Current electricity

ZIMSEC A Level · Physics · Form 5

Create a free account to download All Physics topics and practice questions

Basics

I = ΔQ/Δt; V = W/Q; R = V/I; P = VI = I²R = V²/R.

Resistivity: R = ρL/A.
I = nAvq links current with charge carrier density n and drift speed v.

I–V characteristics

Ohmic resistor (straight line through the origin), filament lamp (resistance increases with temperature), semiconductor diode (conducts in one direction only), thermistor (NTC – resistance falls as temperature rises), LDR (resistance falls with light intensity).

Kirchhoff's laws

First law: the sum of currents into a junction equals the sum out (conservation of charge).
Second law: around any closed loop, the sum of e.m.f.s equals the sum of p.d.s (conservation of energy).

E.m.f. and internal resistance

E = I(R + r), so V = E − Ir. The terminal p.d. falls as current increases.

Worked example: A cell of e.m.f. 1.5 V and internal resistance 0.50 Ω is connected to a 2.5 Ω resistor. Find the current and terminal p.d.

I = 1.5 ÷ (2.5 + 0.50) = 0.50 A; V = 0.50 × 2.5 = 1.25 V

Potential dividers

Vout = Vin × R₂/(R₁ + R₂). With a thermistor or LDR, they make temperature and light sensors.

Worked example: Vin = 9.0 V; R₁ = 4.0 kΩ; R₂ = 2.0 kΩ. Find Vout across R₂.

Vout = 9.0 × 2.0/6.0 = 3.0 V

Exam tip: For internal resistance questions, write E = I(R + r) first – it organises the whole calculation.

Check yourself

Find the resistance of a 2.0 m wire of resistivity 1.7 × 10⁻⁸ Ω m and area 1.0 × 10⁻⁶ m².
State Kirchhoff's first law.
How does the resistance of an NTC thermistor change as it warms?

Show answers

0.034 Ω
The total current entering a junction equals the total current leaving it.
It decreases.

Questions about this topic (0)

No questions about this topic yet.