Chapter 3: Current ElectricityClass 12 Physics — summary, notes, extra questions & MCQ quiz

Summary

Electric current is the rate of flow of charge, \(I=\dfrac{dq}{dt}\), arising from the drift of free electrons under an applied field. The average drift speed \(v_d=\dfrac{eE\tau}{m}\) leads to \(I=neAv_d\). Ohm's law states that for many conductors current is proportional to potential difference, \(V=IR\), where resistance \(R=\rho\dfrac{l}{A}\) depends on geometry and the material's resistivity \(\rho\). Resistivity rises with temperature for metals and falls for semiconductors. Resistors combine in series as \(R_s=R_1+R_2+\cdots\) and in parallel as \(\tfrac{1}{R_p}=\tfrac{1}{R_1}+\cdots\). A real cell has an emf \(\varepsilon\) and an internal resistance \(r\), so its terminal voltage is \(V=\varepsilon-Ir\). Kirchhoff's junction rule (charge conservation) and loop rule (energy conservation) solve complex networks. The Wheatstone bridge is balanced when \(\dfrac{R_1}{R_2}=\dfrac{R_3}{R_4}\), a principle exploited by the meter bridge and potentiometer for precise measurement. Electrical power dissipated is \(P=VI=I^2R=\dfrac{V^2}{R}\), and the heat produced obeys Joule's law.

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