Electricity

Electric current is the rate of flow of electric charge through a conductor. If charge Q flows in time t, the current is I = Q/t. A closed loop that lets charge flow is a circuit; if it breaks anywhere, the current stops.

By convention, current flows from the + terminal to the − terminal (opposite to the actual electron flow).

Charges flow only when there is a potential difference (V) across the conductor — supplied by a cell or battery. It is the work done to move a unit charge between two points: V = W/Q.

A voltmeter measures potential difference and is always connected in parallel across the component; an ammeter measures current and is connected in series.

At constant temperature, the current through a conductor is directly proportional to the potential difference across it: V ∝ I, so V = IR. The constant R is the resistance. A graph of V against I is a straight line through the origin, and its slope is R.

The resistance of a wire depends on four things:

• Length (L): longer wire → more resistance (R ∝ L).
• Area (A): thicker wire → less resistance (R ∝ 1/A).
• Material: captured by the resistivity ρ.
• Temperature: resistance of metals rises as they get hotter.

Series: components share one path. The same current flows through each, and resistances add: R = R₁ + R₂ + R₃. One break stops everything.

Parallel: components sit on separate branches with the same voltage across each, and 1/R = 1/R₁ + 1/R₂ + … The total resistance is less than the smallest branch. Homes use parallel so every appliance gets the full 220 V and can switch independently.

When current flows through a resistor, electrical energy turns into heat — the heating effect. Joule's law: H = I²Rt. This runs heaters, irons, and the filament of a bulb.

Electric power is the rate of using energy: P = VI = I²R = V²/R. Energy used = power × time. The bill is charged in kilowatt-hours (kWh) — "1 unit".