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A student writes for a junction: $I_1+I_2+I_3+I_4+I_5+I_6=0$ using only magnitudes of current. Why is this incorrect?
ABecause Kirchhoff's Current Law applies only to two branches
BBecause current magnitudes can never be added
CBecause KCL requires an algebraic sum with directions, or equivalently entering current equals leaving current
DBecause only outgoing currents should be counted
Answer & Solution
Correct answer: C. Because KCL requires an algebraic sum with directions, or equivalently entering current equals leaving current
KCL is about signed currents. You may either assign signs to currents and write the algebraic sum as zero, or write total entering current equals total leaving current. Adding all magnitudes directly and setting them to zero is meaningless because magnitudes are positive quantities.