Electricity and circuits

Charge is a property of matter, carried by electrons and protons, measured in coulombs. Current is the rate of flow of charge, measured in amperes. Voltage, or potential difference, is the energy transferred per unit charge, measured in volts. Resistance opposes current and is measured in ohms.

Ohm's law ties three together: voltage equals current times resistance. It holds for components whose resistance is constant, which is why a filament lamp, whose resistance rises as it heats, does not produce a straight-line graph.

• In series there is one path: current is the same everywhere, voltages add, and total resistance is the sum.
• In parallel there are multiple paths: voltage is the same across each branch, currents add, and total resistance is less than the smallest branch.
• Adding a parallel branch always lowers total resistance, because it opens another route for charge.
• Household wiring is parallel so that each appliance gets full mains voltage and can be switched independently.

The water analogy helps: voltage is like pressure, current like flow rate, resistance like pipe narrowness. It correctly predicts that increasing pressure increases flow and narrowing the pipe reduces it.

It also misleads in two ways. Charge is not consumed by a component, so the same current leaves a bulb as enters it; what the bulb consumes is energy, which is voltage. And electrons drift remarkably slowly, on the order of millimetres per second, while the electric field that sets them moving propagates near light speed. That is why a light comes on instantly despite slow-moving electrons.

Electrical power is voltage times current, and combining with Ohm's law gives current squared times resistance, which is the form that explains heating in wires. Because loss rises with the square of current, power is transmitted at very high voltage and low current, then stepped down by transformers near its destination.

Energy consumed is power times time. Domestic bills use kilowatt hours, which is simply a kilowatt sustained for an hour. Converting device ratings into kilowatt hours makes it easy to compare what actually dominates a bill, which is usually heating rather than electronics.

Danger comes from current through the body, not from voltage alone, but higher voltage drives more current through a given resistance. Dry skin has high resistance; wet skin has much less, which is why water and mains electricity are a serious combination.

Protection layers work in sequence. A fuse or circuit breaker cuts the supply when current exceeds a limit. Earthing gives fault current a low-resistance path away from the user. Double insulation removes the conducting path entirely. Each addresses a different failure, which is why appliances use more than one.