Electricity is the set of physical phenomena associated with the presence, accumulation and motion of electric charge. In a circuit, a source of electromotive force sets charge in motion, and that motion transfers energy to loads such as lamps, motors and electronic devices.
Learning objectives
- Define electricity in terms of electric charge and its motion.
- Describe the parts of a complete circuit and the role of the source and the load.
- Calculate charge from current and time with Q = I × t.
- Calculate energy transferred from charge and voltage with W = Q × V.
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
A simple circuit needs three things: a source that pushes charge (a battery or generator), a conductor that gives the charge a path, and a load that uses the energy. The source does not create charge; it moves charge that is already in the conductor. The battery or generator supplies energy, and the load converts that energy into light, heat, motion or computation.
Intermediate How it behaves in a circuit
We normally draw conventional current as flowing from the positive terminal through the external circuit to the negative terminal. Electron flow is opposite to that convention. Both descriptions give the same circuit results as long as the convention is used consistently.
Advanced Limits, assumptions, engineering detail
In practice, real sources have internal resistance and real conductors have resistance, so a circuit is a network of energy conversions and losses. Separating the ideal behaviour (what the source tries to do) from the non-ideal losses (what the wiring and internal resistance actually do) is the core engineering skill.
Formulas and variables
Units of measurement
| Quantity | Symbol | Unit |
|---|---|---|
| Electric charge | Q | coulomb (C) |
| Electric current | I | ampere (A) |
| Potential difference | V | volt (V) |
| Energy | W | joule (J) |
Worked examples
Charge moved by a lamp current
Problem. A lamp draws 2.5 A for 40 s. How much charge passes through it?
Solution. Q = I × t = 2.5 A × 40 s = 100 C. Dividing by the elementary charge gives N = 100 / 1.602 × 10⁻¹⁹ ≈ 6.24 × 10²⁰ electrons.
Energy delivered to a load
Problem. How much energy is transferred when 5 C of charge moves through a potential difference of 12 V?
Solution. W = Q × V = 5 C × 12 V = 60 J.
Practical applications
- Batteries and charging systems
- Vehicle wiring, lighting and electronics
- Industrial motors and control panels
- Energy metering and electricity billing
Automotive applications
In a vehicle, a 12 V battery supplies charge to the starter, lighting, sensors and control modules. The battery stores chemical energy and releases it as electrical energy, while the alternator replenishes the charge. A small parasitic drain of a few hundred milliamperes can flatten a battery over days, whereas a starter can draw hundreds of amperes for a few seconds.
Industrial applications
Industrial installations move the same charge at a much larger scale. A three-phase motor may draw hundreds of amperes continuously, and switchgear, busbars and cables are sized to carry that current while limiting heating and voltage drop. Protective devices interrupt current when a fault creates an unintended path.
Measurement procedure
- Measure current with an ammeter or clamp meter in series with the conductor, never in parallel with a voltage source.
- Measure potential difference with a voltmeter connected in parallel across the two points of interest.
- Check the instrument category rating and range before measuring on mains or high-energy circuits.
- A clamp meter reads the current in the one conductor it encircles; clamping the whole cable reads the net current, which is normally zero.
Common mistakes
- Assuming electrons travel at the speed of light: their drift velocity in a conductor is usually far slower, while the electric field change propagates quickly.
- Confusing conventional current with electron flow: conventional current is treated as positive charge moving from the positive terminal to the negative terminal.
- Thinking a voltage source alone is enough: current needs a complete path back to the source.
- Treating static electricity as unrelated to current: both are described by electric charge, electric fields and energy.
Safety considerations
- Treat every unknown conductor as live until it has been proved de-energised, tested and locked out.
- Low-voltage batteries can still deliver very high fault currents that overheat tools, wiring and metal jewellery.
- Capacitors and battery packs can retain dangerous charge after the supply is switched off.