Ch 3 covers current electricity in depth — drift velocity, Ohm's law, resistivity, colour code, Kirchhoff's laws, Wheatstone bridge, meter bridge, and potentiometer.
Current I = dQ/dt = neAv_d. Drift velocity: average velocity of electrons. Resistance R = ρL/A; ρ = 1/σ (resistivity). Temperature dependence: for metals ρ increases with T. EMF vs terminal voltage: V = ε − Ir (internal resistance). Kirchhoff's junction rule: ΣI = 0. Loop rule: ΣV = 0.
Wheatstone bridge: P/Q = R/S → galvanometer shows zero (balanced). Meter bridge: practical Wheatstone using uniform wire. Potentiometer: compares EMFs without drawing current. Principle: V ∝ L (for uniform wire carrying constant current). Applications: compare EMFs, find internal resistance.
Download: https://ncert.nic.in/textbook/pdf/leph103.pdf | Part I: https://ncert.nic.in/textbook/pdf/leph1ps.zip
A voltmeter draws some current from the circuit, so it measures terminal voltage (ε − Ir), not true EMF. A potentiometer works on the null method — at the balance point, no current flows through the cell being measured. So it measures the true EMF, making it more accurate.
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