Electricity
Charge and current
Electric charge is measured in coulombs C. Electrons carry a charge of −1.6 × 10⁻¹⁹ C.
Electric current (I) is the rate of flow of charge: I = Q / t (A = C/s). In metals, current is the flow of electrons. Conventional current flows from + to − (opposite to electron flow).
Voltage (potential difference)
Potential difference (p.d.) or voltage (V) is the energy transferred per unit charge: V = W / Q (volts = J/C).
A 12 V battery transfers 12 J of energy for every coulomb of charge that passes through it.
Electromotive force (e.m.f.) is the energy supplied per unit charge by a source (cell, battery, generator).
Resistance
Ohm's law: V = IR (for ohmic conductors at constant temperature). Resistance = voltage / current: R = V / I (Ω).
Ohmic conductors (e.g. metal wire at constant temperature): I ∝ V → straight line through origin on IV graph. Non-ohmic devices:
- Filament bulb: resistance increases with temperature → IV graph curves (gradient decreases at high I).
- Diode: only allows current in one direction → IV graph shows near-zero current for reverse voltage and rapid rise for forward voltage above ~0.7 V.
- Thermistor: resistance decreases as temperature increases (negative temperature coefficient, NTC). Used in temperature sensors and thermostats.
- LDR (light-dependent resistor): resistance decreases as light intensity increases. Used in automatic lighting.
Series circuits
- Same current through every component.
- Total voltage = sum of individual voltages: V_total = V₁ + V₂ + …
- Total resistance: R_total = R₁ + R₂ + … (increases with more resistors).
Parallel circuits
- Same voltage across each branch.
- Total current = sum of branch currents: I_total = I₁ + I₂ + …
- Total resistance is less than the smallest individual resistance: 1/R_total = 1/R₁ + 1/R₂ + …
- Adding more branches in parallel decreases overall resistance and increases total current from the supply.
Electrical power
P = IV = I²R = V²/R (W = J/s)
Energy: E = Pt = IVt (J)
For electricity bills, energy is measured in kilowatt-hours: 1 kWh = 1000 W × 3600 s = 3.6 × 10⁶ J.
Safety and fuses
A fuse contains a thin wire that melts if current exceeds a rated value, breaking the circuit. Choose the fuse with the lowest rating that exceeds normal operating current. Circuit breakers (RCCBs) and earth wires protect against electric shock and fire.
⚠Common mistakes
- Confusing voltage and current — voltage is the "push" (energy per charge); current is the "flow" (charge per second).
- Adding resistances in parallel directly — use the reciprocal formula.
- Forgetting to convert kW to W and hours to seconds in energy calculations.
- IV graph for filament bulb — the curve is because resistance increases with temperature, not because it breaks Ohm's law in principle.
AI-generated · claude-opus-4-7 · v3-ccea-physics