Understanding Physics: Magnetism and electromagnetism: Edexcel IGCSE Science Double Award Explained

Magnetism and electromagnetism connects two fundamental forces of nature. Understanding how magnetic fields arise from currents, and how changing fields induce voltages, gives you the physics behind motors, generators and transformers.

The edexcel igcse science double award physics: magnetism and electromagnetism section of the 4SD0 specification covers four topics: units, magnetism, electromagnetism, and electromagnetic induction. This is one of the more conceptually demanding areas of the physics paper, but the underlying ideas are logical and the exam questions follow predictable patterns. These edexcel igcse science double award revision notes take an analytical approach, building from first principles through to the applications you need for confident exam answers.

Units

Magnetism

A magnet has two poles: north and south. Like poles repel; unlike poles attract. This is the fundamental law of magnetism. The region around a magnet where a magnetic force can be detected is called a magnetic field. Field lines run from north to south outside the magnet. They never cross, and their density indicates the field strength: closer lines mean a stronger field.

Magnetic materials include iron, steel, nickel and cobalt. These can be attracted by a magnet and can be magnetised. Non-magnetic materials (such as copper, aluminium and wood) are not attracted to magnets. The igcse specification distinguishes between hard and soft magnetic materials:

• Soft magnetic materials (e.g. iron): Easily magnetised but lose their magnetism quickly. Used in electromagnets and transformer cores, where the magnetism needs to switch on and off rapidly.
• Hard magnetic materials (e.g. steel): Difficult to magnetise but retain their magnetism once magnetised. Used in permanent magnets.

To magnetise a material, you can stroke it repeatedly with a magnet (always in the same direction) or place it inside a solenoid carrying direct current. To demagnetise, you can heat it strongly, hammer it repeatedly, or place it inside a solenoid carrying alternating current that is gradually reduced to zero.

The edexcel igcse science double award specification also covers induced magnetism: when a piece of unmagnetised magnetic material is placed near a magnet, it becomes a temporary magnet. The end nearest the magnet's north pole becomes a south pole (by attraction). Remove the magnet, and the induced magnetism disappears (if the material is soft).

Electromagnetism

When an electric current flows through a wire, it produces a magnetic field around the wire. This is the connection between electricity and magnetism that gives this topic its name. The field around a straight wire forms concentric circles. The direction of the field can be found using the right-hand grip rule: grip the wire with your right hand so that your thumb points in the direction of the current, and your fingers curl in the direction of the magnetic field.

A solenoid (a coil of wire carrying current) produces a magnetic field pattern that resembles a bar magnet, with a north pole at one end and a south pole at the other. The field inside the solenoid is strong and uniform. The field can be strengthened by increasing the current, increasing the number of turns, or inserting a soft iron core. An electromagnet is a solenoid with an iron core: it can be switched on and off, which makes it useful in applications like scrapyard cranes, circuit breakers and electric bells.

The motor effect: When a current-carrying conductor is placed in a magnetic field, it experiences a force. This is the motor effect. The direction of the force depends on the direction of the current and the direction of the magnetic field. You can predict the direction of the force using Fleming's left-hand rule: point your first finger in the direction of the field (north to south), your second finger in the direction of the current, and your thumb points in the direction of the force (motion).

The force can be increased by increasing the current, increasing the magnetic field strength, or increasing the length of wire in the field. The force is at its maximum when the wire is perpendicular to the field and zero when the wire is parallel to the field.

The DC motor: A simple DC motor uses the motor effect to produce rotation. A rectangular coil of wire is placed in a magnetic field. When current flows through the coil, one side experiences a force upward and the other side experiences a force downward (because the current flows in opposite directions on each side). This creates a turning effect. A split-ring commutator reverses the current direction every half turn, ensuring the coil continues to rotate in the same direction. The speed of the motor can be increased by increasing the current, using a stronger magnet, or adding more turns to the coil.

Electromagnetic induction

Electromagnetic induction is the reverse of the motor effect. Instead of using current and magnetism to produce motion, it uses motion and magnetism to produce current. When a conductor moves through a magnetic field (or when a magnetic field changes around a conductor), a voltage (electromotive force, or EMF) is induced across the conductor. If the conductor is part of a complete circuit, a current flows.

The edexcel igcse science double award explained specification requires you to know the factors that affect the induced EMF:

• The speed of movement (faster movement induces a greater EMF).
• The strength of the magnetic field (a stronger field induces a greater EMF).
• The number of turns on the coil (more turns induce a greater EMF).

The direction of the induced current can be found using Fleming's right-hand rule (or Lenz's law, which states that the induced current always flows in a direction that opposes the change producing it).

The AC generator: A generator converts kinetic energy into electrical energy. A coil rotates in a magnetic field, and the changing flux through the coil induces an alternating EMF. The output is alternating current (AC) because the direction of the induced current reverses every half turn. Slip rings and brushes maintain the connection to the external circuit without the commutator's reversing action. The output voltage can be increased by rotating the coil faster, using a stronger magnetic field, or adding more turns to the coil.

Transformers: A transformer changes the voltage of an alternating current supply. It consists of two coils (primary and secondary) wound on a shared soft iron core. An alternating current in the primary coil creates a changing magnetic field in the iron core, which induces an alternating EMF in the secondary coil.

The transformer equation is: V_p / V_s = N_p / N_s, where V is voltage and N is the number of turns. A step-up transformer has more turns on the secondary coil than on the primary, so it increases the voltage. A step-down transformer has fewer turns on the secondary, so it decreases the voltage.

For an ideal (100% efficient) transformer, power in = power out: V_pI_p = V_sI_s. If a transformer steps up the voltage, it steps down the current by the same factor. This relationship is used in the National Grid, where step-up transformers increase voltage for long-distance transmission (reducing current and therefore reducing energy losses in the cables, since power loss = I²R), and step-down transformers reduce voltage for safe domestic use.

Exam strategy

The physics: magnetism and electromagnetism edexcel igcse practice questions in the edexcel igcse science double award require a mixture of descriptions, explanations and calculations. Many students find the conceptual questions (explaining how a motor works, or why a transformer only works with AC) more challenging than the numerical ones. The trick is to structure your answer as a sequence of physical events: current flows, field is produced, force acts, coil rotates, commutator reverses current, rotation continues.

Review these edexcel igcse science double award notes until you can describe each device's operation without hesitation. Use the Green Bridge CBT platform for edexcel igcse science double award practice questions and edexcel igcse science double award revision notes and past questions to practice under timed conditions. Precision in language and logic is what earns full marks in this section.