Exploring Magnets · Lesson 4 of 6
Attraction and Repulsion Between Magnets
“Learn how magnetic poles attract or repel, why repulsion reveals a true magnet, and how magnetic effects can act through materials.”
• Predict and explain attraction and repulsion between the poles of two magnets. • Use repulsion to distinguish a magnet from an ordinary magnetic material. • Explain why a compass needle responds to a nearby bar magnet. • Investigate whether magnetic effects can act through thin non-magnetic materials.
What Happens When Two Magnets Come Close?
A magnet does more than attract pieces of iron. When two magnets approach each other, they can either move closer together or push apart. Which behaviour occurs depends on the poles that face one another.
Place one bar magnet on several round pencils so it can roll easily. Bring one pole of a second magnet near one end without touching it. Observe the motion. Then turn the second magnet around and bring its other pole near the same end. Compare the two motions.
When a North pole faces a South pole, the magnets attract. When North faces North or South faces South, the magnets repel. The rule can be remembered through the relationship between the pole types, but it should be understood through the observed motion.
A magnetic interaction in which objects or opposite poles move towards each other.
A magnetic interaction in which like magnetic poles move away from each other.
| Facing poles | Interaction |
|---|---|
| North–North | Repulsion |
| South–South | Repulsion |
| North–South | Attraction |
| South–North | Attraction |
Why Repulsion Is a Better Test for a Magnet
An iron bar can be attracted by either pole of a magnet. This means that attraction alone does not prove that the unknown object is itself a magnet; it might simply be a magnetic material. Repulsion gives stronger evidence because an ordinary iron bar is not expected to repel a pole in this test.
Problem
An unknown metal bar is brought near the North pole of a known magnet. One end of the unknown bar is attracted, but when the bar is turned around, its other end causes repulsion. What does this show?
- 1.Attraction by itself could occur if the bar were simply made of magnetic material.
- 2.Repulsion requires a like pole facing the known North pole.
- 3.The repelling end therefore behaves as a North pole.
- 4.The unknown bar is a magnet.
A piece of iron can be attracted to both poles of a magnet. To identify a magnet using another known magnet, look for repulsion as decisive evidence.
A Compass Needle Is Also a Magnet
The needle inside a magnetic compass behaves like a tiny bar magnet. If the North pole of a bar magnet is brought near the North end of the compass needle, the needle turns away. If the South pole is brought near that same North end, the needle turns towards it.
This deflection links the compass lesson with attraction and repulsion: the compass needle is not simply an arrow showing direction. It is a real magnet whose poles interact with nearby magnetic poles.
Can Magnetic Effect Pass Through Other Materials?
A further investigation places a sheet of wood between a magnet and a compass without moving either one. Similar trials can be made with cardboard, thin plastic and thin glass. The compass needle still shows essentially the same magnetic response.
Set up a bar magnet close enough to deflect a compass needle. Without changing their positions, place a sheet of wood between them and observe. Repeat with cardboard, thin plastic and thin glass. Record whether the deflection changes appreciably.
| Material placed between magnet and compass | Expected observation from the chapter |
|---|---|
| Wood | No appreciable change in deflection |
| Cardboard | No appreciable change in deflection |
| Thin plastic | No appreciable change in deflection |
| Thin glass | No appreciable change in deflection |
The conclusion is that magnetic effect can act through these non-magnetic materials. This does not mean every possible barrier has no effect under all conditions; it means that in the chapter's setup, the listed thin non-magnetic sheets do not stop the interaction from being observed.
Quiz
What happens when a North pole is brought near a South pole?
Which pair of facing poles will repel?
Why is repulsion useful when identifying whether an unknown bar is a magnet?
What should the North end of a compass needle do when a North pole is brought close?
What did the wood/cardboard/plastic/glass investigation show?
Practice Problems
- For each pair N–N, N–S, S–S and S–N, state whether you expect attraction or repulsion.
- Explain why attraction alone cannot prove that an unknown iron-like bar is a magnet.
- Design a two-step test using a known magnet to decide whether an unknown bar is a magnet. State which observation would be decisive.
- Predict what the North end of a compass needle will do when first a North pole and then a South pole are brought near it.
- Describe how you would test whether a thin sheet of cardboard blocks the magnetic effect between a bar magnet and a compass.
- A steel paper clip is attracted to both poles of a magnet. Does this prove that the paper clip itself is a magnet? Explain.
Key Takeaways
• Unlike poles attract and like poles repel. • An ordinary iron bar can be attracted by either pole, so attraction alone does not identify a magnet. • Repulsion is a reliable sign that two like magnetic poles are interacting. • A compass needle is itself a small magnet and responds to nearby poles. • Magnetic effect can be observed through thin non-magnetic materials such as wood, cardboard, plastic and glass.