Voltage, Current and Resistance
The physical meaning of the three core electrical quantities and how they appear in real circuits.
In this lesson
- Learning outcomes
- Core theory
- Trade application
- Worked example
- Workshop task
- Fault-finding notes
- Revision questions and answers
Learning outcomes
- Explain the purpose of this topic in everyday electrical work.
- Identify the circuit conditions that must be checked before relying on a reading.
- Apply the relevant calculation, test or drawing interpretation in a supervised training scenario.
- Recognise common apprentice mistakes and unsafe assumptions.
- Record evidence in a form that another tradesperson can understand.
Core theory
Direct current theory is the simplest place to learn circuit behaviour because polarity is constant and the mathematical relationships are visible. Voltage is electrical pressure, current is the rate of charge flow, and resistance is opposition to that flow. The important point is not to memorise the words, but to predict what will happen when one variable changes.
In a series path, the same current flows through every part because there is only one route for charge. In a parallel network, the voltage is common across branches but current divides according to branch resistance. Most real installations combine both ideas: a supply and protective device feeding multiple loads, each with controls, terminals and conductors that add small but important resistance.
Measurement changes the circuit only slightly when the correct meter function is selected. A voltmeter is placed in parallel because it has high internal resistance. An ammeter is placed in series because current must pass through it. An ohmmeter must not be used on an energised circuit because it injects its own small test signal and can be damaged or give nonsense readings.
Core law
Use the formula only after identifying which part of the circuit the values refer to. A voltage measured across a load, a switch, a fuse or an open conductor does not mean the same thing. The apprentice should always say out loud: “voltage across what, current through what, resistance of what?”
Worked example
A 24 V training circuit supplies a 12 Ω lamp. Current is 24 ÷ 12 = 2 A. If a poor terminal adds 3 Ω in series, total resistance becomes 15 Ω and circuit current falls to 1.6 A. The lamp may still glow, but dimly. The fault symptom is reduced load performance; the cause is unwanted series resistance.
Textbook depth: applying Ohm’s Law without guessing
Ohm’s Law is not only a classroom formula. It is the first diagnostic model for any simple resistive part of a circuit. The key word is part. You can apply V = I × R to the whole circuit, a single load, a conductor, a switch contact, a fuse element, a relay contact or an unwanted high-resistance joint, but only if the voltage, current and resistance refer to the same section.
For example, a lamp circuit may have 230 V at the circuit origin, 229 V at the switch feed, 228 V at the load terminal with the lamp disconnected, and only 170 V at the lamp when the lamp is connected. The unloaded readings may look normal because almost no current is flowing. Under load, a poor termination or damaged conductor drops voltage. This is why electricians test both presence of voltage and circuit behaviour under realistic load conditions where safe and permitted.
| Reading | Likely meaning | Next check |
|---|---|---|
| Correct voltage with load disconnected, low voltage with load connected | High resistance in supply or return path | Measure voltage drop across switches, joins and conductors under load |
| Voltage present on both sides of an open switch | May be induced/back-fed or no effective return path | Confirm reference point and test with an appropriate load/test method |
| Fuse/MCB operates immediately | Possible short or earth fault | Isolate and perform dead testing before re-energising |
Trade application
On site, this topic is rarely isolated. It connects to safety, drawings, protection, cable selection, terminations, testing and documentation. A good apprentice does not ask only “does it work?” They ask whether it is correctly supplied, correctly protected, correctly controlled, mechanically sound, suitable for the environment, and verifiable by inspection and test.
When using this material, build a notebook of standard methods. For each topic, write the normal value, the likely fault value, the test points, the instrument setting, and the action to take if the result is abnormal. This becomes a practical diagnostic map rather than a collection of memorised definitions.
Workshop practical
Build three low-voltage circuits on a training board: one series lamp circuit, one parallel lamp circuit, and one combined series-parallel circuit. Predict voltage drops and currents before measuring. Record where your prediction did not match and explain whether the difference came from lamp resistance, battery sag, connection resistance or meter resolution.
Evidence to collect: labelled sketch, predicted readings, actual readings, explanation of differences, supervisor feedback and one improvement to your method.
Fault-finding notes
- Confirm the complaint or task requirement in plain language.
- Compare the installation against the drawing, label or expected circuit arrangement.
- Prove whether supply is present at the correct point and under the correct condition.
- Divide the circuit into smaller sections instead of testing random points.
- After repair, test the protective measure, not just the load operation.
Common apprentice mistakes
| Mistake | Why it matters | Better habit |
|---|---|---|
| Measuring voltage without a reference plan | The reading may be real, induced, back-fed or meaningless without a return path. | State the exact two points being measured and the expected value first. |
| Assuming a device is faulty because it is not operating | The fault may be supply, control, protection, return path, settings or mechanical load. | Prove each section of the circuit in sequence. |
| Recording only pass/fail | Future workers cannot see whether results were strong, marginal or abnormal. | Record actual values, conditions and instrument details. |
Assessment standard
The assessor will expect correct meter placement, correct units, safe low-voltage practice, and an explanation that links readings back to Ohm’s Law and Kirchhoff’s laws.
Revision questions
- What should be proven before this task is attempted on real equipment?
- Which measurement would best confirm the main idea of this lesson?
- What reading or symptom would make you stop and ask for supervision?
- How could a poor termination change the behaviour of this circuit?
- What information should be recorded for handover or assessment evidence?
Suggested answers
- Isolation, correct circuit identification, suitable supervision, correct instrument condition and an agreed safe work method.
- The measurement depends on the lesson: voltage across a component, current through a load, resistance/continuity of a path, insulation resistance between conductors, or operation time of a protective device.
- Unexpected voltage, unstable readings, signs of heat, damaged insulation, repeated protective-device operation, or any result that conflicts with the drawing.
- It can add resistance, create heat, reduce load voltage, cause intermittent operation, distort test results or prevent protective devices operating as expected.
- Circuit ID, test conditions, instrument used, actual readings, corrective actions, variations from the drawing and supervisor sign-off where required.