The science idea
A permanent magnet is always magnetic. An electromagnet is magnetic only while an electric current flows through its coil, which is why it is so useful: a scrapyard crane can pick up a car body when the current is on and drop it when the current is off. In this project you build a small version and measure what changes its strength.
In 1820 the Danish scientist Hans Christian Oersted noticed that a compass needle moved when a wire carrying current was placed near it. A moving charge makes a magnetic field around it. The field around a straight wire is weak, but if you wind the wire into a coil, the fields of all the turns add up inside it and make a strong, nearly uniform field along the axis. The coil behaves like a bar magnet: one end is a north pole and the other a south pole. To find which is which, use the right-hand grip rule: curl the fingers of your right hand in the direction of the conventional current (from the + terminal, round the coil) and your thumb points to the north pole.
Putting a soft iron core inside the coil makes the magnet much stronger, often by hundreds of times. Iron is made of tiny regions called domains, each a little magnet. Normally they point in random directions and cancel out. The coil’s field lines them up with it, so the iron adds its own magnetic field to the coil’s. When the current is switched off, a core of soft iron loses almost all of its magnetism; a core of hard steel keeps a little (a nail is usually a mild steel and keeps a small amount).
The strength of the electromagnet depends on the number of turns of wire (N), the current through the coil (I), and the core. The field depends on the product N × I, called the ampere-turns. Doubling the turns, or doubling the current, roughly doubles the field. In this project the current comes from cells, so you can test N, the number of cells, and the effect of the core, one at a time, using the number of paper clips lifted as your measurement.
A coil of thin wire has a low resistance, so the current is large, and the wire gets warm because electrical energy is turned into heat (heating effect, power = I²R). That is why you keep the switch on for only a few seconds at a time. The coil is also a good lesson in why we insulate wire: the enamel or plastic stops current from jumping between turns or along the nail, which would short-circuit the coil.
Materials and tools
Cost: Under Rs 100
The cardboard, string, nail, paper clips and used cells are free. You need about 3 to 4 metres of thin insulated copper wire (from an electronics or hardware shop, a few rupees to about Rs 50) unless you can unwind wire from a dead transformer or an old toy motor with an adult.
| What you need | How many | Free substitute or where to find it |
|---|---|---|
| Iron nail, 8 to 10 cm long (a plain steel or iron nail, not a short roofing nail) | 1 (and a second for comparison) | A long nail or a bolt from the toolbox. A galvanised (zinc-coated) nail also works. |
| Thin insulated copper wire (enamelled wire or fine plastic-coated wire, about 0.3 mm thick) | 3 to 4 metres | Unwound from an old transformer, a dead motor, or a speaker coil, by an adult. Otherwise a small reel from an electronics shop. |
| AA cells (1.5 V) | 2, plus a plastic holder if you have one | Cells from a TV remote or clock that still work. Fresh alkaline cells give the strongest pull. |
| Steel paper clips for the load | 20 or more | From the stationery box. They must be steel, not plastic-coated brass. |
| Corrugated cardboard for the base, mast and boom | A box 25 cm × 15 cm for the base; a strip 30 cm × 4 cm for the boom | Cartons and tubes from the kitchen or tissue paper rolls. |
| Cardboard tube for the mast, 30 cm tall | 1 | A kitchen foil roll or a rolled-up sheet of thick card glued into a tube. |
| Wooden skewer and a thin straw for the winch spool and crank | 1 skewer, 1 straw | Leftover from the kitchen. |
| Strong thread or string | 1 metre | Sewing thread, doubled. |
| Two metal paper clips and two split pins (paper fasteners), or thumb tacks, for the switch | 2 clips, 2 pins | Stationery box. |
| Tape and glue | 1 roll | Any strong tape. |
Tools
- Scissors
- Sandpaper or the back of a blade to remove enamel from the wire ends (with adult help)
- A pen or pencil
- A ruler
- A pair of pliers for bending paper clips (optional)
Safety first
Read this before you start
- Use only 1.5 V AA cells (two at most). Never connect the coil to a mains socket, a phone charger, a power bank, a lithium cell or a car battery.
- A coil of thin wire has a low resistance, so it draws a large current from the cells. The wire and the cells get warm. Keep the switch on for no more than about 10 seconds at a time, and let everything cool for half a minute between tries. If the coil is too hot to touch, disconnect it at once.
- Never touch the two free ends of the wire to the two ends of a cell directly (a short circuit). They heat up quickly and drain the cells.
- Be careful with scraped wire ends and the nail point. An adult should do the scraping of enamel with a blade.
- Keep magnets, paper clips and small parts away from small children. Do not hold the electromagnet near a phone, watch, memory card or bank card.
Parts and layout
These drawings show the parts and how they fit together. Keep them beside you while you build.
Scroll sideways to see the whole diagram.
Scroll sideways to see the whole diagram.
Step-by-step build
12 steps. Do them in order, and test each part before you go on to the next.
Make the base
Take the box (about 25 cm × 15 cm × 5 cm) and cut a hole for the mast, 3 cm from one short edge. Put a few coins or small stones inside as a counterweight so the crane does not tip when it lifts a load.
Common mistake: A light base. The crane then tips over as soon as the boom is loaded. Add weight near the mast.
Make and fix the mast
Push a 30 cm tube through the hole in the box lid so it stands upright, and glue or tape it from the inside. Check that it stands vertical with a pencil or a ruler beside it.
Tip: A tube is stiffer than flat card. If you only have flat card, roll a 30 cm × 10 cm piece around a pencil and glue the seam.
Fix the boom
Fold a strip of card 30 cm × 4 cm lengthways into a U-shape to make it stiff. Tape or glue it across the top of the mast so it sticks out about 24 cm over the front of the box. Fix a card brace under the boom to the mast.
Tip: Push the end of the boom through a slot in the mast top so it cannot twist.
Build the winch
Push a skewer sideways through two holes near the top of the mast, 10 cm below the boom. Slip a piece of straw over the middle as a spool and tape the thread to it. Bend the skewer end into a crank handle with a bit of tape.
Common mistake: Making the holes too small. The skewer must turn freely, or the winch jams when it carries a load.
Run the string
Pass the thread from the spool up to the boom, along it, and through a small paper-clip loop taped at the tip of the boom. Let it hang down for about 15 cm.
Tip: A loop at the tip keeps the thread from sliding off sideways when the crane swings.
Wind the electromagnet
Leave 20 cm of wire free at the start (lead 1) and tape it to the head end of the nail. Wind the wire neatly around the nail in one direction without gaps, keeping the turns side by side. Count the turns. After 20 turns, tape the winding down and test it (steps 7 to 10). Keep winding in the same direction, testing at 40, 60, 80 and 100 turns. Do not cut the reel until you finish; the wire still on the reel is lead 2.
Common mistake: Winding some turns one way and some the other way. Their magnetic fields then cancel. Always wind in the same direction.
Prepare the wire ends
Remove about 2 cm of enamel from the start of the wire before you wind it, and from the end that comes off the reel each time you test, using sandpaper (an adult does this with a blade). The bare ends must look shiny copper. If the wire has plastic insulation, strip the plastic instead.
Common mistake: Skipping this. Enamel is an insulator, so no current flows through an unscraped end.
Make the paper-clip switch
On the base, push a split pin through the card and bend a paper clip around it so that it can pivot. Put a second split pin where the free end of the clip lands when you press it down. Connect one wire to each pin (wrap the wire end round the pin legs under the card).
Tip: The clip should spring back off the second pin by itself, so the switch is normally off.
Wire the circuit
Join the parts in a loop as in the circuit diagram: the positive end of the cells to one switch pin; the other switch pin to lead 1 of the coil; lead 2 of the coil to the negative end of the cells. Hold the wires on the cell ends with tape or in a holder.
Common mistake: Leaving a gap somewhere in the loop. Current only flows round a complete, closed circuit.
Test the lifting power
Hold the electromagnet over a heap of paper clips. Press the switch for about 5 seconds and count the paper clips that stay attached. Let go of the switch and see what happens. Repeat three times and write the numbers down.
Tip: Clips can hang from each other in a chain. Count all the clips that hang from the magnet.
Investigate: turns and cells
Use the table to test 20, 40, 60, 80 and 100 turns with two cells. Then test 100 turns with one cell. Finally, take the nail out of the coil and test the coil alone.
Common mistake: Changing the turns and the cells together. Change one thing at a time so the result is fair.
Mount the magnet and lift
Tie the electromagnet to the string at the boom tip with the nail hanging vertically and the wires running up along the boom and down the mast to the base. Wind the winch to lower it to the clips, press the switch, wind it up, swing the boom, and release the load by letting go of the switch.
Tip: Leave a little slack in the wires near the magnet so they do not pull it sideways, and tape them along the boom every few centimetres.
How it should look when it is finished
A small crane about 30 cm tall standing on a heavy cardboard box. A string over the boom carries a nail with a neat coil of wire wound around it. When you press the paper-clip switch, the nail sticks to a handful of steel paper clips and lifts them; when you release the switch, they drop off.
This is a description, not a photograph. Compare your own model with it and with the figures above.
How it works
When you press the switch, the cells push a current through the coil. Each turn of wire makes a magnetic field, and the fields of all the turns add together inside the coil, so the coil acts like a bar magnet with a north pole at one end and a south pole at the other. The pole at each end depends on the direction of the current (right-hand grip rule).
The iron nail lies inside the coil. Its tiny magnetic domains line up with the field of the coil and add their own field to it, so the electromagnet is much stronger than the coil alone. A paper clip near the nail is magnetised by induction: the end nearest the nail becomes the opposite pole, so it is attracted. The attraction is strong enough to lift several clips.
When you release the switch, the current stops, the coil’s field disappears, and the nail loses most of its magnetism. The clips fall. That control, switching the force on and off, is the great advantage of an electromagnet over a permanent magnet.
Scroll sideways to see the whole diagram.
Troubleshooting
| What you see | Likely cause | What to do |
|---|---|---|
| The nail picks up nothing | The enamel was not scraped off, a joint is loose, the cells are weak, or the circuit is open. | Re-scrape the wire ends, tape them firmly to the cell ends, try fresh cells, and check the switch touches the second pin. |
| It lifts one clip only | Too few turns, weak cells, or the wire is wound in alternating directions. | Add turns in one direction, use fresh alkaline cells, and keep the turns tight and in the same direction. |
| The wire or the cells become hot | The coil has a low resistance, so a large current flows. The switch was on for too long. | Press the switch for 10 seconds at most, and let it cool. Use thinner or longer wire to cut the current, and never use more than two cells. |
| Clips stay stuck after the switch is released | The steel nail keeps a little magnetism (residual magnetism). | Tap the nail or use a nail of softer iron. Use light clips so that they fall off. |
| The crane tips over | The base is too light or the boom is too long. | Add weight to the base on the side opposite the boom, or shorten the boom. |
| The string tangles | The winch spool is uneven or the string is loose. | Wind the string in neat turns on the spool and keep it tight. |
Test and record
Use two cells, the same nail and the same paper clips. Press the switch for no more than 10 seconds each time. For each row count the clips lifted three times.
| Number of turns | Clips lifted (1st try) | Clips lifted (2nd try) | Clips lifted (3rd try) | Average |
|---|---|---|---|---|
| 20 | ||||
| 40 | ||||
| 60 | ||||
| 80 | ||||
| 100 (two cells) | ||||
| 100 (one cell) | ||||
| 100 turns, no nail |
Copy the table into your notebook and write each reading as you take it, in pencil, with the date. Repeat every test at least three times and use the average. Then draw a graph or chart of your results.
Viva questions and answers
A teacher or a judge will ask you how your project works. Practise these eight questions and answer them in your own words.
- What is an electromagnet?
A magnet that is made by an electric current flowing through a coil, usually around an iron core. It is magnetic only while the current flows.
- Who discovered that a current makes a magnetic field, and how?
Hans Christian Oersted in 1820, when a compass needle turned near a wire carrying current.
- Why is an iron core used?
Iron is a magnetic material. The coil’s field lines up the iron’s tiny domains, and they add their own field, so the electromagnet becomes much stronger. Without the core the coil is far weaker.
- What factors decide the strength of an electromagnet?
The number of turns, the current, and the core material. More turns and more current give a stronger magnet.
- How can you find the north pole of the electromagnet?
Use the right-hand grip rule: curl your right hand’s fingers along the direction of the current, and your thumb points to the north pole. Or use a compass: its north-pointing end is attracted to the electromagnet’s south pole.
- What happens if you reverse the cells?
The current reverses, so the north and south poles swap. A piece of iron is attracted either way, because it is magnetised by induction.
- Why does the wire get warm?
The current in a wire with resistance turns electrical energy into heat (the heating effect of current, heat = I²Rt).
- Where are electromagnets used in real life?
In scrapyard cranes, electric bells, relays, loudspeakers, electric motors, door locks and MRI scanners.
Written report outline
Use these headings in your project file, and write each part in your own words. Add a title page, your diagrams, the precautions you took and the books or sites you used.
- Aim
- To make an electromagnet and a crane, and to find how the strength depends on the number of turns in the coil.
- Apparatus
- Iron nail, insulated copper wire, two AA cells, switch, steel paper clips, cardboard, string, skewer, tape.
- Procedure
- Describe how you wound the coil on the nail, connected it through a switch to two cells, and counted the paper clips lifted for 20, 40, 60, 80 and 100 turns, then with one cell and with no nail. Add the labelled diagram and the circuit diagram.
- Observation
- The table of paper clips lifted for each number of turns, three tries each, and the averages.
- Result
- The number of clips lifted increased as the number of turns increased (write the numbers). With the nail removed, the coil lifted far fewer clips.
- Conclusion
- The strength of an electromagnet increases with the number of turns and the current, and an iron core makes it much stronger. It is magnetic only while the current flows.
Take it further
- Plot a graph of average paper clips lifted against the number of turns. Does it look like a straight line?
- Test different cores: a nail, a screw, an aluminium rod and a plastic rod. Which works best, and why?
- Use a compass to map the field around the coil and find the north and south poles. Reverse the cells and watch the needle swing.
- Make the boom turn by mounting the mast on a bottle cap, so the crane can swing the load to another place.
- Wind a second coil on a different nail with thicker wire and compare the heating and the lifting power.
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