D.C. Circuits · revision map · A-Level 9702, Topic 10

TOPIC 10 · D.C. CIRCUITS
CAMBRIDGE INTERNATIONAL AS & A LEVEL PHYSICS 9702 · ONE-PAGE REVISION MAP
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D.C. CIRCUITS10
Practical Circuits & E.M.F.

A real cell is not a perfect source.

  • E.m.f. E is the energy given per unit charge: E = W / Q.
  • Internal resistance r wastes the ‘lost volts’ Ir inside the cell.
  • Terminal p.d. V falls below E once a current flows.
  • E = V only when I = 0 (open circuit).
V = E − Ir E = I(R + r)
E r I R terminal p.d. V across R
Kirchhoff's Laws

Two conservation laws rule every circuit.

  • First law (charge): current into a junction equals current out of it. Charge is conserved.
  • Second law (energy): around any loop the sum of e.m.f.s equals the sum of p.d.s. Energy is conserved.
ΣIin = ΣIout ΣE = ΣIR
I₁ I₂ I₃ I₁ = I₂ + I₃ at the junction
Combining Resistances

The two laws give the rules for adding resistors.

  • Series: same current, p.d.s add (second law).
  • Parallel: same p.d., currents add (first law).
  • Series total is larger; parallel total is smaller.
series: R = R₁ + R₂
parallel: 1/R = 1/R₁ + 1/R₂
series R₁ R₂ parallel R₁ R₂
Potential Dividers

Split a voltage in the ratio of resistances.

  • Two series resistors share the supply by ratio.
  • Swap R₂ for a thermistor or LDR to sense and switch.
  • A potentiometer compares p.d.s by a null method: balance when the galvanometer reads zero.
Vout = R₂/(R₁ + R₂) × Vin
Vin R₁ R₂ Vout
BUILDS ON · Electricity · Energy → D.C. CIRCUITS → LEADS TO · Electric Fields · Capacitance · A.C.
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