Current Electricity
Current electricity tests steady currents, drift velocity, resistance and resistivity, series and parallel networks, Kirchhoff rules, cells, Wheatstone bridge, meter bridge, and potentiometer. The exam rewards correct circuit models and careful treatment of internal resistance.
Core rules and concepts
- Current I = nAev_d, where n is carrier density, A cross-section, e charge, and v_d drift speed.
- Ohm law V = IR applies to ohmic conductors at constant temperature. Resistance R = rho L/A; resistivity depends on material and temperature.
- Series: same current, R_eq = R1 + R2 + ... Parallel: same voltage, 1/R_eq = 1/R1 + 1/R2 + ...
- Kirchhoff junction rule conserves charge; loop rule conserves energy. Assign signs consistently.
- A real cell has emf epsilon and internal resistance r: terminal voltage V = epsilon - Ir on discharge.
- Power P = VI = I^2R = V^2/R. A potentiometer compares emfs without drawing current at balance; a meter bridge uses a balanced Wheatstone network.
Worked example
A cell of emf 12 V and internal resistance 0.5 ohm is connected to a 5.5 ohm resistor. Total resistance is 6.0 ohm, so I = 12/6.0 = 2.0 A. Terminal voltage is V = 12 - (2.0)(0.5) = 11 V. External power is I^2R = 4 x 5.5 = 22 W; internal power loss is I^2r = 2 W.
Traps that cost marks
- Treating terminal voltage as equal to emf when current flows.
- Putting an ammeter in parallel or a voltmeter in series.
- Ignoring internal resistance in power calculations.
- Reversing Kirchhoff signs when traversing a battery or resistor.
- Assuming resistivity is independent of temperature for all materials.