Magnetic Fields and Electromagnets

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1. Key words

2. Magnets — a quick reminder

• Every magnet has a north pole (N) and a south pole (S). The forces are strongest at the poles.
• Like poles repel (N–N or S–S push apart). Unlike poles attract (N–S pull together).
• Magnets attract magnetic materials: iron, steel, nickel and cobalt. Most metals (copper, aluminium, gold) are not magnetic.
• Only a magnet can repel another magnet — so repulsion is the true test of whether an object is a magnet. A piece of iron is attracted by both poles.

3. What is a magnetic field?

A magnetic field is the region around a magnet where it exerts a force on magnetic materials and on other magnets. You cannot see a magnetic field, but you can show its shape with iron filings or with plotting compasses. We draw it with magnetic field lines.

Plotting the field with a compass:

1. Put the bar magnet in the middle of a sheet of paper and draw round it. Label N and S.
2. Place a small plotting compass near the north pole. Mark a dot at each end of the compass needle.
3. Move the compass so that its tail is on the dot you just made at its head. Mark a new dot at the head.
4. Repeat until you reach the south pole (or the edge of the paper). Join the dots with a smooth curve and add an arrow pointing away from N.
5. Start again at a different point near the north pole to draw more field lines.

Iron filings are quicker: place paper over the magnet, sprinkle filings and tap gently. The filings line up along the field lines and crowd together at the poles. But filings do not show the direction of the field — a compass does.

4. Fields acting on other fields

A magnetic field exerts a force on other magnetic fields. When two magnets come close, their fields overlap and combine. This is why magnets attract or repel, even without touching.

The same idea explains why a compass works: the compass needle is a tiny magnet, and the Earth's magnetic field exerts a force on its field and turns it to point north. (You will study the Earth's field in another lesson.)

5. Electromagnets

When an electric current flows through a wire, it makes a magnetic field around the wire. The field of a straight wire is weak, but if the wire is wound into a coil, the fields of all the turns add together. Putting an iron core inside the coil makes the field much stronger. This is an electromagnet.

How to make an electromagnet:

1. Take a length of insulated copper wire (the plastic coating stops the current taking a short cut between the turns).
2. Wind it tightly round an iron nail (the core) many times, always in the same direction.
3. Connect the two ends of the wire to a cell (or low-voltage power supply) and a switch.
4. Close the switch: current flows, and the nail becomes a magnet that can pick up paper clips.
5. Open the switch: the current stops, the magnetism disappears and the paper clips drop.

The field of an electromagnet has the same shape as a bar magnet's field, with a N pole at one end of the coil and a S pole at the other. Reversing the current (turning the cell round) swaps the poles.

Why iron and not steel for the core? Iron becomes strongly magnetised while the current flows and loses its magnetism as soon as the current stops. Steel would stay magnetised (it would become a permanent magnet), so the electromagnet could not be switched off properly.

6. Uses of electromagnets

Electromagnets are used wherever we need a magnet that can be switched on and off or controlled:

7. Factors that change the strength of an electromagnet

Measuring strength. The usual method is to count how many identical paper clips the electromagnet can pick up (or how long a chain it holds). Other methods: measure the mass of iron filings it lifts, or the distance from which it attracts a paper clip.

8. Planning the investigation

Question: How does the number of turns on the coil affect the strength of an electromagnet?

Prediction (using scientific knowledge): the more turns, the more paper clips will be picked up, because each turn adds to the magnetic field.

Method: wind 10 turns, switch on, lower the nail into a pile of clips, lift it, count the clips, switch off. Repeat three times and calculate a mean. Then wind more turns and repeat.

Risk assessment: the wire can get hot when the current flows, especially with a short wire and a large current. Control the risk: use a low-voltage supply or a single cell, switch off between readings and do not hold the coil while it is switched on. Never connect a bare wire straight across a cell (short circuit).

Reliability: repeat each reading at least three times. If the repeats are close together, the results are reliable. If one repeat is very different (anomalous), check it and leave it out of the mean.

Accuracy and precision: counting paper clips only gives whole numbers, so small changes in strength may not be noticed. A more precise method is to measure the force needed to pull a piece of iron off the electromagnet with a newton meter, or to weigh the iron filings it lifts.

Presenting results: number of turns is a continuous number variable, so plot a line graph with the number of turns on the x-axis and the number of clips on the y-axis. (If the independent variable were the type of core — iron, steel, wood — you would use a bar chart, because it is a category.)

Conclusion: as the number of turns increases, the number of paper clips picked up increases — the electromagnet gets stronger. The points lie close to a straight line, so doubling the number of turns roughly doubles the number of clips. Limitation: the conclusion is only true for the range tested (10 to 50 turns) with this current and this core.

9. Worked examples

Worked example 1 — Describe what is meant by a magnetic field. [1] Describe how a plotting compass can be used to show the direction of the field around a bar magnet. [2]

1. A magnetic field is the region around a magnet where a magnetic material or another magnet feels a force (1).
2. Place the compass near the magnet and mark where the needle points; move the compass along in that direction, marking dots, and join them (1).
3. The compass needle's north end points in the direction of the field — away from N and towards S (1).

Worked example 2 — Two bar magnets are placed end to end. The field lines between them go straight across the gap from one magnet to the other. Do the magnets attract or repel? Explain. [2]

1. They attract (1).
2. Field lines going straight across the gap show a north pole facing a south pole; unlike poles attract (1).
3. Common mistake: thinking lines across the gap mean the fields are fighting. When like poles face each other, the lines bend away and there is a neutral point with no field.

Worked example 3 — A scrapyard uses an electromagnet on a crane, not a permanent magnet. Give two reasons. [2]

1. The electromagnet can be switched off, so the crane can drop the scrap where it is needed (1).
2. Its strength can be changed (by changing the current) to suit the load (1).

Worked example 4 — Aya wants to find out whether the size of the current affects the strength of an electromagnet. Name the independent variable, the dependent variable and two control variables. [3]

1. Independent variable: the current (1).
2. Dependent variable: the number of paper clips picked up (or another measure of strength) (1).
3. Control variables, any two: number of turns, the core (same nail), size/type of paper clips, the wire (1).

Worked example 5 — Use the graph in section 8. (a) Estimate how many paper clips 25 turns would pick up. [1] (b) A student says "the line shows that 100 turns would pick up about 35 clips". Comment on this claim. [1]

1. (a) Read up from 25 turns to the line, then across: about 8 or 9 clips (accept 8–9).
2. (b) 100 turns is far outside the range tested (10–50). We cannot be sure the pattern continues — for example, the core might not keep getting stronger at the same rate — so this prediction may not be accurate; it should be tested.

Worked example 6 — Explain why an electromagnet used in a door lock has a core made of iron and not steel. [2]

1. Iron loses its magnetism when the current is switched off (1).
2. Steel would stay magnetised, so the lock would not release when the current stops (1).

Practice

All pages in this course

Start here

• Stage 8 Science: The Tests and How to Prepare
• Thinking and Working Scientifically: Models, Investigations and Analysing Results

Biology

• Joints and Antagonistic Muscles
• Blood and Its Components
• Breathing, Gas Exchange and Diffusion in the Lungs
• A Balanced Diet, Energy Stores and Lifestyle
• Aerobic Respiration
• Ecosystems, Bioaccumulation and Invasive Species

Chemistry

• The Structure of the Atom
• Purity, Concentration and Chromatography
• Word Equations, Exothermic and Endothermic Reactions
• The Reactivity of Metals
• Impure Products and Solubility of Salts

Physics

• Speed and Distance–Time Graphs
• Balanced and Unbalanced Forces, and Moments
• Pressure, Particles and Diffusion
• Reflection and Refraction
• Dispersion and Colour
• Magnetic Fields and Electromagnets

Earth and space

• The Earth's Magnetic Field and Earth's Resources
• Climate, Weather and Climate Change
• Galaxies and Asteroids

Practice tests

• Stage 8 Science — Practice Test 1
• Stage 8 Science — Practice Test 2
• Stage 8 Science — Practice Test 3