🧩 Electrical Symbols
Learn the common electrical symbols used in circuit diagrams — these show components such as resistors, bulbs, switches, cells, and wires.
- A cell provides energy to the circuit.
- A battery is two or more cells connected together.
- A switch opens or closes the circuit.
- A resistor limits current.
- A lamp/bulb converts electrical energy to light.
- A diode allows current to flow in one direction only.
- A thermistor changes resistance with temperature.
- A LDR (Light Dependent Resistor) changes resistance with light intensity.
🔄 Current in Circuits
Electric current is the rate of flow of charge around a circuit.
In series circuits, current is the same everywhere.
In parallel circuits, current splits between branches.
Formula:
I = Q / t
(Current = Charge / Time)
⚡ Charge in a Circuit
Charge is measured in coulombs (C) and represents how much electric charge passes through a point.
Formula:
Q = I × t
(Charge = Current × Time)
🧱 Resistance
Resistance is a measure of how much a component opposes the flow of current.
- High resistance = less current.
- Measured in ohms (Ω).
Factors affecting resistance: - Length of wire
- Thickness of wire
- Material
- Temperature
Formula:
R = V / I
(Resistance = Voltage / Current)
🐇 Voltage, Current and Resistance
V = I × R
This famous formula links voltage (V), current (I) and resistance (R).
🔋 Voltage in Circuits
In series circuits: voltage is shared between components.
In parallel circuits: voltage across each branch is the same.
🔗 Resistors in Circuits
- Series: Total resistance = R1 + R2 + R3 …
- Parallel: 1/Rtotal = 1/R1 + 1/R2 + 1/R3 …
Adding resistors in parallel reduces total resistance.
🌞 Light Dependent Resistor (LDR)
An LDR’s resistance decreases as light intensity increases.
Used in phones and streetlights to detect light levels.
❄️ Thermistor
A thermistor’s resistance decreases as temperature increases.
Used in thermostats, ovens, and car sensors.
🔁 Alternating Current
AC (Alternating Current) changes direction and voltage repeatedly.
DC (Direct Current) flows in one direction only.
The UK mains supply is:
- Frequency = 50 Hz
- Voltage = 230 V
🔌 Mains Electricity
Three-pin plug wiring:
- Brown wire: Live
- Blue wire: Neutral
- Green/Yellow wire: Earth
Fuses and circuit breakers protect against excessive current.
⚡ The National Grid
The National Grid transfers electricity from power stations to homes using transformers:
- Step-up transformers increase voltage to reduce current (less energy lost as heat).
- Step-down transformers lower voltage for safe domestic use.
💡 Energy Transfer in Appliances
Appliances transfer electrical energy to useful forms like heat, light or motion.
Energy transferred = Power × Time
E = P × t
🔋 Power in Circuits
Power is the rate at which energy is transferred.
Formulas:
P = E / t
P = I × V
P = I² × R
⚙️ The Power Trinity
Remember the three power equations:
- P = E / t
- P = I × V
- P = I² × R
Use whichever fits the data you’re given!
🧪 Required Practical: Resistance in a Wire
Aim: Investigate how the length of a wire affects its resistance.
- Set up a circuit with an ammeter and voltmeter.
- Measure voltage and current for different wire lengths.
- Calculate resistance using R = V / I.
- You should find resistance increases with wire length.
🧪 Required Practical: IV Characteristics
Aim: Explore how current changes with voltage in components like resistors, bulbs, and diodes.
- Plot graphs of current (I) against voltage (V).
- Resistor: straight line (ohmic).
- Filament bulb: curved (resistance increases when hot).
- Diode: current flows one way only.
