Topic 4 of 8 · Form 5
Electricity and DC circuits
I = ΔQ/Δt; Q = ne; I = nAvq; V = W/Q; R = V/I; P = VI = I²R = V²/R; resistivity R = ρL/A; I–V characteristics of resistors, filament lamps, diodes, thermistors and LDRs.
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Electric current and potential difference
I = ΔQ/Δt; Q = ne; I = nAvq; V = W/Q; R = V/I; P = VI = I²R = V²/R; resistivity R = ρL/A; I–V characteristics of resistors, filament lamps, diodes, thermistors and LDRs.
DC circuits
Kirchhoff's laws (charge and energy conservation); resistors in series and parallel; e.m.f. and internal resistance (E = I(R + r)); potential dividers and potentiometers.
Worked example: A battery of e.m.f. 9.0 V and internal resistance 1.5 Ω supplies a 7.5 Ω resistor. Find the current, terminal p.d. and power wasted inside the battery.
I = 9.0 ÷ 9.0 = 1.0 A; V = 7.5 V; power in r = 1.0² × 1.5 = 1.5 W
Worked example: Find the resistance of a 1.5 m wire of diameter 0.40 mm (ρ = 4.9 × 10⁻⁷ Ω m).
A = 1.26 × 10⁻⁷ m² → R = 4.9 × 10⁻⁷ × 1.5 ÷ 1.26 × 10⁻⁷ = 5.8 Ω
Check yourself
- State Kirchhoff's second law.
- How does the resistance of an NTC thermistor change with temperature?
- Three 6 Ω resistors are in parallel. Find the total resistance.
Show answers
- The sum of e.m.f.s round a closed loop equals the sum of the p.d.s (conservation of energy).
- It decreases as temperature increases.
- 2 Ω
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