Physics · Current Electricity · NEET
No. The junction rule (KCL) is about CURRENT at a node — total current in = total current out (charge conservation). The loop rule (KVL) is about VOLTAGE around a closed loop — the sum of voltage changes is zero (energy conservation). NEET uses both together: KCL to relate currents at junctions, KVL to write equations for each loop. Do not mix them up in the exam.
Pick a direction to walk the loop. For a cell: if you go from the minus (-) terminal to the plus (+) terminal, the emf is +E (potential rises); if you go from + to -, it is -E. For a resistor: if you walk in the SAME direction as the assumed current, the drop is -IR (potential falls); if you walk against the current, it is +IR. Add all these up and set the total to zero.
Electric potential is a single value at each point. If you start at point P, walk all the way around, and return to P, you are back at the same potential you started with. So the net change must be zero. This is the same idea as walking around a hill and returning to your starting height — total height change is zero. That is why ΣV = 0 for any closed loop.
Yes. KVL holds for the instantaneous voltages at any moment. For a capacitor use V = Q/C, for a resistor V = IR, for a cell use E. In a steady-state DC circuit a capacitor carries no current, so the resistor in series with it has zero IR drop, and the full loop voltage appears across the capacitor. In NEET, most KVL numericals are pure resistor + cell loops, but the rule itself is general.
Choose any direction (clockwise or anticlockwise) and any assumed current direction — it does not matter. Apply the sign rules consistently all the way round. If your final current comes out negative, it simply means the real current flows opposite to your assumed arrow; the magnitude is still correct. So a wrong initial guess never gives a wrong answer, only a minus sign to interpret.
Current of 2 A flows from A to B through a 2 ohm resistor, then a 3 V cell (crossing - to +), then a 1 ohm resistor. The potential difference (V_A - V_B) between points A and B is:
A single loop contains resistances 2 ohm, 1 ohm and 7 ohm in series, and two cells of emf 10 V and 5 V that oppose each other. The magnitude and direction of the current is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Around any closed loop of a circuit, the algebraic sum of all potential changes (emfs and IR drops) is zero: ΣV = 0.
KVL is based on conservation of energy. A unit charge returning to its starting point in a loop must have the same potential energy, so the net voltage change is zero.
If you move along the loop in the same direction as the current, the resistor gives a drop of -IR. If you move against the current, it gives +IR.
Moving from the negative terminal to the positive terminal inside the loop gives +E (rise). Moving from positive to negative gives -E (fall). Internal resistance r adds an IR drop like any resistor.
Write KVL for enough independent loops so that, together with the junction (KCL) equations, the number of equations equals the number of unknown currents. For a simple two-loop network that is usually two KVL equations.
