D2.8 · Determine device efficiency and identify energy transformations

Ontario Grade 9 Science

Physics: Principles and Applications of Electricity

Ontario Grade 9 Science — D2.8

What you will learn

• Identify the input energy and useful energy output of a device.
• Describe energy transformations in a device.
• Calculate device efficiency using supplied energy values.
• Explain what an efficiency result does and does not show.

1. Follow energy through a device

• A device changes input energy into one or more output forms.
• Useful output depends on the task being considered.
• An energy-flow model is a simplified way to show transformations.

2. Determine efficiency

• Divide useful output energy by total input energy, then multiply by 100%.
• Use matching energy units for input and output.
• Efficiency describes a specific output for a specific task.

3. Use evidence carefully

• The result depends on the quality of the values and the definition of useful output.
• A model can show important transformations without showing every detail.
• Efficiency alone does not make a social or ethical decision.

Energy pathways for different tasks

Worked example

Flashlight efficiency

1. Choose the useful output
   The stated task is to provide light, so use 24 J as the useful output. The total input is 80 J.
2. Calculate the percentage
   Divide useful output by total input and multiply by 100. Both values are in joules, so the units match.
   $$\frac{24\,\mathrm{J}}{80\,\mathrm{J}}\times100\%=30\%$$
3. Interpret the result
   Using these hypothetical values, 30% of the input is counted as useful light output. The other energy was not counted as useful light; it was not destroyed.

Worked example

A small motor

1. Identify input and useful output
   The motor’s total input is 150 J of electrical energy. Its stated useful output is 90 J of motion energy.
2. Calculate efficiency
   Divide the useful output by the total input, then express the fraction as a percentage.
   $$\frac{90\,\mathrm{J}}{150\,\mathrm{J}}\times100\%=60\%$$
3. Describe another output
   The motor could also produce thermal energy as its parts warm, or sound. These are possible outputs even though the example counts motion as useful.

Worked example

Compare two devices

1. Calculate Device A
   Use 50 J as the useful output and 200 J as the total input.
   $$\frac{50\,\mathrm{J}}{200\,\mathrm{J}}\times100\%=25\%$$
2. Calculate Device B
   Apply the same method because the input and stated task are the same for both devices.
   $$\frac{110\,\mathrm{J}}{200\,\mathrm{J}}\times100\%=55\%$$
3. Compare the results
   Device B has greater efficiency for this task: 55% compared with 25%. This comparison does not tell us other things, such as cost or suitability.

Common mistakes and how to avoid them

Lesson summary

• Devices transform input energy into one or more output forms.
• Efficiency compares useful energy output with total energy input.
• Use the same energy unit for both values and express the result as a percentage.
• A calculation depends on the stated task and the quality of the information supplied.

Check your understanding

Key terms

Energy transformation

A change from one form of energy to another.

Input energy

The total energy supplied to a device.

Useful energy output

The output energy form that does the intended task.

Efficiency

The percentage of input energy that becomes the chosen useful output.

Model

A simplified representation that helps explain a system but may leave out details.

Continue through SNC1W

View the complete SNC1W Ontario Grade 9 Science curriculum and lessons

• D2.7 · Distinguish electricity from electrical energy
• E1.1 · Evaluate the impacts of space observation and exploration
• D1.1 · Assess benefits and challenges of electrical energy sources
• D1.2 · Evaluate electricity impacts on communities and sustainable practices
• D1.3 · Plan action on an electricity issue, including conservation
• D1.4 · Analyse emerging technologies for electricity and storage

About this lesson and its review

Published by DoAssignment. This reviewed lesson follows Ontario Grade 9 Science (SNC1W), expectation D2.8. 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.

Official curriculum reference

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