How voltage pushes current through resistance in a circuit
An electric current is a flow of charge, measured in amperes with an ammeter connected in series. Charge flows only when there is a potential difference between two points, just as water flows from a higher level to a lower one. Potential difference is the work done per unit charge, V = W/Q, measured in volts with a voltmeter connected in parallel.
Ohm’s law states that, at constant temperature, the current through a metallic conductor is proportional to the potential difference across it, so V = IR. Resistance increases with the length of a wire and decreases with its thickness. Resistors in series add directly, R = R₁ + R₂ + R₃, while in parallel their reciprocals add, so the combined resistance is smaller than the smallest one. Electrical power is P = VI, and the energy used is often measured in kilowatt-hours, the unit on household bills. For example, a 2 kW heater running for 3 hours uses 6 kWh.
Why it matters
Understanding current, voltage and resistance explains why household appliances are connected in parallel, why fuses melt when too much current flows, and how to calculate the cost of running a fan or geyser. The heating effect of current is used in irons, toasters and filament bulbs. This topic prepares you for magnetic effects of current and for circuit analysis in higher classes.
Key terms
- Potential difference
- The work done in moving one coulomb of charge between two points in a circuit.
- Ohm’s law
- At constant temperature, the current through a conductor is directly proportional to the voltage across it.
- Resistance
- The opposition a conductor offers to the flow of current, measured in ohms.
- Kilowatt-hour
- The energy used by a one-kilowatt appliance running for one hour, equal to 3.6 million joules.