Current Electricity notes

Current Electricity studies charges in motion and the circuits through which they flow. The chapter begins with electric current, current density, drift velocity and mobility, connecting microscopic electron motion with measurable current. Ohm’s law, resistance, resistivity, conductivity and temperature dependence explain how materials oppose current. Electrical energy and power show how circuits convert electrical energy into heat and useful work. Cells introduce emf, terminal voltage and internal resistance. Kirchhoff’s laws allow systematic analysis of complex circuits using charge and energy conservation. Wheatstone bridge and metre bridge are important applications used to compare and measure resistance. For NEET, this chapter is highly scoring because most questions are formula-based, circuit-based and numerically direct.

Electric current is the rate of flow of charge through a cross-section of a conductor. In metals, free electrons are the charge carriers. Without an electric field, electrons move randomly and no net current exists. When an electric field is applied, electrons acquire a small average velocity opposite to the field, called drift velocity. Conventional current is taken in the direction of positive charge flow, opposite to electron drift. Current density J describes current per unit area and points along conventional current. Mobility measures how easily charge carriers drift under an electric field. The relation I = neAvd connects microscopic electron motion with measurable current, making it a frequent NEET formula.

Ohm’s law states that at constant temperature and physical conditions, current through a conductor is directly proportional to potential difference across it: V = IR. Resistance measures opposition to current and depends on material, length, area and temperature. For a uniform wire, R = ρL/A, where ρ is resistivity, a material property. Conductivity is reciprocal of resistivity. Metals usually have positive temperature coefficient of resistance, so resistance increases with temperature. Semiconductors often show the opposite trend. V-I characteristics help identify ohmic and non-ohmic devices. For NEET, this topic commonly appears as direct I = V/R numericals, V-I graph slope questions and resistance-temperature calculations.

Electrical work is done when charges move through a potential difference. If charge q moves through voltage V, work done is W = qV. Since current is charge per unit time, electrical power is P = VI. For a resistor, using Ohm’s law gives P = I²R and P = V²/R. Joule’s law of heating states that heat produced in a resistor is H = I²Rt. Electrical energy consumed is power multiplied by time. The commercial unit of electrical energy is kilowatt-hour, where 1 kWh = 3.6 × 10⁶ J. NEET questions often involve power ratings, heater calculations, fuse selection and electricity bill calculations.

An electric cell converts chemical energy into electrical energy and maintains potential difference in a circuit. EMF is the work done per unit charge by the cell in driving charge through the complete circuit when no current is drawn. A real cell has internal resistance due to opposition inside the electrolyte and electrodes. When the cell supplies current, some voltage is lost inside the cell, so terminal voltage becomes V = ε - Ir. During charging, terminal voltage can be greater than emf: V = ε + Ir. Cells may be connected in series to increase voltage or in parallel to increase current capacity and reduce effective internal resistance. NEET often asks terminal voltage and cell combination problems.

Kirchhoff’s laws are used to solve circuits that cannot be simplified easily using simple series and parallel rules. The junction rule states that the algebraic sum of currents at a junction is zero, or total current entering equals total current leaving. It is based on conservation of charge. The loop rule states that the algebraic sum of potential changes around any closed loop is zero, based on conservation of energy. Correct sign convention is essential: crossing a resistor in the direction of current gives a potential drop -IR, while crossing a cell from negative to positive terminal gives +ε. These laws are important for multi-loop circuit problems and bridge circuits.

Wheatstone bridge is a network of four resistances used to compare or measure unknown resistance. In the balanced condition, no current flows through the galvanometer because the potentials of its two terminals are equal. The balance condition is P/Q = R/S. A metre bridge is a practical form of Wheatstone bridge using a uniform 1 m wire. At balance, the ratio of resistances equals the ratio of balancing lengths: R/X = l/(100 - l), depending on which gap contains the unknown. The bridge method is accurate because it detects null deflection, so the result does not depend on galvanometer resistance. NEET commonly asks bridge balance, unknown resistance and metre bridge length calculations.