Current ElectricityNEET Physics · Class 12 · NCERT Chapter 3

You can expect 2 questions from this chapter in NEET 2027. The chapter has the highest PYQ frequency in Physics. Resistor combinations, Kirchhoff's laws, Wheatstone bridge, EMF with internal resistance and electrical power are repeated every year.

For most metallic conductors at constant temperature, the current I through a conductor is directly proportional to the potential difference V across it. The constant of proportionality is called resistance: V equals I R. Materials that follow this law are called ohmic; semiconductors, gases and vacuum tubes are non-ohmic.

Free electrons in a metal move randomly at high speeds (about 10 to the 5 m per s). When you apply a field, they pick up a tiny additional drift velocity (about a millimetre per second) in the direction opposite to the field. The current is I equals n A e v_d, where n is the number of free electrons per cubic metre, A is cross-section, e is electron charge.

Series: same current through each, voltages add. R_eff equals R_1 plus R_2 plus R_3. Parallel: same voltage across each, currents add. 1 over R_eff equals 1 over R_1 plus 1 over R_2 plus 1 over R_3. Series gives larger total R; parallel gives smaller R, less than the smallest.

Two rules for solving circuits. KCL (junction rule): the sum of currents flowing into any junction equals the sum flowing out. This is conservation of charge. KVL (loop rule): the algebraic sum of voltage changes around any closed loop is zero. This is conservation of energy.

A Wheatstone bridge has four resistors P, Q, R and S arranged in a diamond, with a galvanometer between the two midpoints. The bridge is balanced (no current through the galvanometer) when P over Q equals R over S. NEET often gives three of the four resistors and asks for the fourth.

EMF (epsilon) is the voltage of a cell when no current flows: the maximum voltage it can provide. When the cell drives current I through external resistance R, internal resistance r drops some voltage. Terminal voltage V equals epsilon minus I r. So when current flows, V is less than epsilon. When the cell is being charged, V is greater than epsilon.