What does Kirchhoff's (junction) rule state?

• A. Sum of voltages around a closed loop equals zero
• B. The sum of currents entering a junction equals the sum leaving
• C. Sum of currents around a closed loop equals zero
• D. Product of currents at a junction equals zero

Answer: (B)

Kirchhoff's current law says that at any junction, charge cannot accumulate, so the total current flowing into the junction must equal the total current flowing out. In other words, the algebraic sum of all branch currents at a node is zero. If you treat currents entering as positive and leaving as negative, you can write I_in1 + I_in2 + ... = I_out1 + I_out2 + ...; for example, if two branches bring in I1 and I2 and one branch carries away I3, then I1 + I2 = I3. This reflects charge conservation at a point in a circuit.

The other statements describe different rules: one is Kirchhoff's voltage law, which concerns the sum of voltages around a closed loop, not currents at a junction; the remaining option isn’t a general rule for junctions since currents can all be nonzero and their product being zero isn’t a requirement.