16.1 DC Circuits, Ohm’s Law & Power
Key Takeaways
- EPC 5 is a critical capability: determine resistance, voltage, current and power in any part of a DC circuit using theory and actual measurement
- Ohm’s law V = I × R and power P = VI = I²R = V²/R are the four relationships every capstone calculation reduces to
- Series circuits share one current and divide voltage; parallel circuits share one voltage and divide current, with total resistance always smaller than the smallest branch
- An open circuit gives zero current and full source voltage across the break; a short circuit gives near-zero resistance, very high current and is what protective devices exist to clear
- Ammeters connect in series and voltmeters in parallel — reversing them either reads nothing useful or creates a deliberate short across the supply
DC Circuits, Ohm’s Law & Power
Quick Answer: A practical circuit needs a source, a protective device, a switch, conductors and a load. V = I × R and P = VI = I²R = V²/R solve any part of it. Series circuits share current; parallel circuits share voltage. An open circuit means no current; a short circuit means very high current and is exactly what your protection is sized to clear.
Why This Sits in the Critical List
Candidates who spend all their revision on AS/NZS 3000 tables get caught here. EPC 5 is classified as critical, and UEEEL0039’s performance evidence explicitly requires safely measuring the parameters for the whole or any part of a d.c. circuit. That is a hands-on item: you will be asked to build or interrogate a circuit, take readings and explain them.
The Five Components of a Practical Circuit
| Component | Purpose | Failure consequence |
|---|---|---|
| Energy source | Provides the electromotive force (EMF) that drives current | No EMF, no current |
| Protective device | Disconnects on overload or fault | Cable damage, fire, shock risk |
| Switch | Provides intentional control of the load | Load cannot be turned off safely |
| Conductors | Carry current with acceptable voltage drop | Overheating, excessive drop |
| Load | Converts electrical energy to heat, light, motion or chemical change | Circuit does nothing useful |
Expect a written stem that shows a sketch and asks you to name each element and its function. Answer with purpose, not just the name.
Ohm’s Law and the Power Family
V = I × R, rearranged as I = V / R and R = V / I.
P = V × I, and by substitution P = I²R and P = V² / R.
Worked fragment: a 240 Ω heating element on a 240 V DC supply draws I = 240 / 240 = 1 A and dissipates P = 240 × 1 = 240 W. Halve the resistance to 120 Ω and, at the same supply voltage, current doubles to 2 A and power doubles to 480 W (P = V²/R).
Always state which quantity is held constant before reasoning about a change. At fixed voltage, halving R doubles the power. At fixed current, halving R halves the power (P = I²R). Mixing the two up is the most common arithmetic trap in this EPC.
Series Circuits
- The same current flows through every component.
- Rₜ = R₁ + R₂ + R₃ …
- Source voltage divides across the components in proportion to their resistance.
Worked example: 10 Ω, 20 Ω and 30 Ω in series across 120 V.
- Rₜ = 10 + 20 + 30 = 60 Ω
- I = 120 / 60 = 2 A (through all three)
- V₁ = 2 × 10 = 20 V; V₂ = 2 × 20 = 40 V; V₃ = 2 × 30 = 60 V
- Check: 20 + 40 + 60 = 120 V ✓
Series behaviour is why a poor termination matters. An unintended 5 Ω of joint resistance in a 2 A circuit drops 10 V and dissipates 20 W at that joint — a hot spot in a wall cavity.
Parallel Circuits
- The same voltage appears across every branch.
- 1/Rₜ = 1/R₁ + 1/R₂ + 1/R₃ …
- Branch currents add to give the total.
Worked example: 12 Ω and 24 Ω in parallel across 24 V.
- I₁ = 24 / 12 = 2 A; I₂ = 24 / 24 = 1 A
- Iₜ = 2 + 1 = 3 A
- Rₜ = 24 / 3 = 8 Ω — note this is smaller than the smallest branch (12 Ω)
Sanity rule: total parallel resistance is always less than the smallest individual branch. If your answer is bigger, you inverted something.
This is exactly why adding more points to a final subcircuit increases total current: the loads are in parallel, so total resistance falls and current rises. Division of circuits (Chapter 8) exists because of this arithmetic.
Open, Closed and Short Circuits
| Condition | Resistance | Current | What you measure |
|---|---|---|---|
| Open | Effectively infinite | Zero | Full source voltage across the break; zero across the load |
| Closed (normal) | Load resistance | Design current | Rated voltage across the load |
| Short | Near zero | Very high, limited only by source and conductor impedance | Collapsed voltage; protective device operates |
An open circuit is why a continuity test reads infinity on a broken protective earthing conductor. A short circuit is the condition that sets breaking capacity (Chapter 7) and drives the adiabatic conductor check (Chapter 10).
Measuring — the Practical Half of EPC 5
- Ammeter in series with the circuit element whose current you want. Break the circuit and insert the meter.
- Voltmeter in parallel across the element whose voltage you want. Never break the circuit.
- Ohmmeter only on a de-energised, isolated circuit — and disconnect parallel paths, or you measure the network, not the component.
- Select the range before connecting; start high and work down on analogue instruments.
- Confirm the instrument is within calibration and the leads are proved on a known source.
The classic capstone practical failure is connecting an ammeter across a supply. Because an ammeter is deliberately near-zero resistance, that is a bolted short — a serious defect and a genuine flash hazard.
Calculating Circuit Behaviour When a Parameter Changes
UEEEL0039 requires you to determine circuit behaviour for a variation in any of the parameters from measured and calculated values. The method:
- Write down what is fixed (usually supply voltage).
- Recalculate total resistance after the change.
- Recalculate total current from the fixed voltage.
- Work back out to individual branch voltages and currents.
- State the practical consequence — more current means more voltage drop, more heating and closer to the protective device rating.
A 15 Ω and a 30 Ω resistor are connected in parallel across a 60 V DC supply. What is the total current drawn from the supply?
What will a voltmeter read across the break in an open-circuited series lighting circuit that is still energised?
Why must an ammeter never be connected directly across a supply?
A 20 Ω element is supplied at 240 V. What power does it dissipate?