P27: Magnets and Magnetic Fields
Permanent and induced magnets, magnetic field lines, the behaviour of north and south poles, Earth's magnetic field, compasses and magnetic materials.
Permanent Magnets
A permanent magnet produces its own magnetic field. It always has a north pole and a south pole. Like poles repel and unlike poles attract.
- North-North: repel
- South-South: repel
- North-South: attract
- A magnet is the only object that can attract magnetic materials without touching them
- The force between two magnetic poles decreases with distance
- The force between poles is an example of a non-contact force
You can test whether an object is a permanent magnet by repulsion. If one end of a known magnet repels the object, it must be a magnet. Attraction alone is not sufficient proof, because a magnet also attracts unmagnetised magnetic materials.
Induced Magnets
An induced magnet is a material that becomes magnetic when placed in a magnetic field. When the magnetic field is removed, the induced magnet loses most or all of its magnetism quickly.
- Induced magnetism is always temporary
- An induced magnet always has a pole that attracts the inducing magnet (the near end becomes the opposite pole)
- For example, if the north pole of a permanent magnet is brought near an iron nail, the end of the nail nearest the north pole becomes a south pole and is attracted
- When the permanent magnet is removed, the nail loses its magnetism
- An induced magnet can never repel the magnet that induced it
Remember: induced magnets always attract the inducing magnet. They never repel. This is because the near end becomes the opposite pole to the inducing pole.
Magnetic Materials
Only a few elements are magnetic. The magnetic elements are iron, nickel and cobalt. Alloys containing these elements (e.g. steel) can also be magnetic.
| Material | Magnetic? | Type |
|---|---|---|
| Iron | Yes | Soft magnetic material (easily magnetised and demagnetised) |
| Steel | Yes | Hard magnetic material (difficult to magnetise but retains magnetism) |
| Nickel | Yes | Magnetic element |
| Cobalt | Yes | Magnetic element |
| Aluminium | No | Non-magnetic |
| Copper | No | Non-magnetic |
| Wood | No | Non-magnetic |
| Gold | No | Non-magnetic |
Soft magnetic materials (like iron) are easy to magnetise but lose their magnetism easily. They are used in electromagnets and transformer cores. Hard magnetic materials (like steel) are difficult to magnetise but retain their magnetism, making them suitable for permanent magnets.
Magnetic Fields
A magnetic field is the region around a magnet where a magnetic force can be detected. Magnetic field lines show the direction and strength of the magnetic field.
- Magnetic field lines always go from north to south (outside the magnet)
- The field lines are closest together where the magnetic field is strongest (near the poles)
- The field lines never cross each other
- The direction of the magnetic field at any point is the direction of the force on a north pole placed at that point
- The strength of the magnetic field decreases with distance from the magnet
Drawing Magnetic Field Lines
You can plot magnetic field lines using a plotting compass:
- Place the magnet on a sheet of paper and draw around it
- Place a plotting compass near the north pole of the magnet
- Mark the direction the compass needle points (from the magnet's north towards south)
- Move the compass to the tip of the arrow and repeat
- Join the dots to show the field line
- Repeat for different starting points to build up a complete field pattern
You can also use iron filings to reveal the magnetic field pattern. Sprinkle iron filings onto a sheet of paper placed over the magnet and tap gently. The filings align along the field lines.
Magnetic Field Patterns
Single Bar Magnet
Field lines curve from the north pole around to the south pole. They are closest together at the poles where the field is strongest. Inside the magnet, field lines go from south to north.
Two Like Poles (N-N or S-S)
When two like poles face each other, the field lines push apart. There is a neutral point between the poles where the fields cancel. The magnets repel.
Two Unlike Poles (N-S)
When two unlike poles face each other, field lines go directly from the north pole to the south pole. The field lines are concentrated between the poles, showing a strong uniform field. The magnets attract.
Earth's Magnetic Field
The Earth has a magnetic field. The Earth's core acts like a giant bar magnet. The geographic North Pole is actually near the magnetic south pole of the Earth's internal magnet.
- A compass needle points towards the geographic North Pole because it is attracted to the Earth's magnetic south pole located nearby
- The Earth's magnetic field is thought to be generated by convection currents in the liquid outer core, made of molten iron and nickel
- The magnetic poles are not at the same location as the geographic poles and they slowly drift over time
- The angle between true north (geographic) and magnetic north is called the angle of declination
- The Earth's magnetic field protects us from harmful charged particles from the Sun (solar wind)
- Charged particles are deflected by the magnetic field towards the poles, where they create the aurora (northern and southern lights)
Compasses
A compass contains a small, freely pivoting bar magnet (the needle). The north-seeking end of the needle points towards the Earth's magnetic south pole (near the geographic North Pole).
- Compasses are used for navigation because they always align with the Earth's magnetic field
- Near other magnets or magnetic materials, a compass needle may not point north
- A compass placed near a bar magnet will align with the combined field of the Earth and the bar magnet
- The closer the compass is to the bar magnet, the stronger the magnet's field compared to the Earth's field
Practice Questions
1. Explain the difference between a permanent magnet and an induced magnet. [3 marks]
A permanent magnet produces its own magnetic field and always has north and south poles. An induced magnet only becomes magnetic when placed in an existing magnetic field and loses its magnetism when the field is removed.
2. Two bar magnets are placed side by side with north poles facing each other. Describe the magnetic field pattern between them and state whether they attract or repel. [2 marks]
The field lines push apart between the two north poles, showing a neutral point where the fields cancel. The magnets repel each other.
3. List three magnetic elements. [1 mark]
Iron, nickel, cobalt.
4. Explain why a compass needle points north. [2 marks]
The compass needle is a small bar magnet that aligns with the Earth's magnetic field. The north-seeking end of the needle points towards the Earth's magnetic south pole, which is near the geographic North Pole.
5. Explain the difference between soft and hard magnetic materials, giving one use for each. [4 marks]
Soft magnetic materials (like iron) are easily magnetised and demagnetised, making them suitable for electromagnet cores and transformers. Hard magnetic materials (like steel) retain their magnetism once magnetised, making them suitable for permanent magnets.
6. Explain how you would use a plotting compass to map the magnetic field around a bar magnet. [4 marks]
Place the magnet on paper and draw around it. Put a plotting compass near one pole and mark dots at the needle tips. Move the compass so the tail needle sits on the previous head dot and mark the new position. Repeat and join the dots to form a field line. Repeat from different starting points to build a complete field pattern.
7. A magnet attracts both an iron nail and another magnet. Explain how you could determine which object is a permanent magnet and which is an induced magnet. [3 marks]
Bring each end of the known magnet towards both ends of the other object in turn. If both ends attract (no repulsion), the object is an induced magnet (or unmagnetised material). If one end repels, the object is a permanent magnet because only like poles of permanent magnets can repel each other.
Maths Skills
This topic has minimal mathematical content, but you should be comfortable with the following:
- Interpreting diagrams: Reading the direction of field lines from diagrams to determine the poles of a magnet and the strength of the field (closer lines = stronger field).
- Counting and comparing: Understanding that the force between poles decreases with distance. If the distance doubles, the force decreases significantly (inversely proportional to the square of the distance for point poles).
- Proportional reasoning: More turns on a coil, or a larger current, produces a stronger magnetic field. These are directly proportional relationships.
Field Strength Comparison
At point X, 2 cm from the north pole of a magnet, the field strength is 40 mT. At point Y, 4 cm from the same pole, the field strength is approximately 10 mT. Explain why.
The distance has doubled (from 2 cm to 4 cm). For a point magnetic pole, field strength is inversely proportional to the square of the distance. Doubling the distance reduces the field strength by a factor of 2² = 4. So 40 / 4 = 10 mT.
Common Misconceptions
"All metals are magnetic." Only three elements are magnetic at room temperature: iron, nickel and cobalt. Most metals — including aluminium, copper, gold, silver, zinc and lead — are not magnetic. A common error is to assume that because something is a metal it must be attracted to a magnet. You can test this by bringing a magnet near different metals: only iron, nickel and cobalt will be attracted.
"Magnetic field lines are real physical things that exist in space." Magnetic field lines are a model used to represent the magnetic field. They are a visual tool, not physical objects. The field itself is real — it exerts forces on magnetic materials and moving charges — but the lines are simply a way of drawing the field's direction and strength. In reality, the field is continuous throughout the region; it does not consist of discrete lines. Iron filings align with the field, but they do not reveal actual lines — they simply show the direction of the field at each point.
"A magnet always attracts any metal object." A magnet only attracts magnetic materials (iron, nickel, cobalt and their alloys). It does not attract non-magnetic metals such as aluminium, copper or gold. Attraction alone also cannot prove an object is a permanent magnet, because a magnet also attracts unmagnetised iron. Only repulsion proves an object is a permanent magnet.
6-Mark Extended Question
Compare permanent and induced magnets. Explain how to plot the magnetic field pattern of a bar magnet. [6 marks]
Permanent magnets produce their own magnetic field all the time. They always have a north pole and a south pole, and they retain their magnetism over long periods. Examples include bar magnets and compass needles. Induced magnets only become magnetic when placed in an existing magnetic field. They lose most or all of their magnetism when the external field is removed. An induced magnet always attracts the magnet that induced it and can never repel it, because the near end becomes the opposite pole. Permanent magnets can both attract and repel other permanent magnets, depending on which poles face each other. To plot the magnetic field pattern of a bar magnet: place the bar magnet on a sheet of paper and draw around its outline. Place a plotting compass near the north pole of the magnet and mark two dots at the tips of the compass needle. Move the compass so that the tail of the needle sits on the previous dot made by the head, and mark the new position of the needle tip. Continue this process until the line of dots reaches the south pole. Join the dots with a smooth curve and add an arrow pointing from north to south. Repeat from several different starting points around the north pole to build up a complete field pattern. The field lines should be closest together near the poles, where the field is strongest, and should never cross each other.
AO3: Analyse and Evaluate
Three students set up different magnet configurations and draw the resulting field line patterns. The diagrams are described below:
- Pattern X: Field lines curve from a north pole directly into a south pole. The lines are concentrated and nearly parallel between the two poles.
- Pattern Y: Field lines push apart between the two facing ends. There is a neutral point in the middle where the field is zero.
- Pattern Z: Field lines form a single set of curves from one end of the combined shape to the other, as if there is only one bar magnet.
(a) Identify which pattern is produced by two unlike poles facing each other (N-S), and which by two like poles facing each other (N-N).
(b) Pattern Z was produced by placing two magnets end-to-end. Explain why the field pattern looks like a single larger bar magnet.
(c) A student claims that where field lines cross, the magnetic field is twice as strong. Evaluate this claim.
(a) Pattern X shows two unlike poles facing each other (N-S). The field lines go directly from the north pole to the south pole, and they are concentrated between the poles, forming a nearly uniform field. Pattern Y shows two like poles facing each other (N-N or S-S). The field lines repel each other and push apart, with a neutral point between them where the fields cancel.
(b) When two magnets are placed end-to-end with a south pole touching a north pole, the touching poles effectively cancel because the field lines go from north directly into south. The result is a combined magnet with a north pole at one outer end and a south pole at the other, just like a single longer bar magnet. The two magnets behave as one.
(c) The claim is incorrect. Magnetic field lines never cross each other. At any point in space, the magnetic field has exactly one direction. If field lines were to cross, it would mean the field points in two different directions at the same point, which is impossible. Where two magnets overlap their fields, the fields add as vectors (they combine), but the resultant field has a single direction at every point, so the field lines of the combined field never cross.
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