Class 11 Physics — Chapter 305: Electric Current Through Conductors

Key concepts: Electric Current Through Conductors (Class 11 Physics)

Charge in motion through conductors: current and drift velocity, Ohm's law with resistance and resistivity and their temperature dependence, series and parallel networks, EMF and internal resistance, Kirchhoff's two rules, the Wheatstone bridge with the metre bridge and potentiometer, and electrical power and Joule heating.

Electric current

The rate of flow of charge through a cross-section. Conventional current is taken along the flow of positive charge, opposite to the actual electron drift.

Drift velocity

The slow net velocity electrons acquire in a field, of order 10⁻⁴ m s⁻¹ — far slower than the near-light-speed at which the electric signal itself travels.

Current and drift velocity

Related by I = neAv_d, which is what links the macroscopic current to the microscopic motion of the charge carriers.

Mobility

The drift velocity acquired per unit electric field. It is higher for electrons than for holes, which is why n-type devices tend to be faster.

Ohm's law

Current is proportional to potential difference at constant temperature. It is an empirical rule, not a universal law — diodes and semiconductors disobey it.

Resistance and resistivity

Resistance depends on the sample's dimensions, while resistivity is the material property independent of shape and size.

Temperature dependence

Resistivity of a metal RISES with temperature as lattice vibrations scatter electrons; for a semiconductor it FALLS, since more carriers are freed.

Series and parallel circuits

In a series circuit the resistances add and the current is shared; in a parallel circuit the reciprocals add and the voltage is shared, always giving less than the smallest resistor.

EMF and terminal voltage

EMF is the full work per unit charge done by the cell; terminal voltage is what is left after the drop across the internal resistance, so it is lower while current flows.

Internal resistance

The resistance of the cell's own electrolyte. An ideal cell would have zero, which is why the terminal voltage of a fresh cell barely dips under load.

Cells in series and parallel

Series connection adds EMFs and internal resistances, giving higher voltage; parallel connection keeps the EMF and lowers internal resistance, giving higher current.

Kirchhoff's junction rule

The total current entering a junction equals the total leaving it — a direct statement of conservation of charge.

Kirchhoff's loop rule

The algebraic sum of potential changes around any closed loop is zero — a direct statement of conservation of energy.

Wheatstone bridge

Four resistors in a loop with a galvanometer across the middle. At balance no current flows through the galvanometer and the ratios of the arms are equal.

Metre bridge and potentiometer

Practical forms of the balance principle. A potentiometer measures EMF without drawing current, so it beats a voltmeter, which always loads the cell.

Power and Joule heating

Electrical power is VI, and in a resistor it appears as heat at the rate I²R — the basis of every heater, fuse and filament lamp.