Permanent and induced magnetism and magnetic fields

What you'll learn

This revision guide covers everything you need to know about magnetism for AQA GCSE Physics. You'll understand how magnets work, why some materials are magnetic while others aren't, and how to interpret magnetic field patterns. These concepts are fundamental to understanding motors, generators, and transformers that appear later in the course.

Key terms and definitions

Permanent magnet — a material that produces its own magnetic field continuously without requiring an external power source, such as a bar magnet made from steel.

Induced magnet — a material that becomes magnetic only when placed in a magnetic field and loses most or all of its magnetism when removed from that field.

Magnetic field — the region around a magnet where a magnetic force acts on magnetic materials or other magnets.

Magnetic field lines — lines drawn to represent the direction and strength of a magnetic field, where the direction shows the force on a north pole and closer spacing indicates stronger fields.

Magnetic poles — the two ends of a magnet (north-seeking and south-seeking) where the magnetic forces are strongest.

Magnetic flux density — a measure of the strength of a magnetic field, measured in tesla (T), indicating how many magnetic field lines pass through a given area.

Soft magnetic material — a material like iron that is easily magnetised and demagnetised, making it useful for temporary magnets and electromagnets.

Hard magnetic material — a material like steel that is difficult to magnetise but retains its magnetism for a long time, making it suitable for permanent magnets.

Core concepts

Magnetic materials and poles

Only certain materials are magnetic. The three main magnetic materials are:

• Iron
• Cobalt
• Nickel

Steel, which is an alloy containing iron, is also magnetic. Other common materials like copper, aluminium, wood, and plastic are non-magnetic.

Every magnet has two poles:

• A north pole (north-seeking pole)
• A south pole (south-seeking pole)

The fundamental rule of magnetic poles states:

• Like poles repel (north-north or south-south)
• Unlike poles attract (north-south)

These forces act at a distance without the magnets touching, demonstrating that magnetism is a non-contact force. The strength of the force increases as magnets move closer together.

Permanent vs induced magnetism

Permanent magnets produce their own magnetic field constantly. Common examples include:

• Bar magnets made from steel
• Horseshoe magnets
• Neodymium magnets (used in hard drives and headphones)
• Fridge magnets

The magnetism in permanent magnets results from the alignment of magnetic domains within the material. Once magnetised, these domains remain aligned, creating a persistent magnetic field.

Induced magnetism occurs when a magnetic material is placed near or in contact with a permanent magnet. The material becomes temporarily magnetised through a process called magnetic induction:

1. The magnetic field from the permanent magnet causes the magnetic domains in the material to align
2. The material becomes a temporary magnet with its own north and south poles
3. The induced magnet is always attracted to the permanent magnet
4. When the permanent magnet is removed, most or all of the magnetism is lost

The key difference is permanence:

• Permanent magnets retain their magnetism indefinitely
• Induced magnets lose their magnetism when the external field is removed

Soft magnetic materials like iron are easily magnetised and demagnetised, making them ideal for induced magnets. Hard magnetic materials like steel are difficult to magnetise but retain magnetism well, making them suitable for permanent magnets.

Magnetic fields and field lines

A magnetic field is the region around a magnet where magnetic forces can be detected. We represent magnetic fields using magnetic field lines, which follow specific conventions:

Direction rules:

• Field lines always point from the north pole to the south pole (outside the magnet)
• The arrow on a field line shows the direction a north pole would experience a force
• Field lines never cross each other

Strength indicators:

• Closer field lines indicate a stronger magnetic field
• Field lines further apart indicate a weaker magnetic field
• The field is strongest at the poles where lines are most concentrated

Plotting magnetic field patterns

You can plot magnetic field patterns using two methods:

Method 1: Using iron filings

1. Place a bar magnet under a sheet of paper
2. Sprinkle iron filings evenly over the paper
3. Tap the paper gently to allow filings to align
4. The filings align with the magnetic field, revealing its pattern

Each iron filing becomes a tiny induced magnet, aligning with the field direction.

Method 2: Using a plotting compass

1. Place a plotting compass near the magnet's north pole
2. Mark the direction the compass needle points
3. Move the compass so its south pole is where the north pole was pointing
4. Repeat to trace a complete field line
5. Draw multiple field lines starting from different points

The plotting compass method is more accurate and shows direction clearly.

Common magnetic field patterns you must recognise:

Single bar magnet: Field lines emerge from the north pole, curve around through space, and enter the south pole. The field is strongest at the poles.

Two bar magnets attracting (unlike poles facing): Field lines run directly from the north pole of one magnet to the south pole of the other. The field between the magnets is strong and uniform.

Two bar magnets repelling (like poles facing): Field lines curve away from each other. There's a neutral point between the magnets where the fields cancel out.

Uniform magnetic field: Created between the opposite poles of two magnets placed close together. Field lines are parallel, equally spaced, and point in the same direction, indicating constant field strength and direction.

The Earth's magnetic field

The Earth has a magnetic field similar to that of a giant bar magnet placed at its centre. Important features include:

• The Earth's core generates the magnetic field through movement of molten iron
• The magnetic north pole is actually near the geographic South Pole
• The magnetic south pole is near the geographic North Pole
• Compass needles align with Earth's field, with the north-seeking pole pointing toward magnetic north

The Earth's magnetic field:

• Protects us from harmful charged particles from the Sun (solar wind)
• Allows navigation using compasses
• Causes auroras (Northern and Southern Lights) when charged particles interact with the atmosphere near the poles

Compasses and navigation

A compass contains a small magnetised needle that is free to rotate. The needle is itself a tiny bar magnet:

• The north-seeking pole (usually coloured red or marked N) points toward magnetic north
• The south-seeking pole points toward magnetic south

When a compass is placed in a magnetic field:

1. The compass needle experiences a force
2. It rotates to align with the field lines
3. The north pole of the needle points in the direction of the field

This is why placing a compass near a bar magnet causes it to point toward the magnet's south pole (unlike poles attract).

In the absence of other magnets, a compass aligns with Earth's magnetic field, providing a reliable method for navigation. This principle has been used for centuries by sailors, explorers, and travellers.

Worked examples

Example 1: Explaining magnetic behaviour

Question: A steel paperclip is attracted to a bar magnet. When the bar magnet is removed, the paperclip can pick up other paperclips for a short time. Explain these observations. [4 marks]

Answer:

The steel paperclip becomes an induced magnet when placed near the bar magnet (1 mark). The magnetic field from the bar magnet causes magnetic domains in the steel to align (1 mark). This creates temporary poles in the paperclip, with the end nearest the magnet's north pole becoming a south pole, causing attraction (1 mark). Steel is a hard magnetic material, so it retains some magnetism temporarily after the bar magnet is removed, allowing it to attract other paperclips briefly before the domains lose alignment (1 mark).

Mark scheme notes: This answer demonstrates understanding of induced magnetism, domain theory, and the difference between hard and soft magnetic materials—all key concepts for higher marks.

Example 2: Interpreting field patterns

Question: A student uses a plotting compass to trace the magnetic field around a bar magnet. Describe how the student should use the plotting compass to draw one complete field line. [3 marks]

Answer:

Place the plotting compass near the north pole of the bar magnet (1 mark). Mark the position of the north pole of the compass needle, then move the compass so its south pole is at the marked position (1 mark). Repeat this process until the field line reaches the south pole of the bar magnet, then draw a smooth curve through all the marked points with an arrow showing direction from north to south (1 mark).

Mark scheme notes: Sequential method marks are awarded for correct procedure. Direction must be specified for full marks.

Example 3: Comparing permanent and induced magnets

Question: Compare the properties of permanent magnets and induced magnets. [4 marks]

Answer:

Permanent magnets produce their own magnetic field continuously, while induced magnets only produce a field when in another magnetic field (1 mark). Permanent magnets retain their magnetism when removed from external fields, whereas induced magnets lose most or all of their magnetism when the external field is removed (1 mark). Permanent magnets can attract or repel other magnets depending on pole orientation, but induced magnets are always attracted to the permanent magnet that induces them (1 mark). Permanent magnets are made from hard magnetic materials like steel, while induced magnets are typically made from soft magnetic materials like iron (1 mark).

Mark scheme notes: Comparative statements earn marks. Simply listing properties of each type separately scores fewer marks than direct comparisons.

Common mistakes and how to avoid them

• Confusing magnetic and non-magnetic materials. Only iron, cobalt, nickel, and steel are magnetic. Aluminium and copper are NOT magnetic despite being metals. Learn the specific list of magnetic materials.

• Drawing magnetic field lines incorrectly. Always draw arrows pointing from north to south pole. Never make field lines cross each other. Space them to show field strength—closer lines mean stronger fields.

• Thinking induced magnets can repel permanent magnets. Induced magnets are ALWAYS attracted to the permanent magnet inducing them, never repelled. The induced poles are always oriented to create attraction.

• Mixing up poles and forces. Remember: like poles repel, unlike poles attract. The north pole of a compass points toward the south pole of a magnet (and toward Earth's magnetic north, which is actually a magnetic south pole).

• Forgetting that magnetism acts at a distance. Magnetic force is a non-contact force. Magnets don't need to touch to exert forces on each other or on magnetic materials.

• Not recognising uniform fields. Between two opposite poles placed close together, field lines are parallel and evenly spaced—this indicates uniform field strength and direction throughout that region.

Exam technique for "Permanent and induced magnetism and magnetic fields"

• Command word "describe" requires method or pattern. When describing how to plot a field or a pattern, give sequential steps or identify key features like field line direction and spacing. Aim for one mark per valid point.

• Command word "explain" needs reasoning. Use terms like "because," "this causes," or "as a result" to link cause and effect. For magnetism questions, reference induced magnetism, domain alignment, or pole attraction/repulsion with justification.

• Diagram questions require precision. When drawing field lines, use a ruler for uniform fields, ensure arrows point from N to S, and make line spacing reflect field strength. Label poles clearly.

• 6-mark questions demand structure. Extended response questions on magnetism should cover multiple points: what happens, why it happens (domain theory or field interaction), and applications or examples. Use scientific terminology throughout.

Quick revision summary

Permanent magnets produce continuous magnetic fields from aligned domains in hard magnetic materials. Induced magnets become temporarily magnetic in external fields and lose magnetism when removed. Magnetic fields are represented by lines pointing from north to south poles—closer spacing indicates stronger fields. Only iron, cobalt, nickel, and steel are magnetic. Like poles repel; unlike poles attract. Compasses align with field lines, enabling navigation using Earth's magnetic field.