Magnetic fields
In this topic you will:
- describe a magnetic field
- understand that magnetic fields surround magnets
- understand how magnetic fields interact.
Key words
- compass
- like poles
- magnet
- magnetic
- magnetic field
- magnetic field lines
The magnetic field
magnet is something that will attract magnetic materials. Magnetic materials include the metals iron, steel, nickel and cobalt. You will probably have used a magnet to attract paperclips.
Magnets have two poles, north and south. They are shown with the letters N and S on diagrams.
When a paperclip is close to one of the poles of a magnet, the paperclip will be attracted to the magnet. As you move the paperclip further away, it stops being attracted.
The paperclip is attracted to a magnet when it is in the magnetic field of that magnet.
A magnetic field is the area around a magnet where the effects of the magnet can be detected.
A magnetic field surrounds all magnets. The magnetic field of a magnet is strongest at the poles.
You can detect a magnetic field in two ways. You can:
- see whether a magnetic object moves because of attraction
- use a compass.
A compass contains a magnetised needle that is free to turn. The needle will turn and point in the direction of a magnetic field. The picture shows a compass. Some mobile devices such as phones have compass apps.
Magnetic field lines
You can draw magnetic field lines around a magnet to represent the magnetic field.
The rules are that magnetic field lines:
- join opposite poles
- have arrows that point N → S
- must not touch each other
- must not cross each other
Common Mistake
Many learners mistakenly believe that magnetic field lines start and end anywhere. In fact, they always point from the north pole to the south pole and never cross or touch.
Following these rules, the magnetic field lines around a bar magnet look like this:
and the magnetic field lines around a horseshoe magnet look like this:
You can tell by looking at magnetic field lines where the magnetic field is strongest. The magnetic field is strongest where the lines are closest together.
If one magnet is stronger than another, the magnetic field of the stronger magnet will be different in two ways.
- All the field lines will be closer together.
- The field lines will extend further away from the magnet.
You can also tell, by looking at magnetic field lines, in what direction a compass will point. When it is in a magnetic field, a compass will point in the direction of the lines.
Important Concept
The magnetic field is strongest where the lines are closest together. Compasses align with the direction of these field lines, helping visualize magnetic effects.
Magnetic fields interacting
You probably already know about the forces between two magnets.
- Two north poles repel.
- Two south poles repel.
- A north and a south pole attract.
These forces are due to the magnetic fields from each magnet.
Look at the field lines between two magnets when their opposite poles are together.
The magnetic field lines between the two magnets are all pointing in the same direction. This means there will be a force of attraction between the magnets.
Now look at the field lines between two magnets when their like poles are together.
The magnetic field lines in the space directly between these two magnets are all pointing in opposite directions. This means the magnets will repel, or try to move away from each other.
Example
If two magnets are placed so that a north and south pole face each other, they will attract because their magnetic field lines point in the same direction. But if two north poles face each other, the field lines oppose each other, causing the magnets to repel.
Questions
Show Answer
A magnetic field is the area around a magnet where magnetic forces can be felt or detected.
State what can be concluded about the strengths of these two magnets.
Show Answer
Magnet A is stronger than magnet B because its magnetic field extends further.
Put arrows on each line.
Show Answer
Field lines should curve from the north pole to the south pole, with arrows pointing from N to S.
Show Answer
Lines should go smoothly from the north pole of one magnet into the south pole of the other, showing attraction.
Show Answer
Lines should bend away from each other between the two south poles, showing repulsion.
Think Like a Scientist
In this investigation, you will investigate ways of detecting a magnetic field. Work in pairs or groups of three.
You will need:
See the diagram. You could also choose some other different types of magnets to investigate.
Steps
- Set the magnet, ruler and paperclip on a smooth surface as shown in the diagram.
- Record whether the north or south pole of the magnet is facing the paperclip.
- Slowly move the paperclip toward the pole of the magnet. If your bar magnet is not marked, can you devise a method to tell which pole is which?
- Record the distance from the pole of the magnet when the paperclip becomes attracted to the magnet. Call this value d for distance.
- Repeat another two times and record all your measurements of d.
- Turn the magnet around so the other pole is facing the paperclip.
- Repeat steps 3–5 for this pole.
- If you have time, repeat the investigation with other, different, magnets.
Follow-Up Questions
Show Answer
Add all three d values for each pole and divide by 3 to find the average distance.
Show Answer
A larger value of d means the magnetic field is stronger, as it can attract the paperclip from farther away.
Show Answer
If the values are similar, both poles have roughly equal magnetic strength. If they differ, one pole is stronger than the other.
Show Answer
A smooth surface reduces friction and allows more accurate detection of when the paperclip is attracted to the magnet.
Show Answer
A small paperclip has less mass and requires less magnetic force to move, improving accuracy of detection.
Show Answer
Repeat the experiment more times, use a digital ruler, reduce sources of error like surface bumps, and ensure the paperclip always starts at the same angle.
Self-assessment
Decide how much you agree with each of these statements. Give yourself 5 if you agree very much and 1 if you do not agree at all.
- I understand what is meant by a magnetic field.
- I can draw the magnetic field lines around a bar magnet.
- I can draw the magnetic field lines between two opposite poles of different bar magnets.
- I can draw the magnetic field lines between two like poles of different bar magnets.