Current Electricity
Current Electricity tests the flow of charge in conductors and circuits: Ohm's law, resistivity, drift velocity, cells, Kirchhoff's rules, Wheatstone bridge and potentiometer. Expect numericals on equivalent resistance and internal resistance.
Core rules
- I = Q/t; conventional current is opposite to electron drift.
- Drift velocity v_d = I/(nAe), so current is small because v_d is very small even when the signal travels near light speed.
- R = ρL/A; resistivity rises with temperature for metals and falls for semiconductors. Conductance is 1/R.
- Kirchhoff: junction rule conserves charge, loop rule conserves energy. For a cell, terminal voltage V = ε - Ir on discharge.
- Series resistances add; parallel resistances add reciprocals. An ideal ammeter has zero resistance and an ideal voltmeter infinite resistance.
Worked example
A 6 Ω and a 3 Ω resistor are connected in parallel across a 12 V supply of negligible internal resistance. Equivalent resistance is (6×3)/(6+3) = 2 Ω. Total current I = 12/2 = 6 A. Branch currents are 12/6 = 2 A and 12/3 = 4 A, which sum to 6 A.
Traps
Forgetting internal resistance when a cell is loaded; using V = IR for the whole circuit when R is only an external resistor; swapping series and parallel formulae; and treating a voltmeter as if it draws no current in every real experiment.