I85PREP
⌘ K
UPMSP Intermediate Physics - Code 151 study materials
UPMSP Intermediate Physics - Code 151 study resources

UPMSP Intermediate Physics - Code 151: Current Electricity

UPMSP Intermediate Physics - Code 151 · Work through your exam, one topic at a time.

All guide sections

Current Electricity

This unit links the microscopic motion of electrons to the macroscopic circuit laws you actually use in the exam: Ohm's law, resistivity, drift velocity, Kirchhoff's rules, the Wheatstone bridge, the potentiometer and the EMF–internal resistance model of a cell. Questions are usually numerical, often one long-answer derivation (drift velocity, or balance condition of a metre bridge) and one shorter calculation on terminal potential difference or power.

Core rules

  • Ohm's law V = IR holds for metallic conductors at constant temperature; R = ρL/A, so resistance doubles with length and halves with doubled area of cross-section.
  • Drift velocity: I = nAev(d); typical values are about 10⁻⁴ m s⁻¹, so clearly the current is not carried at the speed of light.
  • Resistivity of metals rises with temperature (ρ = ρ₀(1 + αΔT)); for semiconductors and electrolytes it falls.
  • Kirchhoff: junction rule (charge conservation), loop rule (energy conservation). Fix one sign convention and keep it.
  • Cell of EMF E and internal resistance r: I = E/(R + r), terminal p.d. V = E − Ir; V = E only on open circuit.
  • Wheatstone balance: P/Q = R/S. Potentiometer at null point draws no current, so it measures EMF, not terminal voltage.

Worked example

A cell of EMF 12 V and internal resistance 1 Ω is connected to a 5 Ω resistor. Find the current, terminal voltage and the power wasted inside the cell. I = E/(R + r) = 12/6 = 2 A. Terminal p.d. V = IR = 2 × 5 = 10 V (equivalently E − Ir = 12 − 2 = 10 V). Power in the external resistor = I²R = 4 × 5 = 20 W; power lost in r = I²r = 4 W. Efficiency = 20/24 = 83 %.

Traps that cost marks

  • Confusing resistance with resistivity; resistivity is a material property and does not change when the wire is cut in half.
  • Mixing up ammeter and voltmeter placement: ammeter in series (low resistance), voltmeter in parallel (high resistance).
  • Sign errors in Kirchhoff loops when a cell is traversed from + to −.
  • Forgetting that a potentiometer's null method draws no current, so it gives EMF rather than terminal p.d.