F2.8 Build and refine a device that uses electromagnetism

Ontario Grade 11 Physics

Electricity and Magnetism

Plan a safe electromagnet, test it fairly, and use evidence to improve its design

An electromagnetic device uses electricity and magnetism together. A simple example is an electromagnet: a coil of wire that produces a magnetic effect when current passes through it. Building the device is only part of the task. You also need a clear purpose, a safe way to test it, and a way to decide whether a design change helped. A proposed procedure is a plan. It becomes measured evidence only after a physical test is carried out and its results are recorded.

What you will learn

Describe how a current-carrying coil can be used in an electromagnetic device.
Plan and safely build a simple electromagnet device.
Test a design in a consistent way and distinguish predictions from measured evidence.
Use a circuit calculation to help guide a design choice.

1. Prerequisite bridge: circuits and fair tests

A circuit is a complete path through which current can pass. Current is measured in amperes, written as

\mathrm{A}

. Voltage is measured in volts, written as

\mathrm{V}

. Resistance describes how much a component opposes current. It is measured in ohms, written as

\Omega

.

Circuit diagrams commonly show conventional current moving from a supply’s positive terminal, through the circuit, and back to its negative terminal. Electrons in metal wires move in the opposite direction. In this lesson, circuit arrows show conventional current.

A fair test uses a consistent method so that a design comparison is useful. Keep important conditions the same, such as the supply, the test objects, and the way you test the device. When possible, change one design feature at a time. Then it is easier to connect an observed difference to the change.

V=IR

Current, voltage, and resistance are different circuit quantities.
Use conventional-current arrows consistently.
Keep test conditions consistent when comparing designs.

2. Plan and build a coil electromagnet

The physical system is the electromagnet circuit and the object it is meant to affect. A simple classroom device can use insulated wire wrapped around an iron nail. The wire forms a coil, the nail is the core, and a suitable low-voltage supply provides electricity. A magnetic field is the region around a magnet or current-carrying coil where magnetic effects can be detected.

When current passes through the coil, the coil produces a magnetic effect. An iron core can make that effect stronger. The number of turns in the coil and the current are also design choices that can affect performance. The actual result depends on the complete design, so a claim that one design is better should be checked with a test.

Before assembly, choose and label the conventional-current direction on your circuit plan: from the positive terminal, through the switch and coil, then back to the negative terminal. The return connection is essential. A circuit must be a complete path for current to pass.

A complete text circuit plan is: positive terminal → switch → coil around iron core → negative terminal → wire returning to positive terminal. The switch opens or closes the circuit. Keep it open while assembling the device. Use insulated wire and a low-voltage supply approved for the activity. Do not use mains electricity. Disconnect the supply between short tests, and stop if a wire or battery becomes warm.

A current-carrying coil produces a magnetic effect.
The core, coil turns, and current are possible design features to investigate.
Draw a complete circuit path, including the return connection.
Use approved low-voltage equipment and watch for heating.

3. Test and refine the device

Start by stating what the device should do in a way you can check. For example: “The electromagnet should lift small steel washers.” Choose a test measure, such as the number of identical washers lifted using the same method each time. Keep the washers, supply setting, test position, and lifting method consistent.

Record results only after carrying out the physical test. A results table might include the number of coil turns, the supply setting, and the number of washers lifted. If you have only planned the test, label the expected outcome as a prediction. If you use a computer simulation, label its output as simulated information, not as a physical measurement.

To refine the device, review the results and select a change with a clear reason. For example, add a small number of turns while keeping the other test conditions the same. Repeat the test and record what happened. Compare the result with the original design. If the chosen measure improves and the device remains safe and practical, the change may be useful.

A single result may not show whether a design change is dependable. Repeat comparable trials and record any variation. Do not claim that a design works better if the results do not support that claim. Keep a brief design record: what changed, what stayed the same, what was observed, and what you would try next.

Choose a measurable purpose before testing.
Separate physical observations from predictions and simulation output.
Make a planned change and compare results using the same test method.
A useful refinement must also be safe and practical.

4. Use a circuit calculation to guide a design choice

Ohm’s law connects voltage, current, and resistance. It can help estimate current before a coil is connected to a supply. For a simple series circuit, components are connected one after another in a single path, and the same current passes through each component.

For a calculation, define the circuit and the conventional-current direction first. State the known values and the unknown. Keep units in the substitution. Volts divided by ohms gives amperes. A calculated value does not replace checking the ratings of the supply, wire, and other components or following the teacher’s safety instructions.

Check the result after calculating. Confirm that the units are correct, that the sign agrees with the chosen current direction, and that the value is suitable for the actual equipment. Do not describe a current as safe or modest without checking relevant component ratings.

I=\frac{V}{R}

Ohm’s law can estimate current in a circuit.
Check equipment ratings; a calculation alone does not establish safety.
Keep units through the calculation and check the result.

A simple design-and-test record

Design feature	Conditions to keep the same	Evidence to record
Number of coil turns	Core, supply, test objects, and method	Number of objects lifted
Supply setting	Coil, core, test objects, and method	Supply setting and number of objects lifted

Worked example

Estimate current for a proposed coil

A proposed coil has a resistance of

12\ \Omega

and is connected to a

3.0\ \mathrm{V}

low-voltage supply. Estimate the current. The system is the supply and coil. Choose conventional current from the positive terminal, through the coil, and back to the negative terminal as the positive direction.

Identify the known values
The voltage is $3.0\ \mathrm{V}$ and the coil resistance is $12\ \Omega$ . The unknown is the current in the chosen conventional direction.
Apply Ohm’s law
Current equals voltage divided by resistance. Include the units in the substitution so the result can be checked.
$I=\frac{3.0\ \mathrm{V}}{12\ \Omega}=0.25\ \mathrm{A}$
Check the estimate
The positive result is in the chosen direction. The units are correct because volts divided by ohms gives amperes. Whether this current is suitable depends on the ratings of the coil and supply, which must be checked before use.

Answer: The estimated current is

0.25\ \mathrm{A}

in the chosen conventional-current direction.

Check: The units are amperes, and the positive sign agrees with the selected direction. The estimate is not proof that the equipment is safe; check its ratings before building.

Worked example

Estimate a series resistor for a current limit

A

6.0\ \mathrm{V}

supply is planned for a series circuit with a coil of resistance

18\ \Omega

. The design goal is to keep current at or below

0.20\ \mathrm{A}

. Find the minimum total resistance and the estimated additional resistance. The system is the supply, resistor, and coil. Positive current is conventional current from the positive terminal through both components.

Find the minimum total resistance
Rearrange Ohm’s law to calculate resistance. Use the maximum allowed current to find the least total resistance that meets the stated current limit.
$R_{\mathrm{total}}=\frac{6.0\ \mathrm{V}}{0.20\ \mathrm{A}}=30\ \Omega$
Find the added resistance
In a series circuit, the component resistances add. Subtract the coil’s resistance from the required total resistance.
$R_{\mathrm{added}}=30\ \Omega-18\ \Omega=12\ \Omega$
Check the current and units
The total resistance is greater than the coil resistance alone, as expected when adding a resistor in series. Recalculate the current using the total resistance. The current is positive in the chosen direction.
$I=\frac{6.0\ \mathrm{V}}{30\ \Omega}=0.20\ \mathrm{A}$

Answer: The minimum total resistance is

30\ \Omega

, so the estimated additional series resistance is

12\ \Omega

.

Check: Volts divided by amperes gives ohms, and volts divided by the total ohms gives amperes. The estimated current is at the stated limit. Check component ratings and test safely.

Worked example

Compare current estimates for two coil designs

A coil with resistance

20\ \Omega

and a proposed redesigned coil with resistance

25\ \Omega

are each connected to the same

4.0\ \mathrm{V}

supply. Estimate the current for each. The system is the supply and coil; positive current is conventional current from the positive terminal through the coil.

Calculate the first current
Use the supply voltage and the first coil’s resistance. The units are included to check the result.
$I_1=\frac{4.0\ \mathrm{V}}{20\ \Omega}=0.20\ \mathrm{A}$
Calculate the redesigned current
Use the same voltage and relationship with the redesigned coil’s resistance.
$I_2=\frac{4.0\ \mathrm{V}}{25\ \Omega}=0.16\ \mathrm{A}$
Interpret what the calculation shows
The redesigned circuit has a lower estimated current. Both values have units of amperes and are positive in the selected direction. This calculation does not show which design lifts more washers; that requires a safe, controlled test with recorded observations.
$0.16\ \mathrm{A}<0.20\ \mathrm{A}$

Answer: The first coil’s estimated current is

0.20\ \mathrm{A}

. The redesigned coil’s estimated current is

0.16\ \mathrm{A}

.

Check: Both estimates have units of amperes and match the chosen current direction. The higher resistance gives the lower estimated current at the same voltage. Performance must still be tested.

Common mistakes and how to avoid them

Calling a prediction or simulation result a physical measurement.

Correction: Label predictions as expected outcomes and simulation output as simulated information. Record physical measurements only after carrying out the test.

Drawing a circuit that stops at the negative terminal.

Correction: Show the complete path, including the return connection from the negative terminal to the positive terminal through the supply.

Changing several design features at once.

Correction: Change one feature at a time when possible, while keeping the test conditions consistent.

Assuming that a larger current is automatically a better design.

Correction: Check component ratings and watch for heating. A design must be safe and practical as well as effective.

Lesson summary

A current-carrying coil produces a magnetic effect, and an iron core can strengthen it.
Build to a clear purpose using a complete circuit and approved low-voltage equipment.
Use a consistent test method and distinguish physical observations from predictions or simulation output.
Refine a device by making a planned change and comparing recorded results.
Use Ohm’s law to estimate current, then check units, direction, component ratings, and reasonableness.

Check your understanding

Question 1

A student adds turns to a coil and keeps the other test conditions the same. What should the student do next?

Record a prediction as if it were a measured result.
Repeat the test using the same method and record the observations.
Change the supply and core at the same time.
Assume the device must now lift more objects.

Show answer and explanation

Repeat the test using the same method and record the observations.

A controlled repeat test provides evidence about the change. The result should be recorded rather than assumed.

Question 2

A

9.0\ \mathrm{V}

supply is connected to a

30\ \Omega

coil. What is the estimated current?

$0.30\ \mathrm{A}$
$3.3\ \mathrm{A}$
$270\ \mathrm{A}$
$0.030\ \mathrm{A}$

Show answer and explanation

0.30\ \mathrm{A}

Using

I=V/R

, the estimate is

9.0\ \mathrm{V}/30\ \Omega=0.30\ \mathrm{A}

. It is in the chosen conventional-current direction. Check equipment ratings before use.

Question 3

A simulation predicts that a new coil design will perform better. Which statement is accurate?

The prediction is a physical measurement.
The prediction is a reason to test the design safely, not proof of the physical result.
The design is proven safe because the simulation predicts improvement.
The design and test conditions do not need to be recorded.

Show answer and explanation

The prediction is a reason to test the design safely, not proof of the physical result.

A simulation can support a prediction, but it is not a physical test. Test the design safely and record the conditions and results.

Key terms

Electromagnet: A device in which current in a coil produces a magnetic effect.
Core: Material placed inside a coil; an iron core can increase the coil’s magnetic effect.
Conventional current: The direction used for current in circuit diagrams, from the positive terminal toward the negative terminal through the circuit.
Refine: Make a planned design change and check its result.
Measured evidence: Information recorded after carrying out a physical test.
Series circuit: A circuit with components connected in one path, so the same current passes through each component.

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About this lesson and its review

Published by DoAssignment. This AI-assisted lesson follows Ontario Grade 11 Physics (SPH3U), expectation F2.8. It is a study resource, not an official curriculum publication.

Before publication, the draft is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability, and then requires administrator approval. Errors can still occur, so corrections are welcomed.

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F2.8 · Build and refine a device that uses electromagnetism

Ontario Grade 11 Physics

Plan a safe electromagnet, test it fairly, and use evidence to improve its design

What you will learn

1. Prerequisite bridge: circuits and fair tests

2. Plan and build a coil electromagnet

3. Test and refine the device

4. Use a circuit calculation to guide a design choice

A simple design-and-test record

Estimate current for a proposed coil

Estimate a series resistor for a current limit

Compare current estimates for two coil designs

Common mistakes and how to avoid them

Lesson summary

Check your understanding

Question 1

Question 2

Question 3

Key terms

Continue through SPH3U

About this lesson and its review