LucidSTEM
A-LEVEL 9702 · AS · TOPIC 9
Electricity
The chain of the topic: charge in motion is current, the work done per unit charge is potential difference and so power, and the opposition a component offers to that flow is resistance, set by its material through resistivity. Around the hexagon are the three ideas; above is what it builds on, below is where it leads.
TOPIC 9: ELECTRICITY
CAMBRIDGE A-LEVEL PHYSICS 9702 · PATHWAYS
TheLucidSTEM · thelucidstem.com
BUILDS ON
T1 Quantities: charge, SI units T5 Energy: work done, W = QV T3 Dynamics: force on carriers 9.1
9.2
9.3
TOPIC 9
ELECTRICITY
charge in motion
1 · ELECTRIC CURRENT
Current is the rate of flow of charge.
Charge is quantised in units of e = 1.60 × 10−¹⁹ C
Current I is the charge passing a point each second
I = Anvq links it to drifting carriers in the wire
Q = I t
I = A n v q
v
conventional I
cross-section A, n carriers per m³, each charge q
2 · POTENTIAL DIFFERENCE & POWER
P.D. is the energy moved per unit charge.
V = W / Q: work done driving charge between two
points, measured in volts (1 V = 1 J C−¹)
Power is energy transferred each second, P = V I
V = W / Q
P = V I = I² R = V² / R
R
Q
W = QV
charge loses energy QV crossing the component
3 · RESISTANCE & RESISTIVITY
Resistance is the opposition to current; resistivity is the material's share of it.
Resistance R = V / I, measured in ohms (Ω)
Ohm's law: for a metal at constant temperature
I is proportional to V, so R stays constant
Resistivity ρ is fixed for a material; R grows with
length L and falls with cross-section A
Thermistors (NTC) and LDRs drop R with heat / light
R = V / I
R = ρ L / A
I
V
ohmic conductor
filament lamp
diode
I–V characteristics
L
A
resistivity ρ: same material, R = ρ L / A
R
T
NTC thermistor: R falls as T rises
LEADS TO
T10 D.C. circuits: Kirchhoff's laws, combining R T18 Electric fields: potential from charge T19 Capacitance: storing charge, Q = CV Each reuses the core idea: current is moving charge, and energy per unit charge (V) drives it against resistance.
← Builds on IGCSE: Electrical quantities