Magnets and magnetic fieldsIB MYP Sciences: Revision notes
Section 1
Poles, attraction and repulsion
Every magnet has two poles: a north-seeking pole (N) and a south-seeking pole (S). The forces are strongest at the poles.
- Like poles repel (N and N, or S and S)
- Unlike poles attract (N and S)
The force between two magnets acts without touching: a non-contact force.
Thinking that two magnets always attract. Two like poles push apart.
Section 2
Magnetic and non-magnetic materials
Only a few materials are magnetic: iron, steel, nickel and cobalt. They are attracted to a magnet. Most other materials, such as copper, aluminium, plastic and wood, are non-magnetic.
Magnetic materials can be separated from waste using a magnet. Steel is used for permanent magnets because it keeps its magnetism; iron loses it quickly, so it makes a temporary magnet.
Section 3
Making a magnet by induction
If a magnetic material, such as an iron nail, is placed close to or touching a magnet, it becomes magnetised. This is induced magnetism.
The end of the nail nearest to the magnet's north pole becomes a south pole, so the two attract. This is why a magnet can pick up a paper clip. If the magnet is taken away, iron loses most of its magnetism but steel keeps more of it.
An induced pole is always opposite to the pole nearest to it. That is why induction always gives attraction.
Section 4
Magnetic fields and field lines
A magnetic field is the region around a magnet where another magnet or magnetic material feels a force. We draw it using field lines.
- Lines go from the north pole to the south pole outside the magnet, so arrows point away from N and towards S
- The field is strongest where the lines are closest together, which is at the poles
- Field lines never cross
The field gets weaker as you move away from the magnet.
Section 5
Plotting fields with a compass
A plotting compass has a small magnetised needle that lines up with the magnetic field. To plot a field line:
- Place the bar magnet on paper and put the compass near its north pole.
- Mark a dot at each end of the needle.
- Move the compass so the tail of the needle is on the last dot, then mark the new position of the head.
- Repeat until you reach the south pole and join the dots with a smooth line.
- Add an arrow from north to south and repeat from different starting points.
Section 6
The Earth's magnetic field
The Earth has a magnetic field like that of a giant bar magnet, caused by moving molten iron and nickel in its core. A compass needle lines up with this field, so its north-seeking pole points towards the geographic north.
Because unlike poles attract, the magnetic pole in the Arctic region is actually a magnetic south pole. Compasses can be disturbed by nearby magnets and iron objects, so they should be used away from them.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Magnets and magnetic fields
- A student in Johannesburg tests four objects with a bar magnet: an iron nail, a copper coin, a steel paper clip and a plastic ruler.Explain why the iron nail is attracted to the magnet but the copper coin is not.2 marks
- A compass contains a small magnetised needle that is free to turn. A hiker in Switzerland uses one to find north on a mountain path, and a science class uses one to investigate the field around a bar magnet.Describe how the class could use a compass to plot the magnetic field line around a bar magnet.2 marks
- A technician at a recycling centre in Singapore separates mixed metal waste with a large magnet. Back in the school laboratory, a science class uses small bar magnets and iron nails to model the same effect.Explain how the magnet separates iron and steel scrap from aluminium, copper and plastic.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).