9702 · 10.2
Kirchhoff's laws — FAQ
Frequently asked questions for 9702 Kirchhoff's laws. Direct answers first, then deeper explanation — then practise with marking.
Why are Kirchhoff's laws considered fundamental in circuit analysis?
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.
How do Kirchhoff's laws relate to Ohm's Law?
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.
What are 'lost volts' and why are they important?
'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.
What's the difference between e.m.f. and terminal potential difference?
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).