Conservation in Circuits


Conservation of Charge

As charge flows through a circuit, it doesn't get used or lost. This means that the current that flows in is the same amount as the current that flows out. (Current is the rate of flow of charge)

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Kirchhoff's first law

The total current entering a junction = the total current leaving it.

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Conservation of energy

Energy is conserved and never lost. In electrical circuits, energy is transferred round the circuit. Energy transferred to a charge is e.m.f (electromotive force), and energy transferred from a charge is potential difference. In a closed loop, these two quantities must be equal if energy is conserved (which it is).

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Kirchhoff's second law

The total e.m.f. around a series circuit = the sum of the p.d.s across each component

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Series Circuit

  • There will be the same current at all points of the circuit (since there are no junctions)
  • The e.m.f. is split between the components (by Kirchhoff's 2nd law)
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Parallel Circuit

  • The current is split at each junction
  • I = I1 +I2 + 13
  • There is the same p.d. across all components - there are three separate loops and within each loop the e.m.f. equals the sum of the individual p.d.s
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