GoalUse the rule for magnetic poles, explain how to prove a rod is a magnet, and describe a magnetic field.
New words
attract · to pull towardsmagnetic field · the space around a magnet where its force actsfield lines · imaginary lines that show the direction of the field
Explanation
Like poles repel and unlike poles attract. Two north poles push apart, two south poles push apart, and a north pole and a south pole pull together. This gives a test: attraction alone does not prove that a rod is a magnet, because iron is attracted too. Only repulsion proves it, since a magnet’s pole can repel another pole. The magnetic field is the space around a magnet where its force acts. We draw imaginary field lines: they leave the north pole, enter the south pole, are closer together near the poles, and never cross. The force acts through paper, which shows that it works at a distance. Police scientists can reveal fingerprints with a magnetic brush and fine magnetic powder. Some trains float on magnetic forces: you can research how they work.
Worked examples
Two bar magnets are placed end to end. When the south pole of the first faces the north pole of the second, they attract. When the south pole faces a south pole, they repel.
Dana has a steel rod and a magnet. The rod is pulled towards the magnet’s north pole. Is it a magnet? She cannot tell yet, because iron is pulled too. She turns the rod round. If one end is pushed away, the rod is a magnet.
Place a sheet of paper over a paper clip and slide a magnet under the paper. The clip moves although the magnet does not touch it. The magnetic force acts through the paper, which shows that it is a force at a distance.
Class activity
Poles and fields, with an adult. Use two small fridge magnets. Try the ends together in different ways and write which pairs push and which pull. Mark the poles with tape: N, S. Slide one magnet under a sheet of paper to move a paper clip on top. Draw the field of a bar magnet using arrows that leave N and enter S. Do not use iron filings: they can harm eyes.
Practice
1
State the rule for magnetic poles.
Like poles repel and unlike poles attract.
2
Why does attraction alone not prove that a steel rod is a magnet?
Iron and steel are attracted to a magnet even when they are not magnets.
3
Which way do field lines run outside a magnet?
From the north pole to the south pole.
4
What does it show that a magnet can move a paper clip through paper?
