Current Electricity
What this domain tests
Current Electricity tests Ohm's law, resistance and resistivity, drift velocity, cells and internal resistance, Kirchhoff's rules, the Wheatstone bridge and the potentiometer.
Core rules
- I = nAev_d; drift velocity is small, often mm per second, although the current is established quickly.
- R = ρL/A; ρ increases with temperature for metals and decreases for semiconductors.
- EMF ε is the cell's open-circuit p.d.; terminal p.d. V = ε − Ir on discharge.
- Kirchhoff: junction rule is charge conservation; loop rule is energy conservation.
- Wheatstone balance: P/Q = R/S. A potentiometer compares EMFs without drawing current.
Worked example
A cell of EMF 12 V and internal resistance 1 Ω is connected to a 5 Ω resistor. Current I = ε/(R+r) = 12/6 = 2 A. Terminal p.d. V = IR = 10 V, which also equals ε − Ir = 12 − 2 = 10 V. Power delivered to the external resistor is I²R = 20 W; power wasted inside the cell is I²r = 4 W.
Traps
- Quoting EMF as terminal voltage when current flows; terminal voltage is lower on discharge.
- Assuming ideal ammeter and voltmeter; a real ammeter has small resistance and a voltmeter large resistance.
- Reversing a sign in a Kirchhoff loop.
- Thinking resistivity changes with length or thickness; it depends on material and temperature only.