Electricity and Circuits

Current, Charge and Potential Difference

Electric current (I): the rate of flow of charge. I = Q/t (amperes, A; charge in coulombs, C; time in seconds).

Conventional current flows from positive to negative terminal (opposite to electron flow).

Potential difference (p.d. / voltage, V): the energy transferred per unit charge between two points. V = W/Q (volts, V; energy in joules).

Resistance (R): opposition to the flow of current. R = V/I (ohms, Ω).

Ohm's Law

Ohm's Law: current through a conductor is directly proportional to the potential difference across it, provided temperature is constant.

V = IR (voltage = current × resistance)

Ohmic conductors: resistors at constant temperature — straight line through origin on an I–V graph. Non-ohmic conductors: filament bulb (resistance increases with temperature), diode (only allows current one way).

Series and Parallel Circuits

Series circuit:

• Same current throughout: I_total = I₁ = I₂ = …
• Potential differences add: V_total = V₁ + V₂ + …
• Resistances add: R_total = R₁ + R₂ + …
• If one component fails, all fail.

Parallel circuit:

• Voltage same across each branch: V_total = V₁ = V₂ = …
• Currents add: I_total = I₁ + I₂ + …
• Total resistance is less than any individual resistance: 1/R_total = 1/R₁ + 1/R₂ + …
• If one branch fails, others continue.

Domestic circuits use parallel — devices operate at the same voltage and can be switched independently.

IV Characteristics

Resistor: straight line through origin → V ∝ I (Ohm's Law obeyed). Resistance = 1/gradient.

Filament bulb: curve — gradient decreases as temperature rises (resistance increases with temperature).

Diode: almost no current in reverse bias; current only flows above a threshold forward voltage (~0.7 V for silicon).

Core Practical 6 — Investigating IV characteristics

Equipment: power supply, variable resistor (rheostat), ammeter (in series), voltmeter (in parallel), connecting leads, component under test (resistor / bulb / diode).

Method:

1. Build the circuit with the ammeter in series with the component and voltmeter in parallel.
2. Vary the p.d. using the variable resistor / power supply.
3. Record I and V. Repeat reversing the connections for a diode.
4. Plot I (y-axis) vs V (x-axis). Identify the type of component from graph shape.

Edexcel tip: For a diode IV, the graph shows no current in reverse bias and rapid current rise in forward bias above ~0.7 V.

Electrical Power

P = IV = I²R = V²/R (power in watts, W)

Energy transferred: E = Pt = IVt (joules, J)

Paying for electricity: energy companies use kilowatt-hours (kWh). 1 kWh = 3,600,000 J. Cost = power (kW) × time (hours) × unit cost (p/kWh).