Electricity — current, voltage, resistance and circuits

Basic electrical quantities

Current (I): the rate of flow of electric charge. Measured in amperes A. Charge flow = current × time: Q = I × t.

Potential difference (p.d.) / Voltage (V): the energy transferred per unit charge. Measured in volts (V). V = E/Q (energy per unit charge).

Resistance (R): how much a component opposes the flow of current. Measured in ohms (Ω).

Ohm's Law

V = I × R (voltage = current × resistance)

Where V is in volts, I is in amperes, A is in ohms.

A component that obeys Ohm's Law at constant temperature is called an ohmic conductor — its I-V graph is a straight line through the origin.

Rearranged: I = V/R; R = V/I.

Example: A bulb with resistance 24 Ω carries a current of 0.5 A. V = 0.5 × 24 = 12 V.

Series circuits

• Components connected in a single loop.
• Current is the same everywhere: I_total = I₁ = I₂ = I₃.
• Voltages add up: V_total = V₁ + V₂ + V₃.
• Resistance adds up: R_total = R₁ + R₂ + R₃.
• If one component fails (open circuit), the whole circuit stops.

Parallel circuits

• Components connected in separate branches.
• Current splits at junctions: I_total = I₁ + I₂ + I₃.
• Voltage is the same across each branch: V_total = V₁ = V₂ = V₃.
• Total resistance is LESS than any individual resistance: 1/R_total = 1/R₁ + 1/R₂ + 1/R₃.
• If one branch fails, the others continue to work.

Non-ohmic components

Filament bulb: resistance increases as temperature increases (the hotter the filament, the more resistance). I-V graph is a curve (not straight).

Diode: only allows current to flow in one direction. Very high resistance in reverse direction.

LDR (light-dependent resistor): resistance decreases as light intensity increases.

Thermistor: resistance decreases as temperature increases. Used in temperature sensors.

Power in circuits

P = I × V (power = current × voltage) Also: P = I² × R; P = V²/R.

Electrical energy: E = P × t = I × V × t.

Units: power in watts (W); energy in joules (J).