Electromagnetism

Electromagnetism is the link between electricity and magnetism. In this lesson you will explore how electric currents produce magnetic fields, how electromagnets work in more detail, and how these principles are applied in real-world devices. This covers AQA GCSE Physics specification 4.7.2.

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The Link Between Electricity and Magnetism

In 1820, Hans Christian Oersted discovered that a wire carrying an electric current deflected a nearby compass needle. This was the first evidence that electricity and magnetism are linked.

Key principle: Whenever an electric current flows, a magnetic field is produced around the conductor.

This is the basis of electromagnetism — using electric currents to create controllable magnetic fields.

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Magnetic Field Around a Single Flat Circular Coil

When current flows through a flat circular coil (a single loop of wire), the magnetic field pattern has these features:

• At the centre of the coil, the field lines are approximately straight and parallel — the field is roughly uniform near the centre.
• At the edges, the field lines curve outward.
• The overall pattern from a distance resembles the field of a short bar magnet.

graph LR
    subgraph "Field of a Flat Circular Coil"
        L["Left side of coil"] -->|"Curved field lines at edges"| C["Centre: nearly uniform field"]
        C -->|"Curved field lines at edges"| R["Right side of coil"]
    end

The strength of the field at the centre of the coil can be increased by:

• Increasing the current flowing through the coil.
• Increasing the number of turns (stacking multiple loops together).
• Decreasing the radius of the coil (concentrating the field in a smaller area).

Exam Tip: The flat circular coil is different from a solenoid. A coil is a single loop (or a few loops), while a solenoid is many loops wound into a long cylinder. The solenoid gives a much more uniform internal field.

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How an Electromagnet Works — Detailed Explanation

An electromagnet consists of a coil of insulated wire wound around a core of magnetically soft material (usually iron).

Step-by-Step Operation

1. When the circuit is switched on, current flows through the coil.
2. The current creates a magnetic field in and around the coil (like a solenoid).
3. The iron core is placed inside the coil and becomes magnetised by induction — the magnetic domains in the iron align with the field.
4. The magnetised iron core greatly increases the overall magnetic field strength (iron has a much higher permeability than air).
5. When the circuit is switched off, the current stops, the magnetic field disappears, and the iron core demagnetises almost immediately.

Component	Role
Coil of wire	Carries the current that produces the magnetic field
Iron core	Becomes an induced magnet; greatly strengthens the field
Power supply	Provides the current; allows the electromagnet to be switched on/off

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Magnetic Domain Theory

Inside magnetic materials, atoms act as tiny magnets arranged in regions called magnetic domains.

• In an unmagnetised material, the domains point in random directions — their fields cancel out and there is no overall magnetism.
• In a magnetised material, the domains are aligned in the same direction — their fields add up to produce a strong overall magnetic field.

graph LR
    subgraph "Unmagnetised Material"
        D1["Domain: right"] --- D2["Domain: up"]
        D2 --- D3["Domain: left"]
        D3 --- D4["Domain: down"]
    end
    subgraph "Magnetised Material"
        E1["Domain: right"] --- E2["Domain: right"]
        E2 --- E3["Domain: right"]
        E3 --- E4["Domain: right"]
    end

When a magnetic material is placed in an external magnetic field:

• The domains that are already aligned with the field grow (their boundaries shift).
• Domains that are not aligned rotate to line up with the field.
• The result is a net magnetisation in the direction of the external field.

Exam Tip: Domain theory explains why magnets can be demagnetised by heating or dropping them — the energy causes the domains to become randomly arranged again, cancelling out the overall field.

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Applications of Electromagnetism

Electromagnetic Relay

A relay is a switch operated by an electromagnet. It allows a small current in one circuit to switch on a much larger current in a separate circuit.

How it works:

1. A small current flows through the coil of the electromagnet.
2. The electromagnet becomes magnetised and attracts an iron armature (a pivoted iron lever).
3. The armature moves and closes (or opens) the contacts in a second, high-current circuit.
4. When the small current is switched off, the electromagnet demagnetises, the armature springs back, and the second circuit opens (or closes).

Advantage	Explanation
Safety	The operator controls a low-voltage circuit while switching a high-voltage circuit
Automation	Relays can be triggered by sensors or timers
Isolation	The two circuits are electrically separate

Electromagnetic Door Lock

An electromagnet holds a metal plate attached to the door. When the electromagnet is energised, the door is held shut. Cutting the power releases the door — this is a safety feature in fire alarm systems.

Magnetic Levitation (Maglev) Trains

Powerful electromagnets in the track and train create magnetic fields that repel each other, levitating the train above the track. This eliminates friction, allowing very high speeds.

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Factors Affecting Electromagnet Strength — Summary Table

Factor	Effect of Increasing It
Current through the coil	Stronger magnetic field
Number of turns on the coil	Stronger magnetic field
Iron core present	Much stronger magnetic field
Distance from the electromagnet	Weaker magnetic field (field weakens with distance)

Exam Tip: If a question asks you to "suggest how" to make an electromagnet stronger, always give specific actions: "increase the current" or "add more turns to the coil." Avoid vague answers like "make it bigger."

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Comparing Electromagnets and Permanent Magnets