Current Electricity — Class 12 Physics

Electric current is the rate of flow of charge, , arising from the drift of free electrons under an applied field. The average drift speed leads to . Ohm's law states that for many conductors current is proportional to potential difference, , where resistance depends on geometry and the material's resistivity . Resistivity rises with temperature for metals and falls for semiconductors. Resistors combine in series as and in parallel as . A real cell has an emf and an internal resistance , so its terminal voltage is . Kirchhoff's junction rule (charge conservation) and loop rule (energy conservation) solve complex networks. The Wheatstone bridge is balanced when , a principle exploited by the meter bridge and potentiometer for precise measurement. Electrical power dissipated is , and the heat produced obeys Joule's law.