10.2.5Electricity and D.C. circuits
Solving Problems with Kirchhoff’s Laws
5 min read 6 parts Lesson 20 of 26 1 quiz
Lesson summary
10.2.5 Solving Problems with Kirchhoff’s Laws — AS PHYSICS 9702 notes: Apply Kirchhoff’s first and second laws to find unknown currents and potential differences.Identify each junction and draw arrows for the currents, from positive to negative.Write total current into the juncti…
- Series and parallel
- Combined resistance
- Kirchhoff’s first law
- Kirchhoff’s second law
- Reciprocal
- R = R1 + R2 +
Section 1 of 6Applying the first law
1
Identify each junction and draw arrows for the currents, from positive to negative.
2
Write total current into the junction = total current out.
3
A meter in series with a known current reads that current.
4
Rearrange for the unknown, for example 80 mA = 27 mA + A2 gives A2 = 53 mA.
5
Repeat at the next junction, for example 270 mA = A3 + 80 mA gives A3 = 190 mA.
Section 2 of 6Applying the second law
1
Choose a closed loop.
2
Write e.m.f. of the supply = sum of the p.d.s around the loop.
3
Identical components with the same current share the p.d. equally.
4
Rearrange for the unknown, for example 24 V = 2 V + V3 gives V3 = 22 V.
Section 3 of 6Combining both laws
Currents
- Currents use the first law at junctions.
P.d.s
- P.d.s use the second law around loops.
Series and parallel
- Series and parallel the p.d. is the same across parallel branches, and the current is the same in series sections.
Section 4 of 6More from the specification
More from the specification
Joule heating dissipation power
- Joule heating dissipation power applying Kirchhoff's loop laws allows current calculation to evaluate electrical power dissipated across each resistor branch.
Ammeter
- Component that measures current.
Voltmeter
- Instrument that measures p.d., connected in parallel.
Charge
- Quantity of electricity in coulombs, Q = It.What the symbols mean: Q = charge (C) · I = current (A) · t = time (s)
Series circuit
- Circuit with components connected one after another in a single loop.
Parallel circuit
- Circuit with branches; same p.d. across each branch.
Closed circuit
- Complete loop with a source in which current can flow
Energy transferred
- W = V Q, in joules.What the symbols mean: E or W = energy transferred (J) · Q = charge (C) · V = potential difference (V) · I = current (A) · t = time (s)
Combined resistance
- Combined resistance in series R = R1 + R2; in parallel 1/R = 1/R1 + 1/R2, so take the reciprocal at the end.
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| Junction | Point where at least three circuit paths meet. |
| Loop | Complete closed path in a circuit. |
| Kirchhoff’s first law | Current in equals current out at a junction. |
| Kirchhoff’s second law | E.m.f. equals the sum of the p.d.s around a loop. |
Section 6 of 6Exam technique
1
Mixing currents in and out of a junction — draw arrows first.
2
Forgetting to include all components in a loop.
3
Splitting the p.d. unequally for identical series components.
4
Forgetting to convert mA to A when using other equations.