Permanent magnets, poles, field lines, induced magnetism and real-world applications
🧲 Describe the properties of permanent magnets, including attraction, repulsion and the concept of poles
📐 Draw and interpret magnetic field line diagrams for bar magnets and pairs of magnets
⚡ Explain induced magnetism and how it differs from permanent magnetism
🔬 Describe how a compass and plotting compass work in terms of magnetic fields
🌍 Explain the Earth's magnetic field and its importance for navigation
🏭 Evaluate real-world applications of magnets including compasses, maglev trains and magnetic storage
🧲 Permanent Magnets and Poles
Permanent magnet: A material that produces its own persistent magnetic field without needing an external field or current. Examples include bar magnets, horseshoe magnets and button magnets made from materials such as iron, steel, nickel and cobalt.
Every magnet has two poles: a north pole (N) and a south pole (S). These poles are where the magnetic force is strongest. The fundamental rule of magnetism is:
Like poles REPEL | Unlike poles ATTRACT
This means two north poles pushed together will repel each other, while a north pole and a south pole will attract. This force acts even without the magnets touching — it is a non-contact force that acts at a distance through the magnetic field.
Magnetic poles always exist in pairs. You cannot isolate a single north or south pole — if you break a magnet in half, each piece has both a north and a south pole.
Magnetic materials are materials that can be attracted to a magnet. These include iron, steel, nickel and cobalt. Note that aluminium, copper, wood and plastic are NOT magnetic materials. Only certain materials can be magnetised or attracted to magnets.
Steel makes a better permanent magnet than iron because once magnetised it retains its magnetism. Iron is an example of a soft magnetic material — it is easily magnetised but also easily demagnetised. Steel is a hard magnetic material — it is harder to magnetise but retains magnetism much longer.
Property
Iron (soft)
Steel (hard)
Ease of magnetisation
Easy
Harder
Retains magnetism
Weakly (temporary)
Strongly (permanent)
Best use
Electromagnet cores
Permanent magnets
📐 Magnetic Field Lines
Magnetic field: A region of space around a magnet (or current-carrying conductor) in which a magnetic force is experienced by another magnet, magnetic material or moving charge.
We represent magnetic fields using field lines (also called flux lines). These are imaginary lines that show the direction and strength of the field. The rules for drawing field lines are:
Field lines run from north to south outside the magnet
Inside the magnet, field lines run from south to north (so lines are always closed loops)
Closer lines = stronger field; further apart = weaker field
Field lines never cross each other
Lines are drawn with arrows showing the direction from N to S
The field is strongest at the poles of a bar magnet — you can see this because the field lines are closest together there.
For a bar magnet, the field lines emerge from the north pole, curve around the outside and re-enter at the south pole, forming closed loops. The field is three-dimensional but we usually show a 2D cross-section.
Between two magnets — unlike poles facing: The field lines link up between the poles, showing attraction. The field between the poles is strong and relatively uniform near the centre.
Between two magnets — like poles facing: The field lines are pushed away from the region between the poles. A neutral point exists between them where the fields cancel and the net field is zero. Field lines never enter the space directly between like poles.
A plotting compass is used to map field lines. The north pole of the compass needle points in the direction of the field line at each point. By moving the compass and plotting the needle direction repeatedly, the full field pattern can be traced out.
⚡ Induced Magnetism
Induced magnetism: The temporary magnetisation of a magnetic material when placed in an external magnetic field. The material becomes a magnet while the external field is present, but may lose its magnetism when the field is removed.
When an unmagnetised iron nail is brought near a magnet, it becomes temporarily magnetised. This is induced magnetism. The pole induced on the iron nail nearest the magnet is always an opposite pole — this is why magnets always attract unmagnetised magnetic materials (they can never repel them).
Magnet (N pole) → attracts → Iron object (S pole induced on near end, N on far end)
This is fundamentally different from the repulsion between two permanent magnets (which requires both objects to already be magnetised with like poles facing).
Magnetic domains: Inside a magnetic material, atoms behave like tiny magnets. In an unmagnetised material these domains are randomly oriented and cancel each other out. When placed in a magnetic field, the domains align with the external field, producing a net magnetic field — this is the microscopic explanation for induced magnetism.
If a permanent magnet is dropped, heated strongly or hammered repeatedly, it can become demagnetised. This is because the domains become randomly oriented again.
Temporary vs Permanent magnets: A soft magnetic material (like iron) will lose its induced magnetism when removed from the external field. A hard magnetic material (like steel) retains significant magnetism — this is how you magnetise a steel bar, by stroking it repeatedly in the same direction with a magnet (the stroking method).
🌍 The Earth's Magnetic Field
The Earth behaves as though it has a giant bar magnet inside it, producing a magnetic field around the planet. This field extends far into space and protects us from harmful charged particles from the Sun (the solar wind) by deflecting them.
The Earth's geographic North Pole is actually near the Earth's magnetic South Pole — this is why the north pole of a compass needle (which points toward geographic north) is attracted there. Like poles repel; unlike poles attract.
The geographic north and magnetic north are not exactly the same location. The angle between them is called magnetic declination. This means compasses give a reading slightly different from true north, though for GCSE purposes we treat them as approximately the same.
The Earth's magnetic field has been essential for navigation for centuries. A compass is simply a freely suspended magnet that aligns with the Earth's magnetic field. The north-seeking pole of the compass points toward magnetic north.
The field is not uniform across the Earth's surface. It is strongest near the poles and weakest near the equator. The field lines enter the Earth near the geographic North Pole and exit near the geographic South Pole (from a magnetic field perspective, thinking of the Earth's internal "south" magnetic pole attracting compass needles' north poles).
🏭 Applications of Magnets
Magnets have a huge range of real-world applications. Here are some key examples relevant to GCSE Physics:
Compass navigation: The most traditional use of magnets. A freely suspended magnetised needle aligns with the Earth's field, indicating direction. Used in hiking, sailing and orienteering.
Magnetic storage: Hard drives in computers use tiny regions of magnetic material to store data. Each region can be magnetised in one of two directions, representing binary 0 or 1. Credit cards use a magnetic strip for data storage.
Maglev trains: Magnetic levitation trains use powerful electromagnets to lift the train off the track and propel it forward. Because there is no friction between train and track, speeds of over 600 km/h can be achieved. The system uses repulsion between like poles to levitate the train.
Electric motors and generators: These rely on the interaction between magnetic fields and electric currents (covered in more detail in the electromagnetism topic). Permanent magnets provide the field in many small motors.
Magnetic resonance imaging (MRI): Medical scanners use extremely powerful magnetic fields (produced by superconducting electromagnets) to image soft tissue inside the body non-invasively.
Loudspeakers and microphones: A permanent magnet combined with a current-carrying coil allows conversion between electrical signals and sound waves.
In all applications, the key principle is the same: magnetic fields exert forces on magnetic materials and on moving charges (electric currents). Understanding the field pattern helps predict these forces.
Example 1: A student places an unmagnetised iron nail near the north pole of a bar magnet. Explain why the nail is attracted to the magnet, referring to induced magnetism and poles.
1 Identify what happens to the iron nail when placed near the magnet. The nail is a soft magnetic material — it undergoes induced magnetism. The magnetic field of the bar magnet causes the domains within the nail to align with the external field.
2 Determine which pole is induced on the end of the nail nearest the magnet. The external field at the north pole of the bar magnet points away from the magnet (field lines leave north poles). This causes a south pole to be induced on the end of the nail closest to the bar magnet's north pole.
3 Apply the rule about poles to explain the attraction. The induced south pole on the nail is adjacent to the north pole of the bar magnet. Unlike poles attract, so there is a magnetic force of attraction between them.
4 Explain why a magnet can never repel an unmagnetised magnetic material. The induced pole on the near end of the nail is always opposite to the pole of the magnet causing it. Therefore the force is always attractive — never repulsive — for unmagnetised materials.
The iron nail undergoes induced magnetism — its domains align with the external field, producing a south pole on the end nearest the magnet's north pole. Unlike poles attract, so the nail is pulled toward the magnet. This always results in attraction, never repulsion.
Example 2: A student uses a plotting compass to map the magnetic field around a bar magnet. Describe the method and explain how the field line pattern shows the strength of the field.
1 Describe the equipment and setup. Place the bar magnet flat on a sheet of paper and draw around it. Place the plotting compass at a point near one of the poles of the magnet.
2 Describe the plotting procedure. Mark a dot at the tip of the compass needle's north pole. Move the compass so its south pole is now at the dot you just marked. Mark the new position of the north pole. Continue this process until the line reaches the other pole or leaves the paper.
3 Explain direction of field lines. The arrows on the field lines point from north to south outside the magnet (in the direction the north pole of the compass points).
4 Explain how spacing shows field strength. Where the field lines are close together (near the poles), the field is strong. Where they are far apart (away from the magnet), the field is weak. The density of field lines represents the magnitude of the magnetic field.
Place compass near a pole; mark dots at the north tip; move compass so its tail is at the last dot; repeat to trace the line. Lines go from N to S outside the magnet. Closely spaced lines = strong field (near poles); widely spaced = weak field (far from magnet).
Example 3: Two bar magnets are placed with their north poles facing each other, 5 cm apart. Describe the magnetic field pattern between them and explain what a neutral point is and where it would be found.
1 Describe what happens to field lines from each north pole. The field lines from both north poles point outward, away from each pole. Between the two magnets, these field lines point in opposite directions — one set pointing right, one set pointing left.
2 Explain the consequence of opposing field lines. The two fields oppose each other in the region between the magnets. At some point, the field from magnet 1 and the field from magnet 2 are equal in magnitude but opposite in direction.
3 Define neutral point. A neutral point is a point in the field where the magnetic field strength from two sources is equal and opposite, so the resultant magnetic field is zero.
4 Locate the neutral point for identical magnets. If both magnets are identical, the neutral point is exactly halfway between the two north poles (i.e. at 2.5 cm from each pole). A compass placed at this point would not point in any particular direction as there is no net field.
Between the like poles, field lines from both magnets point toward each other and cancel. The neutral point — where the resultant field is zero — is located midway between the two north poles (at 2.5 cm from each). Field lines bend around this region rather than passing through it.
Example 4: Explain why the Earth's geographic North Pole corresponds to a magnetic south pole, using knowledge of compass behaviour and magnetic pole rules.
1 State what a compass does. A compass needle is a small permanent magnet that aligns with the Earth's magnetic field. The north-seeking end (marked N) points toward geographic north.
2 Apply the rule of magnetic poles. The north pole of the compass needle is attracted toward geographic north. Since unlike poles attract, the pole of the Earth's field at geographic north must be a magnetic south pole.
3 Reconcile this with the "giant bar magnet" model. The Earth behaves like a bar magnet with its magnetic south pole near geographic north, and its magnetic north pole near geographic south. This is counter-intuitive but is a consequence of the convention that the north pole of a magnet is defined as the pole that points north.
4 Summarise clearly. Geographic North ≈ magnetic south pole of Earth's internal magnet. Geographic South ≈ magnetic north pole of Earth's internal magnet.
The north pole of a compass needle points toward geographic north. Since unlike poles attract, the Earth must have a magnetic south pole near its geographic North Pole. The Earth's internal "bar magnet" has its south end near geographic north and its north end near geographic south.
Question 1: Which of the following is NOT a magnetic material?
Question 2: What does a region of closely spaced magnetic field lines indicate?
Question 3: A plotting compass is placed to the right of a bar magnet's north pole. In which direction does the north end of the compass needle point?
Question 4: Why can a magnet never repel an unmagnetised piece of iron? Write your answer below.
Model answer: When the magnet approaches the iron, it induces magnetism in the iron. The pole induced on the end of the iron nearest the magnet is always the opposite pole to the magnet's pole. Since unlike poles attract, the force is always attractive. Repulsion requires like poles, which cannot be induced — the induced pole always opposes the inducing pole.
Question 5: Explain why steel is preferred over iron for making a permanent magnet. Include reference to magnetic domains in your answer.
Model answer: Both iron and steel can have their magnetic domains aligned by an external magnetic field. However, in iron (a soft magnetic material) the domains easily return to a random arrangement when the external field is removed, so the magnetism is lost. In steel (a hard magnetic material) the domains resist returning to a random orientation — they stay aligned — so the steel retains its magnetism as a permanent magnet.
Challenge 1 (6 marks): A student investigates the field between two bar magnets placed with unlike poles (N and S) facing each other, 8 cm apart. The student claims that the field between the poles is stronger than the field at either pole on its own. Evaluate this claim. In your answer, describe the field pattern between unlike poles and explain the effect on field strength. [6 marks]
Mark scheme (6 marks):
• When unlike poles face each other, the field lines from the N pole link directly to the S pole between them (1)
• The field lines are parallel and evenly spaced between the poles — this is a uniform field (1)
• Field lines from both magnets add together (superpose) in the region between the poles (1)
• More field lines per unit area = stronger field (1)
• The student's claim is broadly correct — the field between unlike poles is stronger than the field at one pole alone (1)
• However, the field is not infinitely stronger — its strength depends on the magnets' individual strengths and the distance between them; near the poles themselves the field is also very strong (1)
Challenge 2 (5 marks): A compass needle always points toward geographic north. However, the Earth's geographic North Pole is approximately 1500 km from the magnetic north pole, and magnetic declination varies around the world. Explain (a) why a compass works as a direction-finding tool despite this discrepancy, and (b) what problems magnetic declination might cause for accurate navigation. [5 marks]
Mark scheme (5 marks):
Part (a):
• The compass needle is a freely suspended permanent magnet that aligns with Earth's magnetic field (1)
• Magnetic north and geographic north are close enough for general navigation — the angle between them (declination) is typically small (1)
• For approximate direction-finding (e.g. hiking), this small difference is acceptable and the compass is reliable (1)
Part (b):
• Magnetic declination means the compass points slightly east or west of true north depending on location (1)
• For precise navigation (e.g. ship or aircraft navigation over long distances), ignoring declination causes significant positional errors that accumulate over distance — navigators must apply a declination correction (1)
Challenge 3 (6 marks): A maglev train uses magnetic repulsion to levitate above the track. (a) Explain using principles of magnetism why repulsion, not attraction, is used for levitation. (b) Suggest one advantage and one disadvantage of using maglev technology compared to conventional trains. (c) Explain why powerful electromagnets rather than permanent magnets are used in maglev systems. [6 marks]
Mark scheme (6 marks):
Part (a):
• Repulsion between like poles pushes the train upward (away from the track), opposing gravity (1)
• Attraction would pull the train toward the track, not lift it — so attraction cannot levitate the train (1)
Part (b):
• Advantage: no friction between train and track so much higher speeds are possible / quieter / less wear and maintenance (1)
• Disadvantage: very high infrastructure cost / requires specially built tracks / large energy consumption for electromagnets / not compatible with existing rail networks (1)
Part (c):
• Electromagnets can be switched on and off and their strength can be varied (by changing the current) to precisely control the levitation height and stability (1)
• Permanent magnets cannot be switched off or easily varied — fine control of a rapidly changing system like a moving train would not be possible (1)
Challenge 4 (4 marks): A student tries to demagnetise a steel bar magnet by cooling it in ice water. A second student suggests heating the magnet to above its Curie temperature. Using your knowledge of magnetic domains, evaluate both methods and explain which is more effective. [4 marks]
Mark scheme (4 marks):
• Magnetism arises from aligned magnetic domains within the material (1)
• Cooling the magnet does not provide enough energy to disrupt domain alignment — the domains remain ordered and the magnet retains its magnetism; this method is ineffective (1)
• Heating above the Curie temperature gives the atoms enough thermal energy to overcome the forces keeping domains aligned; the domains become randomly oriented and the net magnetism is lost (1)
• Heating is far more effective — the second student's method is correct; alternatively, repeated hammering or dropping also works by mechanically disrupting domain alignment (1)