Current Electricity
What the exam tests
This domain tests Ohm law, resistivity, drift velocity, cells with internal resistance, Kirchhoff rules, Wheatstone bridge, potentiometer, power and heating. Numerical questions often mix series-parallel networks with terminal voltage and metering.
Core rules
- Current I = nAev_d; drift velocity v_d = eEτ/m; resistivity ρ = m/(ne^2τ); resistance R = ρL/A.
- Ohm law V = IR holds for metallic conductors at constant temperature. Non-ohmic devices have curved V-I graphs.
- Series resistances add; parallel conductance adds.
- For a cell of emf ε and internal resistance r, terminal voltage is V = ε − Ir while discharging and V = ε + Ir while charging.
- Kirchhoff junction rule: sum of currents entering equals sum leaving. Loop rule: algebraic sum of potential changes around a closed loop is zero.
- Wheatstone balance P/Q = R/S. A potentiometer compares emfs without drawing current at balance.
- Power P = VI = I^2R = V^2/R.
Worked example
A cell of emf 12 V and internal resistance 2 Ω is connected to a 10 Ω resistor. Total resistance is 12 Ω, so I = 12/12 = 1 A. Terminal voltage is V = ε − Ir = 12 − (1)(2) = 10 V. Power in the external resistor is I^2R = 10 W; internal power loss is I^2r = 2 W. This shows why terminal voltage falls as current rises.
Traps
An ammeter is connected in series and should have low resistance; a voltmeter is in parallel and should have high resistance. Reversing cell sign in a loop is a frequent error. Resistivity is a material property, while resistance also depends on length and area. Do not apply V = IR blindly to diodes or bulbs with changing temperature.