Kirchhoff's laws — common mistakes

When applying KVL, always assume a direction for your loop traversal. If you move through a component in the assumed direction of current and it's a resistor, it's a potential drop (negative). If you move from negative to positive terminal of a cell, it's a potential rise (positive e.m.f.). If your calculated current turns out negative, it simply means your initial assumed direction was opposite to the actual flow!

Kirchhoff's laws are fundamental because they are direct applications of the conservation of electric charge and energy, two bedrock principles of physics. They provide a systematic method to analyse any complex circuit, even those that cannot be easily simplified using series and parallel resistance rules alone.

Kirchhoff's laws provide the framework for setting up equations for complex circuits, while Ohm's Law (V = IR) is often used within those equations to express the potential difference across individual resistors. You combine Ohm's Law with KCL and KVL to solve for unknown currents and voltages.

'Lost volts' refer to the potential difference that drops across a power source's internal resistance when current flows. They are important because they reduce the actual voltage available to the external circuit (terminal potential difference), meaning the e.m.f. of the source is not entirely delivered to the load.

E.m.f. (electromotive force) is the total energy supplied per unit charge by the source. Terminal potential difference is the actual voltage measured across the terminals of the source when current is flowing. The terminal p.d. is always less than the e.m.f. for a discharging cell because some voltage is 'lost' across the internal resistance (V = ε - Ir).