Current Electricity
What this domain tests
Current Electricity tests Ohm's law, resistivity, series and parallel circuits, Kirchhoff's rules, cells, Wheatstone bridge and potentiometer. Expect circuit simplification and terminal-voltage reasoning.
Core rules
- Ohm's law: V = IR for ohmic conductors at constant temperature. Resistance R = ρL/A; resistivity depends on material and temperature.
- Series: R = R1 + R2 + ..., same current. Parallel: 1/R = 1/R1 + 1/R2 + ..., same voltage.
- Kirchhoff: junction rule ΣI = 0; loop rule ΣV = 0. Include internal resistance: terminal voltage V = ε - Ir on discharge.
- Wheatstone bridge is balanced when P/Q = R/S; then no current through the galvanometer.
- Potentiometer compares emfs at zero current draw, so it measures emf more accurately than a voltmeter.
Worked example
A 6 Ω and a 3 Ω resistor are connected in parallel across a 12 V battery of negligible internal resistance. Find the total current and branch currents. Parallel resistance: 1/Rp = 1/6 + 1/3 = 1/2, so Rp = 2 Ω. Total current I = V/Rp = 12/2 = 6 A. Branch currents: 12/6 = 2 A and 12/3 = 4 A. They add to 6 A, satisfying the junction rule.
Traps
Do not add parallel resistances directly. An ammeter is connected in series and should have low resistance; a voltmeter is in parallel and should have high resistance. Do not ignore internal resistance when a cell delivers current. In a balanced bridge, the galvanometer reading is zero but the bridge is not disconnected.