Physics electricity

Electric charge (Q)

The property of matter that causes electrical effects; measured in coulombs (C).

Elementary charge

The charge of a proton/electron: 1.6 × 10⁻¹⁹ C.

Conventional current

Flow of positive charge from positive to negative.

Electron flow

The physical movement of electrons from negative to positive.

Electric current (I)

The rate of flow of charge: I = Q / t.

Drift velocity (vₑ)

Average velocity of charge carriers through a conductor.

Current equation with drift velocity

I = n A vₑ q (number density × area × drift velocity × charge).

Potential difference (V)

Energy transferred per unit charge between two points: V = W / Q.

Electromotive force (emf)

Energy transferred to charge by a power source per unit charge.

Work done on charge

W = VQ.

Volt

The potential difference when 1 joule of work is done per coulomb of charge.

Electrical resistance (R)

Opposition to current flow: R = V / I.

Ohm

The resistance when 1 V creates a current of 1 A.

Ohmic conductor

A component where current is directly proportional to voltage (constant resistance).

Non-ohmic conductor

Component where resistance changes (e.g., filament lamp, diode).

Resistivity (ρ)

A material property defining resistance: ρ = RA / L.

Resistance of a wire

R ∝ L and R ∝ 1/A.

Temperature effect on resistance

In metals, resistance increases with temperature due to lattice vibrations.

Ohmic conductor IV graph

Straight line through origin (constant resistance).

Filament lamp IV graph

Curved; resistance increases as temperature increases.

Diode IV graph

No current until threshold voltage; then rapid increase in forward direction.

Thermistor

Resistance decreases as temperature increases (NTC type).

LDR

Resistance decreases as light intensity increases.

Electrical power (P)

P = VI.

Power formula (current form)

P = I²R.

Power formula (voltage form)

P = V² / R.

Electrical energy (E)

E = VIt.

Kilowatt-hour (kWh)

Unit of electrical energy; 1 kWh = 3.6 × 10⁶ J.

Current in series

Same at every point in the circuit.

Voltage in series

Shared between components; sums to supply voltage.

Resistance in series

R_total = R₁ + R₂ + …

Current in parallel

Splits between branches depending on resistance.

Voltage in parallel

Same across each branch.

Resistance in parallel

1/R_total = 1/R₁ + 1/R₂ + …

Kirchhoff’s First Law

Total current into a junction = total current out.

Kirchhoff’s Second Law

Sum of EMFs = sum of potential drops in a loop.

Terminal p.d.

The voltage across a cell’s terminals: V = ε − Ir.

EMF (ε)

Energy supplied per coulomb by the source.

Internal resistance (r)

Resistance inside the power source.

Lost volts

Energy wasted inside the cell: lost volts = Ir.

Maximum power condition

Power output is maximum when external resistance R = internal resistance r.

Number density (n)

Number of free charge carriers per unit volume.

Metals conduction

Conduction due to free (delocalised) electrons.

Semiconductors

Have fewer charge carriers; conductivity increases with temperature.

Insulators

Very few charge carriers → very low conductivity.

Superconductor

A material with zero resistance below a critical temperature.

Critical temperature (T_c)

The temperature below which a material becomes superconducting.

Benefits of superconductors

Used in MRI, particle accelerators, efficient power transmission.

Meissner effect

A superconductor expels magnetic fields when cooled below T_c.

Potential divider

A circuit using resistors to produce a specific output voltage.

Potential divider formula

Vout = (R₂ / (R₁ + R₂)) × Vin.

LDR in potential divider

Produces a voltage that decreases as light increases.

Thermistor in potential divider

Produces a voltage that changes with temperature.

Ammeter

Measures current; connected in series; ideally zero resistance.

Voltmeter

Measures potential difference; connected in parallel; ideally infinite resistance.

Ohmmeter

Measures resistance by applying its own test voltage.