An electrical circuit is an interconnection of electrical elements that provides one or more closed paths for current. It normally contains a source of electromotive force, conductors, a load and, optionally, control and protection devices.
Learning objectives
- Identify the source, load, conductors, control and protection in a circuit
- Distinguish series and parallel connections
- Apply Ohm law and Kirchhoff laws to simple networks
- Calculate power and energy in a circuit
- Recognise the difference between open-circuit and short-circuit faults
Progressive explanation
Start with the plain-language idea, then move into the engineering detail. Each level stands on its own, so you can stop at the depth you need.
Beginner The core idea
Circuits are drawn as diagrams in which lines represent wires and symbols represent components. The source provides the energy, the load converts it into light, heat or motion, and the conductors carry it between them.
Intermediate How it behaves in a circuit
Kirchhoff laws generalise this: the algebraic sum of currents at a node is zero, and the algebraic sum of voltages around any closed loop is zero. Ohm law and these laws solve any linear resistive network.
Advanced Limits, assumptions, engineering detail
Formulas and variables
Units of measurement
| Quantity | Symbol | Unit |
|---|---|---|
| Voltage | V | volt (V) |
| Current | I | ampere (A) |
| Resistance | R | ohm (Ω) |
| Power | P | watt (W) |
Worked examples
Current and power in a simple circuit
Problem. A 12 V battery is connected across a 3 Ω load. Find the current and the power.
Solution. I = V/R = 12/3 = 4 A. P = V × I = 12 × 4 = 48 W.
Parallel branches
Problem. A 12 V source supplies a 6 Ω resistor and a 3 Ω resistor in parallel. Find the equivalent resistance, the total current and each branch current.
Solution. Rp = (6 × 3)/(6 + 3) = 18/9 = 2 Ω. Itotal = 12/2 = 6 A. I6 = 12/6 = 2 A and I3 = 12/3 = 4 A.
Energy used
Problem. The 48 W load runs for 2 hours. How much energy does it use?
Solution. E = P × t = 48 W × 2 h.
Practical applications
- Domestic and industrial power distribution
- Vehicle wiring and lighting
- Printed circuit board design
- Motor control and automation
- Protection and switching
- Battery-powered devices
Automotive applications
A vehicle circuit runs from the battery through fuses, relays, switches and loads, and returns through the chassis or a dedicated ground wire. Poor ground connections and high-resistance joints cause dim lights, slow motors and intermittent faults.
Industrial applications
Industrial circuits include motor starters, contactors, protection relays, control transformers and PLC input and output circuits. Separation between power and control wiring, and correct protective earthing, are essential for safety and noise immunity.
Measurement procedure
- De-energise the circuit and verify zero voltage before checking continuity or resistance.
- Measure voltage in parallel with the component and current in series with the load.
- Check the return path and ground connections when a load behaves erratically.
- Compare measured values with the diagram and the expected calculation.
Common mistakes
- Connecting an ammeter in parallel with a voltage source; it has very low resistance and will draw a large current.
- Assuming the chassis is a perfect conductor regardless of corroded joints.
- Ignoring source and wire resistance when calculating load current.
- Confusing an open circuit with a short circuit.
Safety considerations
- Isolate the circuit and verify dead before working on it.
- Use fuses and circuit breakers rated for the conductor and load.
- Never bypass a protective device; a short circuit can cause fire or explosion.
- Treat automotive batteries with care; accidental short circuits produce very high currents.