Physics · Current Electricity · NEET
Just assume ANY direction for each branch current and mark it with an arrow. You do not need to guess correctly. Solve the equations. If a current comes out positive, your assumed direction was right. If it comes out NEGATIVE, the real current simply flows the opposite way, but the magnitude is still correct. So a wrong guess never gives a wrong answer, it only flips a sign. This is why you should never redo the whole problem just because a value turned out negative.
Pick a direction to walk around the loop, then: (1) Resistor: if you move ALONG the current, the potential DROPS, write -IR. If you move AGAINST the current, potential RISES, write +IR. (2) Cell: if you go from the minus terminal to the plus terminal (- to +) inside the cell, that is a RISE, write +E. If you go from + to -, write -E. Add all these changes around the loop and set the sum to zero. Keep the same walking direction for the whole loop.
Count the unknown currents (branches). You need exactly that many INDEPENDENT equations. For a circuit with J junctions, only (J - 1) junction equations are independent; the rest of the equations must come from the loop rule using independent loops (each new loop must contain at least one branch not used before). Extra loop equations are not wrong, but they give no new information and just waste time in NEET.
Use the junction rule FIRST, at each junction, to reduce the number of unknown currents (for example, at a junction where I1 and I2 enter, label the exit current as I1 + I2 instead of a new I3). Then use the loop rule on closed loops to get equations relating the emfs and IR drops. Junction rule reduces unknowns; loop rule gives the solving equations.
Series/parallel works only when resistors are cleanly in series or parallel. In many NEET networks (like an unbalanced Wheatstone bridge, or a circuit with cells in different branches), no resistor pair is purely series or parallel, so the simple formulas fail. Kirchhoff's rules always work because they come from charge and energy conservation, not from geometry.
The potential difference (V_A - V_B) between points A and B in the figure (current 2 A flows from A to B through a 2 ohm resistor, then a 3 V cell, then a 1 ohm resistor) is:
The magnitude and direction of the current in the single-loop circuit shown (resistors 2 ohm, 1 ohm, 7 ohm in the loop with a 10 V and a 5 V cell) is:
In two circuits with the same 10 V cell and same 10 ohm resistor, using ideal meters, only the order of the voltmeter (V) and ammeter (A) is swapped. The readings will be:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Junction rule: total current entering a junction equals total current leaving it (charge is conserved). Loop rule: the sum of all potential changes (emfs and IR drops) around any closed loop is zero (energy is conserved).
The junction rule comes from conservation of charge (no charge piles up at a point). The loop rule comes from conservation of energy (a charge returning to its start has zero net change in potential).
No. Assume any directions, solve, and a negative answer just tells you the true direction is opposite. The magnitude is always correct, so you never need to restart.
For a network, total independent equations = number of unknown currents. Junctions give (J - 1) independent equations; the remaining equations come from independent loops, where each new loop must include at least one fresh branch.
Yes. NEET regularly asks single-loop current, potential difference between two points, and unbalanced-bridge current questions that need the loop and junction rules. Mastering the sign convention makes these fast and error-free.
