lying beneath it. Which pole does the coil's upper face become?

A magnet's south pole is drawn steadily away from a closed coil's near face. As seen from the magnet's side, which way does the induced current flow?

A magnet's north pole approaches a closed coil, and the coil's near face becomes a north pole. What is the correct reason?