A Level Physics Practice Exam

According to Kirchhoff's first law, what must be true about the current at a junction in a circuit?

1. The sum of the current entering equals the sum of the current leaving

2. The current must double as it enters a junction

3. The current remains constant regardless of pathways

4. The current is split equally among all paths

The correct answer is: The sum of the current entering equals the sum of the current leaving

In electrical circuits, Kirchhoff's first law, often referred to as the junction rule, states that at any junction in an electric circuit, the total current entering the junction must equal the total current leaving the junction. This principle is derived from the conservation of electric charge, meaning that charge cannot accumulate at a junction; whatever charge comes in must go out. When applying this law, if you visualize a junction where multiple branches of a circuit converge, the currents flowing into the junction from different paths must be balanced by the currents flowing out. For example, if three currents enter a junction and two exit, the sum of those entering must equal the sum of those exiting. This ensures that charge is conserved and does not build up at the junction. The other options do not accurately represent the behavior of current flow at a junction according to Kirchhoff’s first law.