GCSE / Physics
Electricity
Revise GCSE Physics Electricity, including circuit symbols, electric current, charge, potential difference, resistance, Ohm’s law, I–V characteristics, thermistors, LDRs, series and parallel circuits, mains electricity, electrical power, energy transfer, the National Grid, static electricity and electric fields.
GCSE Physics Electricity begins with standard electrical symbols and circuit diagrams, and students should recognise and correctly use symbols for cells, batteries, switches, lamps, resistors, variable resistors, diodes, LEDs, thermistors, LDRs, ammeters, voltmeters and fuses when drawing or interpreting circuits.
Electric current is the rate of flow of electrical charge through a circuit and is measured in amperes, A. Charge flow, current and time are linked by , where is charge in coulombs, is current in amperes and is time in seconds.
A current can only flow continuously when there is a closed circuit and a source of potential difference. In a single closed loop, the current has the same value at every point because charge is conserved as it moves around the circuit.
Potential difference is the energy transferred per unit charge between two points and is measured in volts, V. The relationship between potential difference, current and resistance is given by , where resistance is measured in ohms, .
For an ohmic conductor at constant temperature, current is directly proportional to potential difference, so the resistance remains constant. This gives a straight-line I–V characteristic through the origin, which is a key idea in Ohm's law and resistance questions.
The resistance of a component can change depending on its behaviour. A filament lamp becomes hotter as current increases, so its resistance rises, while a diode allows current to pass mainly in one direction and has a very high resistance in the reverse direction.
Thermistors and light dependent resistors are important variable-resistance components. The resistance of a thermistor decreases as temperature increases, while the resistance of an LDR decreases as light intensity increases, making them useful in thermostats, temperature sensors and automatic lighting circuits.
In series circuits, the same current flows through every component, the total potential difference is shared between components and the total resistance is found using . Adding resistors in series therefore increases the total resistance.
In parallel circuits, the potential difference across each branch is the same, while the total current is the sum of the currents in the separate branches. Adding extra parallel branches reduces the total resistance because charge has more paths through which it can flow.
Direct current, DC, flows in one direction only and is supplied by sources such as cells and batteries, while alternating current, AC, repeatedly changes direction. In the UK, mains electricity is approximately AC at a frequency of .
UK mains electricity uses a three-core cable with a brown live wire, blue neutral wire and green-and-yellow earth wire. The live wire carries the alternating potential difference, the neutral wire completes the circuit and the earth wire provides a safety path if a fault makes a metal case live.
Electrical safety depends on correct wiring and protective devices because the live wire can be dangerous even when a switch is open. Fuses, earthing and insulation help reduce the risk of electric shock, overheating and electrical faults in domestic appliances.
Electrical power is the rate at which energy is transferred in a circuit and can be calculated using or . Power is measured in watts, W, and a higher-power device transfers more energy each second.
The energy transferred by an electrical appliance can be calculated using or . Electrical appliances transfer energy into different stores, such as thermal energy in a kettle, kinetic energy in a motor or light energy in a lamp.
The National Grid transfers electrical power from power stations to consumers using cables and transformers. Step-up transformers increase potential difference for transmission so current is lower and less energy is wasted as heating, while step-down transformers reduce the potential difference before electricity reaches homes.
Static electricity is produced when electrons are transferred between insulating materials, often by rubbing. The object gaining electrons becomes negatively charged and the object losing electrons becomes positively charged, so like charges repel and opposite charges attract.
A charged object creates an electric field around itself, and another charged object placed in that field experiences a non-contact force. The electric field is strongest close to the charged object and becomes weaker as distance increases, helping explain attraction, repulsion and electrostatic sparking.