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D2.4 · Investigate current, voltage, and resistance to build a model

Learn to investigate current, voltage, and resistance to build a model through clear examples and targeted practice.

Ontario Grade 9 Science

Physics: Principles and Applications of Electricity

Using safe evidence to build a simple circuit model

A small lamp lights when it is connected to a battery in a complete circuit. A circuit is a closed path through which electric current can flow. If the path is broken, the lamp goes out. This everyday observation raises a science question: What affects the amount of current in a circuit? In this lesson, we investigate current, voltage, and resistance and use their relationship to build a simple model. Electrical investigations should use approved, low-voltage classroom equipment under teacher supervision. Never experiment with wall outlets or damaged electrical equipment.

What you will learn

Define current, voltage, and resistance in a simple circuit.
Describe how changing voltage or resistance can affect current.
Use a simple model to predict a value when the other two quantities are known.
Explain how a fair investigation and its limits help build a useful model.

Three useful ideas about a circuit

Before studying electricity, recall that matter is made of tiny particles. Some particles have electric charge. In a metal wire, some charged particles can move. Their organized movement through a complete circuit is called electric current. Current tells us how much charge passes a point in a given time. We measure current in amperes, written A.

A battery or other power source provides a difference in electric potential between its ends. This difference is called voltage. Voltage is measured in volts, written V. In a simple model, voltage is the push that helps drive current around a complete circuit. The word “push” is an analogy; voltage is not a mechanical force.

A material or component that makes current harder to pass through has resistance. Resistance is measured in ohms, written with the symbol Ω. A lamp has resistance, as does a resistor, a component designed to provide resistance. Wires also have some resistance, but a simple circuit model often treats it as small compared with the resistance of the main component.

These ideas help explain why a lamp may glow more brightly when its circuit is changed. More current can make a lamp brighter, though brightness also depends on the lamp and the circuit. Brightness is an observation, not a direct measurement of current.

Current is the flow of electric charge; its unit is the ampere (A).
Voltage is the electrical push supplied by a source; its unit is the volt (V).
Resistance makes current harder to pass; its unit is the ohm (Ω).

A model and a fair investigation

A model is a simplified way to represent or explain something. For a simple circuit with a fixed resistor, a useful model links voltage, current, and resistance: increasing voltage tends to increase current, while increasing resistance tends to decrease current. The model assumes the circuit stays otherwise the same. Real components can behave in more complex ways, so the model is not a perfect description of every circuit.

One way to picture the relationship is to imagine water moving through a pipe. The water flow is like current, the pressure difference is like voltage, and a narrow or blocked section is like resistance. This picture can help at first, but electricity is not water. The analogy does not show what charged particles are or how a real component works.

A fair investigation changes one factor at a time while keeping other important conditions the same. For example, a class could use a low-voltage power source, a resistor, and suitable meters to compare current at different voltage settings. The class should keep the resistor and circuit arrangement the same. To investigate resistance, it could compare suitable resistors while keeping the source setting and other parts of the circuit unchanged. Follow teacher instructions and equipment limits.

A meter reading is evidence: it is an observation made with a measuring tool. A pattern across repeated readings can support or challenge a model. A model is still a simplification, and readings may be affected by measurement limits or changes in a component. Do not invent results if no investigation was performed. Hypothetical data can help practise reasoning, but it is not experimental evidence.

V = I \times R

Change one factor at a time to make a comparison easier to interpret.
Keep the circuit and measuring method consistent where possible.
A model helps explain evidence but does not replace evidence.

Using the relationship carefully

For a fixed resistor in a simple circuit, the model can be written as voltage equals current multiplied by resistance. In symbols,

V

means voltage,

I

means current, and

R

means resistance. If two quantities are known, the relationship can be rearranged to find the third. For example, current equals voltage divided by resistance. This is a model for the situation described, not a rule that every electrical device behaves exactly the same way.

Units provide a useful check. Volts divided by ohms gives amperes. If a calculation gives a current but the units are volts, something has gone wrong. A sensible answer should also fit the situation: with the same resistor, a greater voltage predicts a greater current in this model.

Do not treat a formula as proof that a real circuit will match it perfectly. The formula makes a prediction under stated conditions. Measurements provide evidence about how well the prediction works. If results differ, check the setup and readings, then consider whether the model's assumptions fit the component.

I = \frac{V}{R}

Use the relationship only for the stated simple-circuit model.
Check that the answer has the correct unit and makes sense.
Predictions and measurements are related, but they are not the same thing.

From evidence to a conclusion

A careful conclusion states what was changed, what was measured, and what pattern appeared. It should not claim more than the evidence supports. For example, evidence from one resistor supports a conclusion about that resistor and those test conditions. It does not automatically prove that every lamp or electronic device behaves in the same way.

The scientific model concerns how current, voltage, and resistance relate in a circuit. It does not decide whether a technology is socially fair, useful, or desirable. Those questions involve values and other kinds of evidence. Keeping scientific evidence separate from social or ethical judgments makes explanations clearer.

Describe the evidence and the conditions of the investigation.
State the limits of the model and avoid overgeneralizing.
A scientific relationship alone does not settle a social or ethical judgment.

Quantities in a simple circuit model

Quantity	Meaning in this lesson	Symbol	Unit
Current	Flow of electric charge	I	ampere (A)
Voltage	Electrical push supplied by a source	V	volt (V)
Resistance	How much a component makes current harder to pass	R	ohm (Ω)

Worked example

Predicting current for a fixed resistor

A simple circuit model has a voltage of 6 V and a fixed resistance of 3 Ω. What current does the model predict?

Simple low-voltage circuit

A schematic model of a closed circuit; component sizes and wire lengths are not to scale.

Choose the relationship
The voltage and resistance are given. For this fixed-resistor model, current is voltage divided by resistance.
$I = \frac{V}{R}$
Substitute and calculate
Divide 6 V by 3 Ω. The units also divide, giving amperes.
$I = \frac{6\ \mathrm{V}}{3\ \Omega} = 2\ \mathrm{A}$

Answer: The model predicts a current of 2 A.

Check: The units are amperes, and the result is consistent with the stated model.

Worked example

Finding a model's resistance

A hypothetical classroom data set reports 0.5 A of current when a fixed resistor is connected to a 4 V source. What resistance does the simple model predict?

Rearrange the model
The voltage and current are known. Divide voltage by current to find the resistance predicted by the model.
$R = \frac{V}{I}$
Use the hypothetical values
The values are practice data, not a claim about a real investigation. Volts divided by amperes gives ohms in this model.
$R = \frac{4\ \mathrm{V}}{0.5\ \mathrm{A}} = 8\ \Omega$

Answer: The model predicts a resistance of 8 Ω.

Check: This is a calculation from hypothetical values. It is not experimental evidence.

Worked example

Comparing two model predictions

Two separate simple circuits have the same 6 V source. Circuit A has a 2 Ω fixed resistor. Circuit B has a 6 Ω fixed resistor. Use the model to compare their predicted currents.

Two circuit arrangements for comparison

Schematic comparison only; component sizes and wire lengths are not to scale, and the drawing does not show measured current.

Predict circuit A
Use the same voltage-current-resistance relationship for the first fixed resistor.
$I_A = \frac{6\ \mathrm{V}}{2\ \Omega} = 3\ \mathrm{A}$
Predict circuit B
The source voltage stays the same, but the resistance is larger. Calculate the current using the model.
$I_B = \frac{6\ \mathrm{V}}{6\ \Omega} = 1\ \mathrm{A}$
Compare the predictions
With the same voltage, the model predicts less current for the greater resistance. This is a model prediction, not a measurement of either circuit.
$3\ \mathrm{A} > 1\ \mathrm{A}$

Answer: Circuit A is predicted to have 3 A, and circuit B is predicted to have 1 A. The model predicts more current in circuit A.

Check: The two predictions use the same voltage. The circuit with greater resistance has lower predicted current.

Common mistakes and how to avoid them

Thinking voltage and current mean the same thing.

Correction: Voltage is the electrical push in the model. Current is the flow of charge.

Assuming more resistance always creates more current.

Correction: In this model, with voltage held the same, greater resistance predicts less current.

Calling a prediction a measurement.

Correction: A calculation gives a prediction. A meter reading is evidence gathered from a circuit.

Assuming the model describes every device perfectly.

Correction: The relationship is being used for a simple circuit with a fixed resistor. State the model's limits.

Lesson summary

Current is the flow of charge and is measured in amperes.
Voltage is the electrical push supplied by a source and is measured in volts.
Resistance makes current harder to pass and is measured in ohms.
For a fixed resistor in a simple circuit, the model links voltage, current, and resistance.
Fair tests and careful measurements provide evidence for judging how useful a model is.

Check your understanding

Question 1

In the simple fixed-resistor model, what happens to predicted current if voltage rises while resistance stays the same?

It rises.
It falls.
It stays the same in every case.
It becomes resistance.

Show answer and explanation

It rises.

The model predicts that greater voltage gives greater current when resistance stays the same.

Question 2

A fixed resistor is 5 Ω and the source voltage is 10 V. What current does the model predict?

0.5 A
2 A
5 A
50 A

Show answer and explanation

2 A

Divide 10 V by 5 Ω. The predicted current is 2 A.

Question 3

Which statement best describes a hypothetical data set?

It is proof that a real circuit was measured.
It is practice information and should not be described as experimental evidence.
It proves that every component follows the model exactly.
It removes the need to describe investigation conditions.

Show answer and explanation

It is practice information and should not be described as experimental evidence.

Hypothetical values can be used to practise reasoning, but they are not readings from a real investigation.

Key terms

Circuit: A closed path through which electric current can flow.
Current: The flow of electric charge through a circuit.
Voltage: The electrical push supplied by a source in the simple model.
Resistance: How much a component makes current harder to pass.
Model: A simplified representation used to explain or predict something.
Evidence: Observations or measurements used to support or challenge an explanation.

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About this lesson and its review

Published by DoAssignment. This reviewed lesson follows Ontario Grade 9 Science (SNC1W), expectation D2.4. It is a study resource, not an official curriculum publication.

Before publication, content is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability. Errors can still occur, so corrections are welcomed.

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