Electricity

Electricity is central to modern physics and technology. At A-Level, you need a thorough understanding of circuits, charge flow, and the relationship between current, voltage, and resistance. This topic covers charge carriers, drift velocity, resistance, circuit analysis using Kirchhoff's laws, and sensor circuits.

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Charge, Current, and Potential Difference

Electric Charge

Key Definition: Electric charge (Q) is a fundamental property of matter, measured in coulombs (C). The fundamental unit of charge is the charge on an electron: e = 1.60 × 10⁻¹⁹ C.

Charge is quantised — it always comes in integer multiples of e.

Current

Current (I) is the rate of flow of charge:

I = ΔQ/Δt

Current is measured in amperes (A). In a metal conductor, current is carried by free (delocalised) electrons, which drift from the negative terminal to the positive terminal. Conventional current flows in the opposite direction — from positive to negative.

Potential Difference

The potential difference (p.d.) between two points is the energy transferred per unit charge:

V = W/Q

It is measured in volts (V). One volt means one joule of energy per coulomb of charge.

Electrical Power and Energy