English:Voltage, Current, and Resistance
Voltage, Current, and Resistance
Introduction
Electrical technicians work with voltage, current, and resistance every day. These three quantities help you understand why a circuit operates normally, why a fuse opens, why a motor receives too little voltage, or why a damaged connection becomes hot. In this course, you will connect the theory of electric circuits with practical measurement, safe workshop routines, and fault-finding.
Learning goals: By the end of the course, you should be able to explain voltage, current, and resistance in clear technical language; use their units and symbols correctly; apply Ohm's law; predict how series and parallel circuits behave; choose an appropriate digital multimeter function; interpret measurements; and use evidence from a circuit to support a troubleshooting decision.
The diagram above shows the essential idea of a closed circuit: a source provides electrical potential difference, a conductive path allows charge to move, and a load uses electrical energy. In real vocational work, the circuit may be part of a control panel, vehicle, machine, building installation, sensor system, or electronic device.
Workshop focus: Use extra-low-voltage training circuits for learner experiments unless your instructor has established a different safe system of work. Work on mains or other hazardous voltages only when you are trained, authorised, supervised as required, and following the applicable workplace rules.
Core Electrical Quantities
Voltage
Voltage is the electric potential difference between two points. It describes how much energy is transferred per unit of electric charge between those points. The symbol is usually V and the SI unit is the volt, also written V.
A voltage measurement always compares two points. For example, a 24 V control supply has a potential difference of approximately 24 volts between its designated terminals. A voltmeter or digital multimeter in voltage mode is therefore connected across the two points of interest.
Useful prefixes include millivolt, where 1 mV = 0.001 V, and kilovolt, where 1 kV = 1000 V. Vocational technicians should read both the number and the prefix before making a decision.
Current
Electric current is the rate at which electric charge passes a point in a circuit. The symbol is usually I and the SI unit is the ampere, also written A.
A current of 1 ampere corresponds to 1 coulomb of charge passing a point each second. In a metal conductor, electrons move through the material, but conventional current direction is defined from positive toward negative through the external circuit.
Current must have a continuous conductive path. If the path is open, steady current cannot flow through that branch. Excessive current can overheat conductors and components, which is why circuits use correctly selected protective devices such as fuses and circuit breakers.
Useful prefixes include milliampere, where 1 mA = 0.001 A, and microampere, where 1 µA = 0.000001 A.
Resistance
Electrical resistance describes how strongly a component or material opposes current for a given voltage. The symbol is R and the SI unit is the ohm, written Ω.
A higher resistance produces less current when the applied voltage is unchanged. Resistance is not simply a fault: many components are designed to provide a particular resistance so that current, voltage, timing, sensing, or heat production can be controlled.
Fixed resistors often use coloured bands to identify resistance and tolerance. In practical work, you should compare the marked or colour-coded value with a measured value and with the permitted tolerance in the component specification.
Useful prefixes include kilohm, where 1 kΩ = 1000 Ω, and megohm, where 1 MΩ = 1,000,000 Ω.
Ohm's Law
Georg Simon Ohm investigated relationships between current and voltage in conductors. The relationship now called Ohm's law is written:
V = I × R
For an ohmic component in the operating range where its resistance is effectively constant, the same relationship can be rearranged to:
I = V / R
R = V / I
You can use Ohm's law when you know two of the three quantities and need to calculate the third. Always include units and check whether the result makes physical sense. For example, increasing resistance while keeping voltage constant should reduce current.
Ohm's law is not a statement that every device always has constant resistance. Lamps, semiconductor devices, heating elements, and other components can have resistance that changes with temperature, voltage, current, or operating state. For these devices, a current-voltage characteristic may be more useful than one fixed resistance value.
A straight current-voltage relationship through the origin is characteristic of an ohmic resistance over the range shown. A curved relationship indicates that the resistance is changing.
Worked Examples
| Situation | Method | Result | Practical meaning |
|---|---|---|---|
| A 24 V supply is connected across a 120 Ω resistor. | I = V / R = 24 V / 120 Ω | 0.20 A | The expected current is 200 mA. |
| A 12 V load draws 2 A. | R = V / I = 12 V / 2 A | 6 Ω | The operating resistance at that point is 6 Ω. |
| A resistor carries 0.30 A and has 80 Ω resistance. | V = I × R = 0.30 A × 80 Ω | 24 V | The voltage across the resistor is 24 V. |
A measured value that is far from a calculated value is useful evidence. It may indicate a wrong assumption, incorrect meter setup, an open circuit, a short circuit, a high-resistance connection, a damaged component, or a changing operating condition.
Measuring with a Digital Multimeter
A digital multimeter can combine several measurement functions in one instrument. The exact controls and limits vary by model, so always check the meter instructions, lead ratings, fuse ratings, measurement category, and maximum input values before use.
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Voltage Measurement
To measure voltage, set the meter to the correct voltage function and place the probes in parallel across the two points being compared. The circuit may need to be energised for a functional voltage measurement, so this task requires an appropriate risk assessment, correct test equipment, and the competence required by your workplace.
For a DC measurement, polarity affects the sign shown by the meter. If the red probe is at a lower potential than the black probe, many digital meters display a negative value.
A useful troubleshooting technique is to compare the expected voltage with the measured voltage at several points. This can show where a voltage drop or loss occurs.
Current Measurement
To measure current with a conventional ammeter function, the meter becomes part of the current path. It is connected in series, which normally means opening the circuit and inserting the meter.
Before connecting, confirm that the leads are in the correct sockets and that the selected current range can safely handle the expected current. Many digital multimeters have separate fused inputs for small current and larger current ranges. A wrong socket or range can damage the fuse, the meter, the circuit, or create a hazard.
A clamp meter can measure current in a single conductor without opening the circuit, but you must still select suitable equipment and follow the manufacturer's instructions.
Resistance and Continuity Measurement
Resistance measurement is normally carried out on a de-energised circuit or isolated component. Remove power, control stored energy as required, and verify the safe condition using the approved procedure for your workplace before taking the resistance measurement.
A resistance meter applies its own small test signal. Measuring resistance on an energised circuit can produce incorrect readings and may damage the meter or create a hazard.
Continuity mode checks whether a low-resistance path exists. A continuity beep does not prove that a circuit can safely carry its normal load current, and it does not replace required insulation, protective conductor, or functional tests.
Safe Working Practice
Electrical testing can expose you to shock, arc, fire, and equipment-damage hazards. Good vocational practice is to plan the task before touching the circuit.
- Risk assessment: Identify the voltage level, available fault energy, environment, exposed conductors, stored energy, and the exact test you need.
- Safe isolation: De-energise whenever possible, prevent unintended re-energisation, and verify the condition using an approved procedure.
- Test equipment: Use a meter, probes, and leads with suitable ratings for the system and measurement category; inspect them before use.
- Meter setup: Select the correct function, range, and input sockets before making contact with the circuit.
- Personal protective equipment: Use the PPE required by your risk assessment, workplace rules, and local regulations.
- Qualified person: Perform hazardous live testing only when you have the required training, authorisation, and competence.
For additional workplace guidance, see the Health and Safety Executive resource Electrical test equipment for use on low voltage electrical systems and the Occupational Safety and Health Administration rule Selection and use of work practices. Your local law, employer procedure, and instructor requirements take priority for your actual workplace.
Series and Parallel Circuits
The way components are connected changes current, voltage, and total resistance.
| Feature | Series circuit | Parallel circuit |
|---|---|---|
| Current | The same current passes through each series component. | Total current divides among branches and is the sum of branch currents. |
| Voltage | Supply voltage is shared among series components. | The same branch voltage appears across parallel branches connected to the same two nodes. |
| Equivalent resistance | Resistances add: R_total = R1 + R2 + ... | For two or more positive resistances, the equivalent resistance is lower than the smallest individual branch resistance. |
| Open fault | An open can stop current through the whole series path. | An open branch can stop only that branch while other branches may continue to operate. |
For two resistors in parallel, you can calculate equivalent resistance with:
1 / R_total = 1 / R1 + 1 / R2
For two resistors only, the product-over-sum shortcut is:
R_total = R1 × R2 / (R1 + R2)
These relationships are valuable in troubleshooting because the location of a fault changes what you expect to measure.
Resistance in Real Components and Cables
Resistance depends on material, length, cross-sectional area, and temperature. For a uniform conductor:
R = ρ × L / A
Here, ρ is resistivity, L is conductor length, and A is cross-sectional area. A longer wire usually has more resistance, while a larger cross-sectional area usually has less resistance if the material and temperature are unchanged.
For many metals, resistance increases as temperature rises. This matters in motors, heaters, cables, and sensors. A cold-resistance measurement may therefore differ from the effective resistance during operation.
When diagnosing a cable or connection, a small unwanted resistance can still matter if current is high. The power converted to heat in a resistance can be estimated by:
P = I² × R
The same electrical power can also be written as P = V × I. This is why a loose or corroded connection can overheat under load even when a low-current continuity test appears acceptable.
Troubleshooting by Reasoning
A good technician does not replace parts only because a reading "looks wrong." First state what you expected, then measure, compare, and explain the difference.
| Observation | Possible interpretation | Useful next check |
|---|---|---|
| Supply voltage is correct but load current is zero. | The current path may be open, or the load may not be enabled. | Check control conditions, continuity on a safely isolated circuit, and voltage across suspected open points. |
| Current is higher than expected. | Effective resistance may be too low, or an unintended parallel path may exist. | Isolate the circuit and compare component resistance and wiring with the specification. |
| A load receives less voltage than expected under load. | There may be excessive series resistance in wiring, terminals, contacts, or the source. | Measure voltage drops across individual parts while following the approved test procedure. |
| A connector becomes hot. | Contact resistance may be creating I²R heating. | Isolate safely, inspect the connection, and compare its condition and resistance with acceptable limits. |
| A fuse opens repeatedly. | The circuit may have excessive current caused by a fault or overload. | Do not bypass the fuse; identify the load current and fault path before replacing components. |
Practical Workshop Routine
Use this routine on an instructor-approved extra-low-voltage training circuit:
- Identify the source voltage, load resistance, circuit arrangement, and expected current from the diagram.
- Calculate the expected current or voltage with Ohm's law before measuring.
- Inspect the meter, leads, sockets, range, and circuit connections.
- Measure one quantity at a time using the correct connection method.
- Record the measured value with its unit and prefix.
- Compare measured and expected values, then explain any difference before changing the circuit.
This predict-measure-explain routine develops both technical accuracy and disciplined troubleshooting.
Interactive Tasks
Quiz: Test Your Knowledge
Which SI unit is used for voltage? (Volt) (!Ampere) (!Ohm) (!Watt)
Which SI unit is used for electric current? (Ampere) (!Volt) (!Ohm) (!Joule)
Which SI unit is used for resistance? (Ohm) (!Ampere) (!Volt) (!Coulomb)
A twelve volt supply is connected across six ohms. What current should flow? (Two amperes) (!One ampere) (!Six amperes) (!Twelve amperes)
How is a voltmeter connected to compare two points in a circuit? (In parallel) (!In series) (!Across a fuse only) (!Without test leads)
How is a conventional ammeter connected to measure branch current? (In series) (!In parallel) (!Across the supply) (!Across an open switch)
What is the normal condition for measuring resistance with a digital multimeter? (Deenergised circuit) (!Energised circuit) (!Maximum current) (!Shorted supply)
What remains the same through components connected in one series path? (Current) (!Voltage) (!Resistance) (!Power)
What is the same across parallel branches connected to the same two nodes? (Voltage) (!Current) (!Resistance) (!Temperature)
At constant voltage what happens to current when resistance increases? (Current decreases) (!Current increases) (!Current doubles) (!Current stays fixed)
Memory Game
| Voltage | Electric potential difference between two points |
| Current | Rate of flow of electric charge |
| Resistance | Opposition to current for a given voltage |
| Voltmeter | Instrument function connected across two points |
| Ammeter | Instrument function inserted in the current path |
| Continuity | Check for a low resistance conductive path |
Drag and Drop
| Match the correct terms. | Topic |
|---|---|
| Connect across two points | Voltage measurement |
| Insert into the current path | Current measurement |
| Deenergise and isolate before testing | Resistance measurement |
| Same current through every component | Series circuit |
| Same voltage across each branch | Parallel circuit |
...
Crossword Puzzle
| Voltage | What quantity describes electric potential difference between two points? |
| Current | What quantity is the rate of flow of electric charge? |
| Resistance | What quantity is measured in ohms? |
| Ampere | What SI unit measures electric current? |
| Parallel | What circuit arrangement connects branches across the same two nodes? |
| Continuity | What meter function checks for a low resistance path? |
LearningApps
Cloze Text
Open-Ended Tasks
Easy
- Circuit Quantity Poster: Create a one-page poster that explains voltage, current, and resistance with symbols, units, one workplace example, and one clear diagram.
- Safe Meter Checklist: Produce a checklist for inspecting a digital multimeter, leads, sockets, function, range, and test environment before an instructor-approved low-voltage measurement.
- Ohm's Law Calculation Card: Make a pocket-sized reference card that shows the three rearrangements of Ohm's law and includes two solved extra-low-voltage examples with units.
- Resistor Photo Hunt: Photograph or sketch at least four resistors from approved training equipment, decode their markings or colour bands, and compare the stated values with component data.
Standard
- Low Voltage Measurement Lab: Build an instructor-approved extra-low-voltage resistor circuit, predict voltage and current, measure them safely, and explain the difference between calculated and measured values.
- Series and Parallel Investigation: Construct one series circuit and one parallel circuit with the same two resistors, record branch or series measurements, and explain how the circuit arrangement changes total resistance and current.
- Technician Interview: Interview an electrician, electronics technician, automotive technician, or maintenance technician about how voltage, current, and resistance measurements support fault-finding, then write a one-page summary.
- Troubleshooting Video: Record a short training video in which you diagnose a teacher-created low-voltage open-circuit or high-resistance fault using a predict-measure-explain method.
Advanced
- Voltage Drop Investigation: Create a low-voltage test setup with a deliberately added safe series resistance, measure voltage drops under two load currents, and explain the results using Ohm's law and power.
- Sensor Circuit Design: Design a safe low-voltage resistive sensor or voltage-divider circuit, calculate expected voltages for at least three conditions, then compare your design predictions with measurements or simulation.
- Maintenance Fault Report: Analyse a realistic machine or control-circuit fault scenario, choose a logical sequence of voltage, current, and resistance checks, and write a professional report that justifies every measurement.
- Workplace Measurement Study: Visit an approved workshop, training centre, laboratory, or maintenance department and document how test instruments, safe isolation, equipment ratings, and measurement records are used in real work; present your findings as a report, infographic, or video.
Learning Assessment
- Diagnostic Reasoning Assessment: Given a circuit diagram, expected values, and several measured values, identify the most likely fault location and justify the conclusion using voltage, current, and resistance relationships.
- Meter Method Assessment: For three different measurement goals, select the meter function, connection method, range strategy, and safety checks, and explain why each choice is appropriate.
- Series Parallel Transfer Assessment: Compare two circuits that use the same resistors in different arrangements and predict how total current, branch current, and voltage distribution will change.
- Measurement Uncertainty Assessment: Compare a calculated resistor current with a measured value and evaluate whether component tolerance, supply variation, meter resolution, temperature, or a fault could explain the difference.
- Safety Decision Assessment: Analyse a proposed electrical test procedure, identify unsafe steps, and rewrite it as a safer procedure that follows the learner's workplace rules and competence limits.
- Fault Report Assessment: Write a short technician report that separates observations, measurements, calculations, interpretation, corrective action, and verification after repair.
Evidence of Learning
| Area | Evidence |
|---|---|
| Knowledge | You accurately explain voltage, current, resistance, Ohm's law, units, prefixes, series and parallel behaviour, and basic power relationships. |
| Skills | You select appropriate meter functions, connect instruments correctly on approved training circuits, calculate expected values, record measurements with units, and follow safe measurement routines. |
| Products | Your portfolio contains calculations, circuit diagrams, measurement tables, a troubleshooting record, a safety checklist, and at least one self-created technical communication product. |
| Reasoning | You compare expected and measured values, distinguish symptoms from causes, and justify fault-finding decisions with electrical evidence. |
| Transfer | You apply the same relationships to unfamiliar vocational contexts such as control systems, electronics, vehicles, machinery, building services, or maintenance tasks while respecting local safety procedures. |
OERs on the Topic
The central relationship connecting voltage, current, and resistance is explained in the English Wikipedia article on Ohm's law.
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