English:Current, Voltage, and Resistance
Current, Voltage, and Resistance
Introduction
Welcome to Current, Voltage, and Resistance. This aiMOOC is designed for learners in Grades 9–10. You will learn how three central electrical quantities describe what happens in simple circuits, how to measure them, and how to use Ohm's law to make predictions.
Electric circuits are part of everyday technology: flashlights, phones, sensors, computers, vehicles, and many other devices depend on controlled movement of electric charge. In this course, you will work mainly with simple direct-current circuits and low-voltage sources such as batteries or classroom power supplies.
A useful starting idea is that voltage provides an electrical potential difference, current describes the rate at which charge flows, and resistance describes how strongly a component opposes current. These ideas are connected, but they are not the same thing.
Learning Goals
By the end of the course, you should be able to explain current, voltage, and resistance in your own words; use the correct symbols and SI units; rearrange and apply Ohm's law; connect an ammeter and voltmeter correctly in a simple circuit; interpret basic current-voltage graphs; compare series and parallel circuits; and plan safe low-voltage investigations.
| Quantity | Symbol | SI unit | What it describes | Typical measuring instrument |
|---|---|---|---|---|
| Electric current | I | ampere, A | Rate of flow of electric charge | Ammeter |
| Voltage | V | volt, V | Electric potential difference between two points | Voltmeter |
| Resistance | R | ohm, Ω | Opposition to electric current | Ohmmeter or multimeter |
Electric Current
Electric current is the rate at which electric charge passes a point or cross-section in a circuit. Its symbol is I, and its SI unit is the ampere, symbol A. One ampere means one coulomb of charge passes a point each second.
In metal wires, electrons are the mobile charge carriers. The conventional direction of current is defined as the direction positive charge would move, from the positive terminal of a source through the external circuit toward the negative terminal. In metals, the electrons drift in the opposite direction. Both descriptions are useful, but circuit diagrams normally use conventional current.
A current requires a complete conducting path. If a switch opens the path, the current stops. If the path is closed and a source maintains a potential difference, charge can continue to move through the circuit.
A higher current means more charge passes a point each second. It does not mean that individual electrons travel around the whole circuit at the same speed as the electrical effect is established.
Voltage
Voltage, also called electric potential difference, compares the electric potential energy per unit charge between two points. Its symbol is V, and its SI unit is the volt, also symbol V.
A battery or power supply can maintain a potential difference between its terminals. When a conducting path connects those terminals, the electric field in the circuit can drive charge. Voltage is therefore measured between two points, not "through" a component.
A useful but limited analogy is water in pipes: a pressure difference can drive water flow, while an electrical potential difference can drive charge flow. The analogy helps you separate the driving difference from the amount of flow, but electrical circuits are not literally pipes filled with moving water.
Resistance
Electrical resistance describes how strongly a component or material opposes electric current. Its symbol is R, and its SI unit is the ohm, symbol Ω. One ohm is one volt per ampere.
In a metal resistor, moving electrons interact with the material. Energy transferred in these interactions can increase the thermal energy of the component. Resistance depends on the material and geometry of the conductor and can also depend on temperature.
Resistors are manufactured in many values. Color bands can encode a resistor's nominal resistance and tolerance. In practical work, a multimeter can be used to check the resistance of an isolated component.
Resistance and Material Properties
For a uniform wire of a given material, a longer wire usually has greater resistance because charge carriers encounter more material along the path. A thicker wire usually has lower resistance because it provides a larger cross-sectional area for current. Different materials have different resistivities, so equal-sized wires made from different materials can have different resistances.
Resistance is not always constant. The resistance of a filament lamp changes strongly as the filament heats, and semiconductor components such as diodes have non-linear current-voltage behavior. This matters because Ohm's law in its simplest constant-resistance form applies only when the component is approximately ohmic under the conditions being studied.
Ohm's Law
For an ohmic component at approximately constant physical conditions, current is proportional to voltage. The relationship is written as:
V = I × R
You can rearrange it as:
I = V ÷ R
R = V ÷ I
The equation connects three quantities. If resistance stays constant and voltage increases, current increases in the same proportion. If voltage stays constant and resistance increases, current decreases.
Worked Examples
Example A: Find current. A 6 V battery is connected across a 12 Ω resistor. Using I = V ÷ R gives I = 6 V ÷ 12 Ω = 0.5 A.
Example B: Find resistance. A component has 9 V across it and carries 0.30 A. Using R = V ÷ I gives R = 9 V ÷ 0.30 A = 30 Ω.
Example C: Find voltage. A 40 Ω resistor carries 0.20 A. Using V = I × R gives V = 0.20 A × 40 Ω = 8 V.
When solving problems, first identify the quantity you need, write the correct form of the equation, substitute values with units, calculate, and then check whether the result is physically reasonable.
Current-Voltage Graphs
An ohmic resistor has a linear current-voltage relationship when its resistance remains constant. If current is plotted against voltage, the graph is a straight line through the origin. A steeper I-versus-V line represents lower resistance because the slope is I divided by V, which equals 1 divided by R.
A curved current-voltage graph shows that the ratio V divided by I is changing. Such a component is non-ohmic under those conditions. A filament lamp is a common school example because its temperature changes as current increases.
Measuring Current, Voltage, and Resistance
A multimeter can measure several electrical quantities, but you must select the correct function, range, ports, and connection method.
To measure current, an ammeter is placed in series so the current being measured passes through the meter.
To measure voltage, a voltmeter is connected in parallel across the component or two points being compared.
To measure resistance, disconnect power from the circuit and measure the component with the meter set to resistance. Measuring the resistance of a powered circuit can give misleading results and may damage equipment.
Measurement Quality
Good measurements include a value and a unit. Choose a range that can safely include the expected value, and record enough significant figures to match the instrument's resolution. Repeating measurements can help reveal random variation. If a result is surprising, check the circuit, meter mode, lead positions, and units before deciding that the physics is wrong.
Series and Parallel Circuits
In a series circuit, components form one path. The same current passes through each series component. The supply voltage is shared among the voltage drops across the components.
In a parallel circuit, components are connected across the same two nodes. Each branch has the same voltage across it, while the total current from the source is the sum of the branch currents.
For resistors in series, the equivalent resistance is the sum of the individual resistances. Adding another resistor in series therefore increases total resistance.
For resistors in parallel, adding another branch provides an additional path for current, so the equivalent resistance becomes smaller than the resistance of any individual branch.
Electrical Power and Energy Transfer
Electric power describes the rate of electrical energy transfer. For a circuit component:
P = V × I
where P is power in watts, V is voltage in volts, and I is current in amperes. Combining this with Ohm's law gives useful forms for resistive components, including P = I²R and P = V² ÷ R.
This helps explain why resistors can become warm: electrical energy is transferred to thermal energy. In engineering, components must be chosen not only for resistance but also for a suitable power rating.
History and Scientific Evidence
Georg Simon Ohm investigated how voltage, current, and conducting paths are related. His experimental work in the nineteenth century helped establish the relationship now called Ohm's law. The unit of resistance, the ohm, is named after him.
Ohm's law is an empirical relationship, meaning it summarizes a pattern found through measurement. It is extremely useful, but it is not a universal rule for every component under every condition. Scientific models are strongest when you know both where they work and where their assumptions break down.
Safe Practical Work
Use batteries or approved low-voltage classroom power supplies for student investigations. Never experiment directly with household mains electricity. Check that leads and components are undamaged, avoid short-circuiting batteries or power supplies, and switch off power before changing resistance measurements or rebuilding a circuit.
Resistors and lamps can become hot. Allow components to cool before touching them, and stay within the voltage, current, and power ratings given by your teacher or the equipment manufacturer.
Interactive Simulation and Exploration
Use the PhET Ohm's Law simulation to change voltage and resistance and observe how current responds. Then use the PhET Circuit Construction Kit: DC to build circuits, switch between lifelike and schematic views, and practice using virtual ammeters and voltmeters.
Before moving a slider, predict what will happen. Afterward, compare the result with your prediction and explain any difference using V = I × R.
Interactive Tasks
Quiz: Test Your Knowledge
What is the SI unit of electric current? (Ampere) (!Volt) (!Ohm) (!Watt)
What does voltage describe? (Electric potential difference) (!Rate of charge flow) (!Opposition to current) (!Rate of energy use only)
Which instrument is connected in series to measure current? (Ammeter) (!Voltmeter) (!Thermometer) (!Barometer)
Which instrument is connected in parallel to measure voltage? (Voltmeter) (!Ammeter) (!Ohmmeter) (!Galvanometer only)
What equation states Ohm's law for an ohmic resistor? (Voltage equals current times resistance) (!Current equals voltage times resistance) (!Resistance equals current times voltage) (!Voltage equals resistance divided by current)
If voltage doubles while resistance stays constant, what happens to current? (It doubles) (!It halves) (!It stays the same) (!It becomes zero)
If resistance doubles while voltage stays constant, what happens to current? (It halves) (!It doubles) (!It stays the same) (!It becomes infinite)
What is the same through all components in an ideal series circuit? (Current) (!Voltage) (!Resistance) (!Power)
What is the same across branches connected in parallel? (Voltage) (!Current) (!Resistance) (!Charge)
Which statement best describes an ohmic component under constant conditions? (Current is proportional to voltage) (!Current is always zero) (!Resistance increases with every voltage) (!Voltage is unrelated to current)
Memory Game
| Current | Rate of flow of electric charge |
| Voltage | Electric potential difference between two points |
| Resistance | Opposition to electric current |
| Ampere | SI unit of electric current |
| Ohm | SI unit of electrical resistance |
| Voltmeter | Instrument connected in parallel to measure potential difference |
| Ammeter | Instrument connected in series to measure current |
Drag and Drop
| Match the correct terms. | Topic |
|---|---|
| Rate of charge flow | Electric current |
| Potential difference | Voltage |
| Opposition to current | Resistance |
| Series measurement | Ammeter |
| Parallel measurement | Voltmeter |
Predict first, then check each match by explaining how the measurement or definition works in a real circuit.
Crossword Puzzle
| Ampere | What SI unit measures electric current? |
| Voltage | What quantity describes electric potential difference? |
| Resistance | What quantity opposes current in a component? |
| Resistor | What component is designed to provide a chosen resistance? |
| Circuit | What complete conducting path can carry electric current? |
| Ohmic | What word describes a component with proportional current and voltage under constant conditions? |
LearningApps
Cloze Text
Open-Ended Tasks
Easy
- Circuit Vocabulary Poster: Create a one-page visual poster that explains current, voltage, resistance, ampere, volt, and ohm in your own words and includes one original circuit sketch.
- Ohm's Law Photo Hunt: Photograph or sketch three safe battery-powered devices and annotate where you think voltage sources, conducting paths, and resistive loads are located.
- Multimeter Interview: Interview a teacher, technician, electrician, engineer, or experienced maker about how a multimeter is used and summarize three practical tips without working on live mains circuits.
- Explainer Video: Record a two-minute video that teaches a younger learner the difference between current and voltage using a simple model or analogy and one limitation of that analogy.
Standard
- Low-Voltage Circuit Investigation: Build a teacher-approved battery-and-resistor circuit, change one variable at a time, measure current and voltage, and present your results in a table.
- Current-Voltage Graph: Use measured or simulated data for a resistor to create an I-versus-V graph, calculate the resistance, and explain what the graph's straightness and slope mean.
- Series and Parallel Comparison: Build or simulate one series and one parallel circuit with the same source and comparable resistors, then compare current, voltage, equivalent resistance, and brightness or power behavior.
- Resistor Color Code Guide: Research the common resistor color code, create an illustrated guide, and verify at least three resistor values with a multimeter or reliable simulation.
Advanced
- Non-Ohmic Component Study: Investigate a filament lamp, diode, or another teacher-approved non-ohmic component using simulation or low-voltage equipment and explain why its current-voltage graph is not a straight line.
- Circuit Design Challenge: Design a low-voltage circuit that keeps the current through a chosen resistor below a specified safe value, show your Ohm's law calculations, and test the design in a simulator or supervised lab.
- Measurement Uncertainty Project: Repeat voltage and current measurements, estimate uncertainty from instrument resolution and variation, and discuss how uncertainty affects a calculated resistance.
- Electricity in a Workplace: Visit a science lab, makerspace, repair workshop, technical school, or engineering workplace with permission, document how current, voltage, and resistance are measured or controlled, and connect your observations to at least three course concepts.
Learning Assessment
- Predict and Explain: A resistor is connected to a fixed-voltage source and its resistance is doubled; predict the change in current and justify your answer mathematically and conceptually.
- Diagnose a Measurement Error: A learner connects an ammeter directly across a battery like a voltmeter; explain why the setup is incorrect, identify the risk, and draw a correct low-voltage measurement arrangement.
- Interpret Experimental Data: Given several voltage-current data pairs, determine whether the component is approximately ohmic, calculate an appropriate resistance value, and justify your conclusion from the pattern.
- Compare Circuit Designs: Compare a two-resistor series circuit with a two-resistor parallel circuit using the same source and resistor values, explaining how current paths, voltage distribution, and equivalent resistance differ.
- Transfer to Engineering: Choose a real low-voltage device and explain how designers must consider voltage, current, resistance, and power when selecting components.
- Evaluate a Model: Critique the water-flow analogy for electricity by identifying two useful similarities and two important limitations.
Evidence of Learning
| Evidence type | What strong evidence looks like |
|---|---|
| Knowledge | You accurately distinguish current, voltage, and resistance; use correct symbols and SI units; and explain when Ohm's law is applicable. |
| Skills | You rearrange V = I × R, solve multi-step problems, connect meters correctly in low-voltage circuits, interpret graphs, and check units and reasonableness. |
| Products | You produce clear circuit diagrams, data tables, graphs, lab notes, explanations, videos, posters, or designs that show scientific understanding. |
| Reasoning | You make predictions before testing, use evidence to support claims, identify assumptions, and explain unexpected results. |
| Transfer | You apply the relationships among current, voltage, resistance, and power to unfamiliar devices, engineering choices, and safe practical situations. |
OERs on the Topic
Use these open or freely accessible resources to review and extend your learning:
- Ohm's law: Explore the relationship among voltage, current, and resistance in an encyclopedia overview.
- Electric current: Review charge flow, conventional current, units, and related concepts.
- Electrical resistance and conductance: Explore resistance, conductance, materials, and measurement.
- OpenStax Physics: Ohm's Law: Read a free textbook treatment with worked examples and graphs.
- NIST SI Units: Electric Current: Check the SI units ampere, volt, and ohm.
- PhET Ohm's Law: Experiment interactively with voltage, resistance, and current.
- PhET Circuit Construction Kit: DC: Build and measure virtual circuits.
Linked Learning Areas
The topic connects directly with Physics, Mathematics, Engineering, Electronics, Energy, data analysis, graph interpretation, scientific measurement, and laboratory safety. These links make the course suitable for secondary-school science and introductory technical or vocational education.
aiMOOC Projects
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