Read a field diagram

A free north pole sits on a field line. Which way is it pushed?

A field diagram and one marked point are all you need to name the force direction on a chosen pole of a small test magnet. This lesson makes the read automatic.

From the last lesson, a field line's arrow shows the force direction on a free north pole. A real test magnet has two poles, and the read depends on which pole sits at the point.

At the dot, the line runs toward the magnet's S end. A north pole there is pushed along the line, toward the S end. A south pole there is pushed against the line, toward the N end.

The read has two steps. First, read the arrow on the drawn line nearest the point: that arrow is the field line's direction at the point. Then apply the pole rule.

A north pole is pushed along the line, the way the arrow points. A south pole is pushed against the line, the opposite way.

That is the whole read. The one new care is the pole. Check which pole sits at the point before you answer.

A test magnet's north pole sits at a point above a bar magnet, where the field line runs toward the magnet's S end. Which way is the force on it?

Write down the field line's direction at the marked point:

The line at the point runs toward the magnet's S end.

Apply the pole rule:

A north pole is pushed along the line.

The force is along the line, toward the magnet's S end.

A test magnet's south pole sits at the same kind of point, where the field line runs toward the magnet's S end. Which way is the force on it?

Write down the field line's direction at the marked point:

The line at the point runs toward the magnet's S end.

Apply the pole rule:

A south pole is pushed against the line.

The force is against the line, toward the magnet's N end.

Quick-fire drill

Here are six fast reads, each on its own diagram.

A test magnet's north pole sits at P. Which way is the force on it?

A test magnet's south pole sits at Q. Which way is the force on it?

A test magnet's south pole sits at R. Which way is the force on it?

A test magnet's north pole sits at T. Which way is the force on it?

A test magnet's north pole sits at V. Which way is the force on it?

A test magnet's south pole sits at U. Which way is the force on it?

Your turn

A test magnet's south pole sits at P, above the magnet. Which way is the force on it?

At one point on a field diagram, the field line points north. Jude places a test magnet's south pole there. Which way is the force on it?

Hana places a test magnet's north pole, then its south pole, at the same marked point on a field diagram. How do the two forces compare?