Electricity · revision map · A-Level 9702, Topic 9
TOPIC 9 · ELECTRICITY
CAMBRIDGE INTERNATIONAL AS & A LEVEL PHYSICS 9702 · ONE-PAGE REVISION MAP
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ELECTRICITY9
Charge & Electric Current
Current is the rate of flow of charge.
- Charge is quantised: every charge is a whole number of e = 1.60 × 10−¹⁹ C.
- Current I is the charge passing a point each second.
- I = nAvq links it to the drift of charge carriers in the wire.
I = Q / t
I = nAvq
n = carrier density, A = area, v = drift speed, q = carrier charge
Potential Difference & Power
P.D. is the energy transferred per unit charge.
- V = W / Q: work done driving charge between two points, in volts (1 V = 1 J C−¹).
- E.m.f. is the energy given per unit charge by a source.
- Power is the energy transferred each second.
V = W / Q
P = VI = I²R = V²/R
Resistance, Resistivity & I-V Characteristics
Resistance is the opposition to current; resistivity is the material's share of it.
- R = V / I, measured in ohms (Ω).
- Ohm's law: for a metal at constant temperature, I is proportional to V, so R is constant.
- Resistivity ρ is fixed for a material: R grows with length L, falls with area A.
- Thermistors (NTC) and LDRs drop R with heat / light.
R = V / I
R = ρL / A
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