CBSE Class 12-science Physics Current Electricity

Ohm's Law

Electric current : Rate of flow of charge through any cross section of the wire.
Ohm's law : Electric current produced through a conductor is directly proportional to the potential difference applied.
Resistance is directly proportional to length and inversely proportional to area cross section of a conductor.

Drift of Electrons and Resistivity

Define drift speed and study its dependence on applied electric field.

Combination of Resistors

To determine equivalent resistance of a series and parallel combination of resistors.

emfs and Internal Resistance

Emf: Work done per unit charge in moving the charge from one terminal of the cell to another in an open circuit.
Internal resistance: Offered by the electrolyte and electrodes of a cell when the electric current flows through it.
During discharging: Terminal p.d. V < emf E. V = E-Ir
During recharging: Terminal p.d. V > emf E. V = E+Ir

Combination of Cells

Study the voltage, current and power output of a circuit when cells are connected in series and parallel combination.

Kirchhoff's Rules

Kirchhoff's Junction Rule: The sum of currents entering any junction must equal the sum of the currents leaving the junction.
Kirchhoff's Loop Rule: The sum of the potential differences across all the elements around any closed loop must be zero.

Electrical Appliances

Wheatstone bridge: Most sensitive when the resistance of all four arms of the bridge is in the same order of the magnitude.
Meter bridge cannot be used to measure very low or very high resistance accurately.
Principle of Potentiometer: Potential drop is directly proportional to the length of wire, if uniform thickness and steady current is maintained.

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