What you'll learn
This revision guide covers the essential principles of magnetism and electromagnetism required for WJEC GCSE Physics. You'll explore magnetic fields, the relationship between electricity and magnetism, and how these principles underpin technologies including motors, generators and transformers. Understanding these concepts is crucial for both exam success and appreciating modern electrical systems.
Key terms and definitions
Magnetic field — the region around a magnet where a magnetic force acts on magnetic materials or other magnets.
Magnetic flux density — the strength of a magnetic field, measured in tesla (T), representing the concentration of magnetic field lines.
Electromagnet — a coil of wire that produces a magnetic field when an electric current flows through it; the magnetic field can be switched on and off.
Motor effect — the force experienced by a current-carrying conductor placed in a magnetic field.
Electromagnetic induction — the process of generating a potential difference (voltage) across a conductor when it experiences a changing magnetic field.
Generator — a device that converts kinetic energy into electrical energy using electromagnetic induction.
Transformer — a device that changes the voltage of an alternating current using two coils wrapped around an iron core.
Solenoid — a cylindrical coil of wire that produces a uniform magnetic field inside when carrying an electric current.
Core concepts
Permanent magnets and magnetic materials
All magnets have two poles: a north pole and a south pole. Like poles repel each other, while unlike poles attract. This fundamental behaviour applies to all magnets, from small bar magnets to the Earth itself.
Magnetic field lines provide a visual representation of magnetic fields:
- They always point from the north pole to the south pole
- The closer together the lines, the stronger the magnetic field
- Field lines never cross each other
- The direction of the field at any point is shown by the tangent to the field line
Magnetic materials include iron, steel, cobalt and nickel. These materials can be magnetised or are attracted to magnets. Iron is magnetically soft (easily magnetised and demagnetised), making it ideal for temporary magnets and transformer cores. Steel is magnetically hard (difficult to magnetise but retains magnetism), making it suitable for permanent magnets.
The Earth generates its own magnetic field, approximately like a bar magnet tilted relative to the Earth's axis. The geographic North Pole is near the magnetic south pole, which is why the north pole of a compass needle points toward geographic north.
Electromagnetism and solenoids
When an electric current flows through a wire, it produces a magnetic field around the wire. The magnetic field forms concentric circles centred on the wire. The right-hand grip rule determines the field direction: if your right thumb points in the direction of conventional current flow, your fingers curl in the direction of the magnetic field.
The strength of this magnetic field increases when:
- The current is larger
- The distance from the wire is smaller
A solenoid is formed by winding wire into a cylindrical coil. When current flows through a solenoid, it produces a magnetic field pattern similar to a bar magnet, with a north pole at one end and a south pole at the other. Inside the solenoid, the field is strong and uniform.
The magnetic field strength of a solenoid increases when:
- The current increases
- The number of turns per unit length increases
- An iron core is placed inside the coil
Adding a soft iron core creates an electromagnet. The iron becomes magnetised when current flows, significantly increasing the magnetic field strength. When the current is switched off, the iron loses most of its magnetism. Electromagnets are used in scrapyard cranes, electric bells, relays and circuit breakers.
The motor effect and electric motors
When a current-carrying conductor is placed in a magnetic field, it experiences a force. This is the motor effect. The force is greatest when the conductor is perpendicular to the magnetic field and zero when parallel.
Fleming's left-hand rule determines the direction of this force:
- First finger points in the direction of the magnetic Field (north to south)
- SeCond finger points in the direction of the Current (positive to negative)
- Thumb points in the direction of the force (motion)
The size of the force depends on:
- Magnetic flux density (strength of the magnetic field)
- Current through the conductor
- Length of conductor in the magnetic field
The force can be calculated using: F = BIL
Where:
- F = force in newtons (N)
- B = magnetic flux density in tesla (T)
- I = current in amperes (A)
- L = length of conductor in field in metres (m)
A simple d.c. motor consists of a rectangular coil of wire positioned in a magnetic field between two poles of a magnet. When current flows through the coil, forces act on the sides of the coil (using the motor effect), causing it to rotate. A split-ring commutator reverses the current direction every half turn, ensuring continuous rotation in the same direction.
Applications of the motor effect include:
- Electric motors in appliances, vehicles and industrial machinery
- Loudspeakers (where varying current creates varying forces, producing sound)
- Moving-coil ammeters and voltmeters
Electromagnetic induction
Electromagnetic induction occurs when a conductor experiences a changing magnetic field, inducing a potential difference (voltage) across it. If the conductor forms part of a complete circuit, an induced current flows.
The induced p.d. can be increased by:
- Moving the wire or magnet faster
- Using a stronger magnet
- Increasing the number of turns of wire (using a coil)
- Placing an iron core inside the coil
The direction of the induced current can be determined using Fleming's right-hand rule:
- Thumb points in the direction of motion
- First finger points in the direction of the magnetic Field
- SeCond finger points in the direction of the induced Current
Alternatively, Lenz's law states that the direction of the induced current opposes the change that caused it. This ensures energy conservation.
Generators convert kinetic energy into electrical energy using electromagnetic induction. In a simple a.c. generator, a coil rotates in a magnetic field, continuously cutting through field lines. This induces an alternating potential difference, producing alternating current. Slip rings maintain continuous electrical contact with the rotating coil.
The induced p.d. varies as the coil rotates:
- Maximum when the coil sides move perpendicular to the field lines
- Zero when the coil sides move parallel to the field lines
Power stations use massive generators driven by turbines (powered by steam, water or wind) to generate electricity for the National Grid.
Transformers
A transformer consists of two coils (the primary and secondary) wound around an iron core. It changes the voltage of an alternating current. Transformers only work with alternating current because they rely on a changing magnetic field.
Operation:
- Alternating current in the primary coil creates a changing magnetic field
- The soft iron core links the magnetic field to the secondary coil
- The changing magnetic field induces an alternating p.d. in the secondary coil
The ratio of voltages is related to the ratio of turns:
Vₚ/Vₛ = nₚ/nₛ
Where:
- Vₚ = primary voltage (V)
- Vₛ = secondary voltage (V)
- nₚ = number of turns on primary coil
- nₛ = number of turns on secondary coil
A step-up transformer increases voltage (nₛ > nₚ), while a step-down transformer decreases voltage (nₛ < nₚ).
For an ideal (100% efficient) transformer:
VₚIₚ = VₛIₛ
This represents power conservation: power input equals power output.
Real transformers are not 100% efficient due to:
- Resistance in the coils causing heating
- Eddy currents in the iron core causing heating
- Energy used to repeatedly magnetise and demagnetise the core
Typical transformer efficiency is 95-99%.
Transformers are essential in the National Grid:
- Step-up transformers increase voltage to 400,000 V for transmission, reducing current and minimising energy losses in cables
- Step-down transformers reduce voltage to 230 V for safe domestic use
The power loss in transmission cables is calculated using: P = I²R
Higher voltages mean lower currents for the same power, significantly reducing energy waste.
Worked examples
Example 1: Motor effect force calculation
Question: A wire of length 8.0 cm carrying a current of 4.5 A is placed perpendicular to a magnetic field of flux density 0.25 T. Calculate the force acting on the wire. [3 marks]
Solution:
Step 1: Convert length to metres L = 8.0 cm = 0.080 m [1]
Step 2: Write the equation F = BIL [1]
Step 3: Substitute and calculate F = 0.25 × 4.5 × 0.080 F = 0.090 N (or 9.0 × 10⁻² N) [1]
Example 2: Transformer voltage calculation
Question: A step-down transformer has 2400 turns on the primary coil and 120 turns on the secondary coil. The primary voltage is 240 V. Calculate the secondary voltage. [3 marks]
Solution:
Step 1: Write the transformer equation Vₚ/Vₛ = nₚ/nₛ [1]
Step 2: Rearrange for Vₛ Vₛ = Vₚ × (nₛ/nₚ) [1]
Step 3: Substitute and calculate Vₛ = 240 × (120/2400) Vₛ = 240 × 0.05 Vₛ = 12 V [1]
Example 3: Transformer efficiency and current
Question: A transformer with 90% efficiency has an input power of 2000 W and an output voltage of 12 V. Calculate the output current. [4 marks]
Solution:
Step 1: Calculate output power Efficiency = (output power/input power) × 100% 90 = (output power/2000) × 100 Output power = (90 × 2000)/100 = 1800 W [1]
Step 2: Write the power equation P = VI [1]
Step 3: Rearrange for current I = P/V [1]
Step 4: Calculate output current I = 1800/12 = 150 A [1]
Common mistakes and how to avoid them
Confusing Fleming's left-hand and right-hand rules. Remember: Left for motors (force), Right for generators (current). The left-hand rule applies when current causes motion (motor effect); the right-hand rule applies when motion causes current (electromagnetic induction).
Forgetting to convert units. Always convert length to metres (cm ÷ 100), especially in F = BIL calculations. Similarly, ensure current is in amperes and magnetic flux density in tesla.
Assuming transformers work with direct current. Transformers only function with alternating current because they require a changing magnetic field to induce voltage in the secondary coil. D.C. produces a constant field after initial switch-on.
Mixing up step-up and step-down transformers. Check the number of turns: if the secondary has more turns than the primary, voltage increases (step-up). Conversely, fewer turns mean voltage decreases (step-down).
Using the wrong Fleming's rule fingers. First finger = Field direction (not force). Take time to orient your hand correctly before determining the answer, ensuring your thumb is perpendicular to both fingers.
Forgetting that efficiency affects transformer power output. If efficiency is less than 100%, output power is less than input power. Calculate output power first, then use it to find current or other quantities.
Exam technique for "Magnetism and Electromagnetism"
Command word recognition: "Describe" requires stating what happens without explanation; "Explain" demands reasoning using physics principles. For Fleming's rules, "state the direction" means identify the direction clearly (e.g., "upwards" or "to the left"), often worth 1 mark.
Show your working in calculations. Write the formula, substitute values with units, then calculate. This earns method marks even if your final answer is incorrect. In 3-mark calculations, typically 1 mark is for the formula, 1 for substitution and 1 for the correct answer.
Sketch field lines carefully. Use a ruler for straight lines where appropriate, include arrows showing direction, and ensure lines don't cross. Field patterns around bar magnets and solenoids are frequently examined — practice drawing these.
Link physics to applications. Questions often ask about real devices (motors, generators, transformers in the National Grid). Explain how the physics principle causes the observed effect, using precise terminology rather than vague descriptions.
Quick revision summary
Magnets produce fields with north and south poles; like poles repel, unlike poles attract. Current-carrying wires generate circular magnetic fields; solenoids produce fields similar to bar magnets. The motor effect (F = BIL) creates forces on current-carrying conductors in magnetic fields, enabling motors to work. Electromagnetic induction generates voltage when conductors experience changing magnetic fields, enabling generators to produce electricity. Transformers use electromagnetic induction to change a.c. voltage according to the turns ratio; they're essential for efficient electrical power transmission in the National Grid.