D.C. circuits
This is where circuit theory becomes rigorous. We start with the circuit symbols and diagrams the examiner expects, then define electromotive force (e.m.f.) properly and separate it cleanly from potential difference. We meet the internal resistance of a real cell and the equation V = E − Ir. Then the two great accounting rules of every circuit: Kirchhoff's first law (conservation of charge) and second law (conservation of energy). From those two laws we DERIVE the combined resistance in series (R = R₁ + R₂ + …) and in parallel (1/R = 1/R₁ + 1/R₂ + …), solve a multi-loop circuit, and finish with the potential divider — including how a thermistor or an LDR turns it into a temperature or light sensor.
By the end you should be able to (NSSCAS Physics (AS) 3.3):
- Recall and use the standard circuit symbols and draw and interpret circuit diagrams containing sources, switches, resistors, ammeters, voltmeters, thermistors and light-dependent resistors
- Define electromotive force (e.m.f.) of a source as the energy transferred per unit charge in driving charge round a complete circuit, and distinguish e.m.f. from potential difference
- Discuss the effect of the internal resistance of a source on the terminal potential difference and output power, and recall and use V = E − Ir
- State Kirchhoff's first law and link it to conservation of charge; state Kirchhoff's second law and link it to conservation of energy
- Derive, using Kirchhoff's laws, the formula for the combined resistance of resistors in series (R = R₁ + R₂ + …) and solve problems with it
- Derive, using Kirchhoff's laws, the formula for the combined resistance of resistors in parallel (1/R = 1/R₁ + 1/R₂ + …) and solve problems with it
- Apply Kirchhoff's laws to solve a multi-loop circuit problem
- Explain the potential divider as a source of variable p.d., recall and use Vout = Vin × R₂ ÷ (R₁ + R₂), and explain the use of thermistors (NTC) and LDRs in potential dividers
Miss Elizabeth and Mike talk through the whole topic — with the figure and working drawn live.