Magnets and Electromagnets: The Invisible Push and Pull
Have you ever stuck a magnet on the fridge, or used a compass on a hike in a Country Park? In this chapter you will learn about the two poles of a magnet, which materials magnets attract, how magnetic force changes with distance, and how an electric current can make a magnet. You will also build a simple electromagnet, test what makes it stronger, and see how electromagnets work in doorbells, recycling cranes and super-fast trains.
How to build and draw circuits is covered in Simple circuit; current and voltage are covered in Current, voltage and resistance.
1. Permanent Magnets: Two Poles, Push and Pull
What is a permanent magnet?
A permanent magnet keeps its magnetism all the time, without electricity. Bar magnets, horseshoe magnets and fridge magnets are all permanent magnets.
North pole and south pole
Every magnet has two ends called poles: a north pole (N) and a south pole (S). The magnetic force is strongest at the poles. Dip a bar magnet into paper clips and most clips stick near the two ends.
A freely hanging bar magnet points roughly north-south; the end pointing north is the north pole. A compass needle is a tiny magnet that can turn freely.
Magnets always have two opposite poles. Break a bar magnet in half and each piece has its own N and S poles.
Did you know? The Earth behaves like a giant magnet. Since unlike poles attract, the magnetic pole near the geographic North Pole is actually a magnetic south pole!
The law of magnetic poles
Like poles repel (push apart): N and N, or S and S.
Unlike poles attract (pull together): N and S.
Memory aid: "Same pushes away, different comes to play."
How to test for a magnet: A magnet attracts an iron nail at both poles, so attraction cannot tell you whether the nail is a magnet. Only a magnet can be repelled by another magnet, so repulsion is the true test.
Magnetic and non-magnetic materials
Magnetic materials are attracted by a magnet. The common ones are iron, steel (mostly iron), nickel and cobalt.
Memory aid: "I Can Stick Nails" = Iron, Cobalt, Steel, Nickel.
Non-magnetic materials are not attracted: wood, plastic, glass, paper, and many metals such as copper, aluminium, gold and silver.
Common mistake: "All metals are attracted by magnets." Wrong! An aluminium drink can is not attracted, but a steel food can is.
Key takeaway: Like poles repel, unlike poles attract. Iron, steel, nickel and cobalt are magnetic materials.
2. Magnetic Force: Working Without Touching
A non-contact force
A magnet can pull a paper clip without touching it, even through paper or a thin table top. So magnetic force is a non-contact force, like gravity.
The magnetic field
The space around a magnet where a magnetic force acts is the magnetic field. To show its pattern, sprinkle iron filings on paper placed over a bar magnet and tap gently. The filings form curved lines from one pole to the other, crowded near the poles where the field is strongest.
Closer means stronger
The closer an object is to a magnet, the stronger the magnetic force. As the distance increases, the force becomes weaker quickly.
Try it: Slide a paper clip slowly towards a magnet along a ruler. Far away, nothing happens; at a certain distance the clip suddenly jumps onto the magnet.
Key takeaway: Magnetic force acts without contact and weakens as distance increases.
3. The Magnetic Effect of Current
A wire with a current acts like a magnet
When an electric current flows through a wire, a magnetic field is produced around the wire. This is the magnetic effect of current.
Demonstration: Put a compass under a straight wire connected to a cell and a switch. Close the switch: the needle turns. Open it: the needle swings back. Reverse the cell and the needle turns the other way.
Did you know? In 1820, the Danish scientist Hans Christian Ørsted noticed a compass needle move when a current flowed in a nearby wire. This was the first clear evidence that electricity and magnetism are linked.
Coil the wire for a stronger effect
The field around one straight wire is weak. Wind the wire into a coil and the effects of all the turns add up. A current-carrying coil behaves like a bar magnet, with N at one end and S at the other. Reverse the current and the poles swap.
Key takeaway: A current produces a magnetic field; a current-carrying coil acts like a bar magnet.
4. Building a Simple Electromagnet
The parts of a simple electromagnet
An electromagnet is a magnet made by passing a current through a coil of wire. A simple one has three parts:
1. A coil of insulated wire, e.g. enamelled copper wire. Insulation makes the current flow round every turn instead of taking a short cut where bare turns touch.
2. An iron core, such as a large iron nail inside the coil. The iron becomes magnetised and makes the electromagnet much stronger. Iron gains and loses magnetism easily; steel would keep some magnetism after switching off.
3. A cell (or battery) and a switch. The cell drives a current through the coil; the switch turns the electromagnet on and off.
Switch on, switch off
An electromagnet is magnetic only when a current flows. Switch on: it picks up paper clips. Switch off: they drop. Its biggest advantage: it can be turned on and off, and its strength can be changed.
Safety tip: The coil and cell get hot if the current flows for a long time, so switch off between tests.
Permanent magnet vs electromagnet
Permanent magnet: always magnetic; cannot be switched off; no electricity needed.
Electromagnet: magnetic only when a current flows; can be switched on and off; strength can be changed; needs electricity.
Key takeaway: Simple electromagnet = coil of insulated wire + iron core + cell. It can be switched on and off.
5. Key Practical Task: What Makes an Electromagnet Stronger?
The factors
1. Number of turns of the coil – more turns, stronger electromagnet.
2. Size of the current – larger current, stronger electromagnet (e.g. two cells instead of one).
3. The iron core – a coil with an iron core is much stronger than with no core, or with a core of wood, plastic or aluminium. The length of the iron bar may also affect the strength – find out with a fair test (below).
Memory aid: The "3 Cs" – more Coils (turns), more Current, an iron Core.
Measuring strength with paper clips
We measure strength by the number of paper clips the electromagnet can pick up. Use identical clips: dip, lift and count.
Planning a fair test
Hypothesis: "The more turns in the coil, the more paper clips the electromagnet can pick up."
Independent variable (change): number of turns, e.g. 10, 20, 30, 40.
Dependent variable (measure): number of paper clips picked up.
Control variables (keep the same): the current (same cells, checked with an ammeter), the same iron core, the same wire and the same paper clips.
To test the length of the iron bar instead, change only the bar length and keep the turns, current and clips the same.
Method
1. Wind 10 turns of insulated wire around an iron nail.
2. Connect the coil, a cell, a switch and an ammeter in series.
3. Close the switch, dip the nail into the clips, lift it and count.
4. Open the switch. Repeat twice more.
5. Repeat for 20, 30 and 40 turns.
Top tip: Use the same total length of wire each time and leave the spare wire unwound, so only the number of turns changes.
Handling your results
Record each trial and the average in a table. If one count is very different from the others (an outlier), repeat that trial. Plot the average number of clips against the number of turns: it should rise as the turns increase.
Common mistake: Changing two things at once, such as adding turns and adding a cell. Then you cannot tell which change made it stronger. Change one variable at a time!
Key takeaway: More turns, a larger current and an iron core make an electromagnet stronger. Test one factor at a time and repeat each measurement.
6. Electromagnets at Work in Daily Life
The electric doorbell
An electric doorbell has an electromagnet, a soft iron armature with a hammer, a gong, a spring and a contact screw.
1. Press the bell push: the circuit is complete, a current flows and the electromagnet is magnetised.
2. The electromagnet attracts the armature and the hammer hits the gong: "ding!"
3. The armature moves away from the contact screw, so the circuit breaks, the current stops and the electromagnet loses its magnetism.
4. The spring pulls the armature back to the contact screw, the circuit is complete again, and it all repeats.
This repeats quickly while you hold the button. Memory aid: "On – pull – ding – break – spring back – again!"
Scrap-metal recycling crane
At a metal recycling factory, a crane carries a big flat electromagnet. Switched on, it lifts heavy scrap iron and steel such as old car parts. The crane moves the load, then the current is switched off to drop it. It also separates iron and steel from non-magnetic metals such as aluminium and copper.
Maglev trains
Maglev means magnetic levitation. Powerful electromagnets lift the train slightly above the track and push it forward. There is very little friction, so the ride is fast and smooth. Shanghai has a maglev line linking Pudong International Airport to the city.
Other uses and your own ideas
Electromagnets are also used in electric door locks and loudspeakers.
Be creative: Could an electromagnet help your school sort steel cans from aluminium cans? Design your own electromagnet gadget!
Key takeaway: Electromagnets are useful because they can be switched on and off: doorbells, scrap-metal cranes and maglev trains all rely on this.
Quick Review
1. A magnet has a north pole and a south pole; the force is strongest at the poles.
2. Like poles repel, unlike poles attract. Repulsion is the true test for a magnet.
3. Iron, steel, nickel and cobalt are magnetic materials; copper, aluminium and plastic are not.
4. Magnetic force is a non-contact force that gets weaker with distance.
5. A current produces a magnetic field: the magnetic effect of current.
6. Electromagnet = insulated coil + iron core + cell.
7. More turns, a larger current and an iron core make it stronger.
8. Electromagnets can be switched on and off.
Great work! Next time you hear a doorbell ring, you will know there is a tiny electromagnet switching on and off inside it.