Electricity and Magnetism

The One Thing

Electric circuits control the movement of charge and the transfer of electrical energy, while magnetic fields interact with currents and changing magnetic fields to produce motors, generators and transformers. Circuit behaviour can be predicted using relationships between charge, current, potential difference, resistance and power, while safety depends on limiting current and preventing contact with live conductors.

Reactions, Processes and Experiments

Key Terms

• Electric charge: A property of matter that can be positive or negative; charge is measured in coulombs (C).
• Current: The rate of flow of electric charge through a conductor, measured in amperes (A).
• Potential difference: The energy transferred per unit charge between two points in a circuit, measured in volts (V).
• Resistance: The opposition a component or material provides to current, measured in ohms (Ω).
• Ohm’s law: For a conductor at constant temperature, current is directly proportional to potential difference, so V = IR.
• Conventional current: The assumed direction of current from the positive terminal to the negative terminal of a source in the external circuit.
• Series circuit: A circuit with one continuous path; the current is the same everywhere, while potential difference is shared between components.
• Parallel circuit: A circuit with more than one path; potential difference is the same across each branch, while current divides between branches.
• Electrical power: The rate at which electrical energy is transferred, measured in watts (W).
• Electrical energy: Energy transferred by an electrical device, often measured in joules (J) or kilowatt-hours (kWh).
• Magnetic field: A region where a magnetic material, magnetic pole, or moving charge experiences a magnetic force.
• Magnetic field lines: Imaginary lines showing the direction and relative strength of a magnetic field; they are closer together where the field is stronger.
• Electromagnet: A temporary magnet produced by passing current through a coil, usually around an iron core.
• Motor effect: The force experienced by a current-carrying conductor placed in a magnetic field.
• Electromagnetic induction: The production of a potential difference when a conductor cuts magnetic field lines or experiences a changing magnetic field.
• Generator: A device that uses electromagnetic induction to convert kinetic or mechanical energy into electrical energy.
• Transformer: A device that changes the size of an alternating potential difference using electromagnetic induction between coils.
• Fuse: A safety device containing a thin wire that melts and breaks the circuit if the current becomes too large.
• Circuit breaker: An automatic switch that opens a circuit when excessive current flows and can usually be reset.
• Earth wire: A protective wire that provides a low-resistance path for fault current, reducing the risk of electric shock.
• Double insulation: A safety design using insulating material and no accessible metal parts connected to earth.

Easily Confused

• Current and potential difference: Current is the rate of flow of charge; potential difference is the energy transferred per unit charge.
• Series and parallel circuits: Series circuits have one current path and shared potential difference; parallel circuits have separate branches, equal branch potential differences and divided current.
• Electrical energy and electrical power: Energy is the total amount transferred; power is the rate of transfer.
• Magnetic field and magnetic field lines: A magnetic field is the region in which magnetic forces act; field lines are a representation of its direction and relative strength.
• Motor effect and electromagnetic induction: The motor effect is a force on a current-carrying conductor in a magnetic field; electromagnetic induction produces a potential difference when a conductor experiences a changing magnetic field.
• Fleming’s left-hand rule and Fleming’s right-hand rule: The left-hand rule gives the direction of motor-effect force; the right-hand rule gives the direction of induced current.
• Generator and transformer: A generator converts kinetic or mechanical energy into electrical energy; a transformer changes an alternating potential difference.
• Fuse and circuit breaker: A fuse melts and must be replaced; a circuit breaker opens automatically and can usually be reset.
• Earth wire and double insulation: An earth wire provides a low-resistance path for fault current; double insulation prevents accessible metal parts from being connected to earth.
• Step-up and step-down transformers: A step-up transformer has more secondary turns and increases potential difference; a step-down transformer has fewer secondary turns and decreases potential difference.

What Gets Asked

• Calculate charge using Q = It, potential difference using V = W/Q, or resistance using R = V/I; marks are lost by using incorrect units or confusing charge with current.
• Apply Ohm’s law, V = IR, for a conductor at constant temperature; the stated constant-temperature condition must not be omitted.
• Compare series and parallel circuits; marks are lost by stating that current is shared in series or that potential difference divides across parallel branches.
• Calculate electrical power or energy using P = IV, P = I²R, P = V²/R or E = Pt = VIt; marks are lost by confusing power with energy or failing to convert 1 kWh = 3.6 × 10⁶ J.
• Describe magnetic fields around bar magnets, straight current-carrying wires and solenoids; marks are lost by reversing the external bar-magnet field direction or drawing field lines that cross.
• Determine motor-effect or induced-current directions using the correct Fleming rule; marks are lost by using Fleming’s left-hand rule for induction or the right-hand rule for the motor effect.
• Explain transformers and safety devices; marks are lost by reversing the turns ratio for step-up and step-down transformers, or by failing to state that transformers require alternating current.