Magnetic Fields and Electromagnets
At a scrapyard, a giant crane lifts a whole car with a flat round magnet, then drops it the instant the operator flicks a switch. No glue, no rope, just a magnetic field you can switch on and off.
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Q1 · A magnet can attract a paper clip across a small gap, even though it never touches it. What do you think fills the space around the magnet where it can do this?
Q2 · A scrapyard crane magnet can be switched off, but the fridge magnet at home cannot. What do you think is different about how the crane magnet is made?
● Know
- What a magnetic field is and how field lines describe it
- That an electromagnet is a temporary magnet made with electric current
- The three things that make an electromagnet stronger
● Understand
- Why field lines point from north to south and never cross
- Why an electromagnet can be switched on and off but a permanent magnet cannot
- Why a fair test only changes one variable at a time
● Can do
- Map a magnetic field using iron filings or a plotting compass
- Build an electromagnet from a coil, a cell and an iron nail
- Plan a fair test to compare electromagnet strength safely
A magnet can pull a paper clip toward it before they even touch. That means the magnet's force reaches out into the space around it. We call this region the magnetic field, the region around a magnet where its force acts on magnetic materials or other magnets.
The field is invisible, so scientists draw it using field lines. There are clear rules for field lines that you must follow:
- Outside the magnet, field lines point from north to south.
- Field lines never cross each other.
- Field lines are closest together where the field is strongest, which is at the poles.
You cannot see a magnetic field, but you can map it in two ways:
- Iron filings: sprinkle tiny iron filings onto a sheet of paper laid over a magnet. The filings line up along the field lines and reveal the pattern.
- A plotting compass: move a small compass around the magnet, step by step, and mark the direction the needle points each time. Joining the marks traces the field lines.
Earth has its own magnetic field, almost as if a giant bar magnet sits inside the planet. This is why a compass works anywhere on Earth: the needle is a small magnet that lines up with Earth's field and points toward the north.
An electromagnet is a temporary magnet made by passing an electric current through a coil of wire. A coil of wire is called a solenoid. When current flows through the coil, the coil produces a magnetic field just like a bar magnet, with a north pole at one end and a south pole at the other.
If you place an iron core, such as an iron nail, inside the coil, the electromagnet becomes much stronger. To build a simple electromagnet you need only three things:
- A length of insulated wire wound into a coil
- An iron nail to slide inside the coil
- A low-voltage cell or battery connected to the ends of the wire
The biggest difference between an electromagnet and an ordinary permanent magnet (like a fridge magnet) is control. An electromagnet can be switched on and off with the current: connect the circuit and it works, disconnect it and the magnetism disappears. A permanent magnet is always on and cannot be switched off. That is exactly why the scrapyard crane uses an electromagnet, it can grab a car and then drop it the instant the operator cuts the current.
Three things make an electromagnet stronger. Each one increases how much metal it can lift:
| Change | Effect on strength |
|---|---|
| More turns of wire in the coil | Stronger field, the electromagnet picks up more |
| More current (more cells) | Stronger field, the electromagnet picks up more |
| Add an iron core | Much stronger than a coil with no core |
To compare strength we count how many paper clips or staples the electromagnet can pick up. The more it lifts, the stronger it is.
To make this a fair test, you must change only one variable at a time and keep everything else the same. For example, if you want to test the effect of more turns, you change only the number of turns and keep the same battery, the same nail and the same type of paper clip. If you changed two things at once, you would not know which change made the difference.
Australian context: the same idea runs the maglev train tested in Australia and overseas, electromagnets lift and guide the train above the track. Electromagnets are also inside electric bells, loudspeakers, electric motors and the powerful MRI scanners in our hospitals.
You build an electromagnet with 20 turns of wire and one cell, and it lifts 4 paper clips. You then wind the coil to 40 turns and keep everything else the same. Predict how many paper clips it will lift now, and explain your reasoning.
How close was your prediction?
Well done, you predicted that more turns increase the strength.
Key insight: more turns of wire make an electromagnet stronger, so it lifts more.
Map the magnetic field around a bar magnet using a plotting compass (or iron filings on paper). Follow these steps, then record what you see.
- Place a bar magnet on a sheet of paper and draw around it.
- Put a small plotting compass near the north pole and mark a dot at each end of the needle. Move the compass a little and repeat, joining the dots into a smooth line.
- Repeat all the way around the magnet to build up several field lines.
- Add an arrowhead to each line showing the direction the compass pointed.
Answer these questions in the box:
- Which way do your field lines point, from north to south or from south to north?
- Where are the lines closest together, and what does that tell you about the field strength there?
Build a simple electromagnet and plan a fair test to compare its strength. Wind insulated wire around an iron nail, then connect the ends to a low-voltage cell. Count how many paper clips the tip can pick up.
Now plan ONE fair test. Choose one variable to change (turns of wire, number of cells, or iron core vs no core) and answer here:
- Which one variable will you change?
- Which things will you keep the same to make it fair?
- How will you measure the strength each time?
Q1. State two rules for drawing magnetic field lines around a bar magnet. (2 marks)
Q2. Describe how you would build a simple electromagnet, and explain one safety rule you must follow. (3 marks)
Q3. A student wants to test whether more cells make an electromagnet stronger. Explain how they should set up a fair test and how they would measure the strength. (3 marks)
Answers
▾MCQ 1
B A magnetic field is the region around a magnet where its force acts on magnetic materials or other magnets. It is not the metal, the weight or stored electricity.
MCQ 2
A Outside a magnet, field lines always point from the north pole to the south pole. They never cross and are closest together at the poles.
MCQ 3
C Sprinkling iron filings on paper over a magnet reveals the field pattern. A plotting compass works too. Weighing, heating or shining light does not map a field.
MCQ 4
B Adding more turns of wire makes the field stronger. Removing the core, using fewer cells or disconnecting the battery all make it weaker or switch it off.
MCQ 5
C An electromagnet can be switched off by cutting the current, so the crane can drop the car. A permanent magnet cannot be switched off.
Short Answer 1
Model answer: Any two of these rules: (1) outside the magnet, field lines point from the north pole to the south pole; (2) field lines never cross each other; (3) field lines are closest together where the field is strongest, which is at the poles.
Short Answer 2
Model answer: Wind insulated wire into a coil around an iron nail, then connect the two ends of the wire to a low-voltage cell. The current through the coil turns the nail into a magnet that can pick up paper clips. One safety rule: use only a low-voltage cell (about 1.5 to 6 volts) and never mains electricity, because mains electricity is dangerous. You should also disconnect the circuit between tests because the wire and battery can get warm.
Short Answer 3
Model answer: The student should change only the number of cells and keep everything else the same: the same coil with the same number of turns, the same iron nail, and the same type of paper clips. For each number of cells, they connect the circuit and count how many paper clips the electromagnet can pick up. Counting more paper clips means a stronger electromagnet. Because only one variable changed, the test is fair and they can be sure the cells caused any difference.
At the start of this lesson you saw the hook: a scrapyard crane that lifts a car with a single round magnet, then drops it the instant the operator flicks a switch. Now you know the secret, the crane uses an electromagnet, and the operator simply cuts the current so the magnetism disappears!
How does your new understanding of electromagnets compare to what you first thought? Explain in your own words why an electromagnet can be switched off but a fridge magnet cannot, and name two ways to make an electromagnet stronger.
- A magnetic field is the region around a magnet where its force acts. We map it with field lines that point from north to south, never cross, and are closest together at the poles.
- An electromagnet is a temporary magnet made by passing current through a coil; an iron core makes it stronger, and it can be switched on and off.
- To compare strength, count the paper clips lifted and run a fair test, changing only one variable (turns, current or core) at a time. Always use a low-voltage cell, never mains.