Ch 2Current Electricity — Class 12 Physics, concept-first.

Current electricity is the branch of physics that studies the flow of electric charge, as opposed to electrostatics (Unit 1), which studies charges that are at rest.

All ordinary matter is built from atoms, each with a positively charged nucleus surrounded by negatively charged electrons.

In a real circuit, the particles that actually move are electrons, flowing from the negative terminal of the battery, through the external circuit, and back into the positive terminal.

With no electric field applied, a conductor's free electrons undergo a purely random zig-zag motion, repeatedly colliding with the lattice's positive ions; on average, exactly as many electrons travel…

Building the current formula from first principles: consider a conductor of cross-sectional area A with an electric field applied along it, containing n free electrons per unit volume, all assumed to…

Starting directly from the microscopic form of Ohm's law, , consider a uniform segment of wire of length l and cross-sectional area A.

The resistance of a conductor, from equation (2.18) (writing explicitly for conductivity), depends on the material used (through ) and separately on the conductor's own geometry (through l and A).

When two or more resistors are connected end to end so that the same current has no alternative path but to flow through every one of them in turn, they are said to be connected in series.

Carbon resistors, used constantly in the physics laboratory, are built from a ceramic core coated with a thin layer of crystalline carbon; they are prized for being inexpensive, electrically stable an…

A material's resistivity depends measurably on its temperature. For a wide range of temperatures, experiment shows that a conductor's resistivity increases linearly with temperature: where is the resi…

When a battery of voltage V drives current through a circuit, it continuously supplies energy to whatever device is connected.

An electric cell converts stored chemical energy directly into electrical energy. A simple example (Figure 2.17) uses two dissimilar electrodes -- carbon (positive) and zinc (negative) -- immersed tog…

A battery or cell is described as a source of electromotive force (emf), usually denoted . The name is, admittedly, something of a historical misnomer -- an emf is not literally a force at all, but a…

Once an external resistance R is switched into the circuit and a current I actually flows (Figure 2.20(b)), the voltmeter no longer reads the full emf -- it instead reads a smaller value V, because a…

Several identical cells can be strung together in series: the negative terminal of one cell is joined to the positive terminal of the next, and so on down the chain, so that the free positive terminal…

Alternatively, n identical cells can be connected in parallel: all of their positive terminals are joined together at one common node, and all of their negative terminals are joined together at a seco…

Ohm's law by itself, , is only directly useful for simple circuits containing a single loop and a single source of emf.

Kirchhoff's first rule, the current rule (or junction rule), states that the algebraic sum of the currents at any junction of a circuit is zero.

Kirchhoff's second rule, the voltage rule (or loop rule), states that in any closed circuit, the algebraic sum of the products of current and resistance for each part of the circuit is equal to the to…

An important, extremely common practical application of Kirchhoff's rules is the Wheatstone's bridge, used both to compare resistances and to determine an unknown resistance in a network.

The metre bridge is simply a practical, laboratory-scale realisation of the Wheatstone's bridge, built around a single one-metre length of uniform manganin wire, AB, stretched along a metre scale on a…

The potentiometer is an instrument built for very accurate, null-method measurement of potential differences, currents and resistances -- more accurate, in fact, than a direct voltmeter reading, preci…

To compare the emf values of two different cells, and , the potentiometer wire CD is connected in series with a battery Bt, a rheostat Rh, and a key K to form the primary circuit (Figure 2.28).

The potentiometer can also directly measure a cell's own internal resistance r. The circuit connections (Figure 2.29) place the potentiometer wire's end C at the positive terminal of a primary battery…

When current flows through a resistor, some of the electrical energy delivered to it is inevitably converted into heat and dissipated into the surroundings -- this is called Joule's heating effect.

If a current I flows through a conductor across a potential difference V for a time t, the total electrical work done -- equivalently, the electric potential energy spent -- is When there is no other…

Joule's heating effect has a wide range of familiar, everyday and industrial applications, all exploiting the deliberate conversion of electrical energy into heat:

Thermoelectric effect is the general name for the reversible, two-way conversion between a temperature difference and an electrical voltage: a thermoelectric device generates a voltage across itself w…

Seebeck discovered that in a closed circuit made from two DISSIMILAR metals joined at two junctions, if those two junctions are maintained at different temperatures, an emf (a potential difference) de…

Peltier, in 1834, discovered the reverse-direction phenomenon: when an electric current is passed THROUGH a thermocouple circuit (rather than a temperature difference producing a current, as in the Se…

Thomson showed that even within a SINGLE, uniform conductor (not a junction of two different metals at all), if two points along it are held at different temperatures, the density of free electrons at…

The current in a conductor is , where dQ is the charge crossing a cross-section in time interval dt; the SI unit of current is the ampere, .

The unit's concept map is rooted at Current Electricity and branches into four main strands. The first strand, Flow of Charges, branches further into drift velocity, current density and mobility -- th…

This evaluation section carries the unit's complete end-of-chapter assessment, split into four parts exactly as printed in the textbook: Part I, Multiple Choice Questions (15 questions, each with four…

Sample papers and previous-year board questions for this subject.