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Science lesson plan · 8th grade (Year 9 in England) · 30 minutes

Conservation of energy and wasted energy

Energy

CCSS NGSS.MS-PS3-5CCSS NGSS.HS-PS3-1England: KS3 Physics: Energy (changes in systems)England: KS4 Combined Science: Energy

Objective and success criteria

Students will: Conservation of energy and wasted energy.

  • I can answer questions on today's skill on my own, without help.
  • I can explain each step and say why it works.
  • I can check my answer and spot a mistake.

Before and after

Needs first: How devices transfer energy

Leads to: Kinetic and gravitational potential energy; Efficiency; Use energy conservation to find speeds and heights

Materials: Wind-up toys or a pendulum, Energy transfer diagrams, Mini whiteboards, A diagram or short video clip of the idea, Printed student sheet (below).

Warm-up · 4 min

  • Quick retrieval from earlier learning (How devices transfer energy). Students answer on mini whiteboards.
  • Show answers together and note who needs support in the main task.
  1. What is the main useful energy output of headphones? (light / sound / heating (thermal) / movement (kinetic)) → soundPrompt: What is the device for?
  2. A battery-powered torch (flashlight) is switched on. Which list shows the energy transfer? (sound → light / light → chemical → sound / chemical → electrical → light (and some heat) / kinetic → nuclear → light) → chemical → electrical → light (and some heat)Prompt: Start with the energy store in the battery.
  3. Which device is mainly designed to give out heating (thermal)? (a doorbell buzzer / a washing machine drum / headphones / an electric kettle) → an electric kettlePrompt: Think about what you use each device for.

Hook · 3 min

  • Wind up a toy or stretch an elastic band and ask where the energy goes.
  • Share the objective: Conservation of energy and wasted energy. Read the success criteria together.

I do: model it · 7 min

  • Work through the example out loud, saying why you do each step, not just what you do.
  • Example: A speaker is supplied with 200 J of energy. It transfers 80 J usefully. How much energy is wasted, in J?
  • Step 1: 200 − 80 = 120
  • Step 2: So the answer is: 120 J
  • Answer: 120 J
  • Pause after each step and ask a student to say what comes next.

We do: guided practice · 6 min

  • Do these together. Students try each step on whiteboards before you reveal it.
  • Use the hints as prompts rather than giving the answer.
  1. A kettle is supplied with 1000 J of energy. It transfers 900 J usefully. How much energy is wasted, in J? → 100 JPrompt: Energy is conserved: input = useful output + wasted output.
  2. How can wasted energy in a machine with moving parts be reduced? (paint it a darker colour / remove the oil / lubricate the parts to reduce friction / make it move faster) → lubricate the parts to reduce frictionPrompt: What causes heating where parts rub?

You do: independent practice · 7 min

  • Students work on the student sheet. Circulate and check the first two answers of every student early.
  • Students who finish: write a question of their own for a partner, with the answer.

The 6 questions are on the student sheet below, with answers in the key.

Exit ticket · 3 min

  • Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.
  1. A speaker is supplied with 1200 J of energy. It transfers 720 J usefully. How much energy is wasted, in J? → 480 JPrompt: Energy is conserved: input = useful output + wasted output.
  2. Conservation of energy: which statement is NOT true? (A machine can give out more energy than it takes in. / No real machine is 100% efficient. / Wasted energy is often transferred to the surroundings by heating. / A Sankey diagram shows useful and wasted energy.) → A machine can give out more energy than it takes in.Prompt: Can energy ever be created or destroyed?

Watch out for

  • Everyday meanings of words getting in the way of the science meaning: define key words explicitly.
  • Remembering facts without the reason: ask "why?" and "what is the evidence?"

Support and stretch

Support

  • Review first: How devices transfer energy.
  • Use concrete materials: wind-up toys or a pendulum.
  • Give the first step of each question, or a partly worked example to finish.
  • Pair with a confident partner for the guided practice.

Stretch

  • Move on to: Kinetic and gravitational potential energy; Efficiency; Use energy conservation to find speeds and heights.
  • Explain a common mistake in writing and how to fix it.
  • Write a word problem that uses today's skill, then solve a partner's.

Homework and more practice

Free online practice with hints and printable worksheets for this skill: eurekami.com/science/grade-8.

Free lesson plan from eurekami.com/lesson-plan-builder

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Conservation of energy and wasted energy

Name

Date

Score/ 6

  1. Question 1

    Conservation of energy: which statement is NOT true?

    • A. Energy is used up and disappears.
    • B. The total energy in a closed system stays the same.
    • C. No real machine is 100% efficient.
    • D. A Sankey diagram shows useful and wasted energy.

  2. Question 2

    A motor is supplied with 1200 J of energy. It transfers 900 J usefully. How much energy is wasted, in J?

    AnswerJ

  3. Question 3

    How can wasted energy in a machine with moving parts be reduced?

    • A. remove the oil
    • B. paint it a darker colour
    • C. make it move faster
    • D. lubricate the parts to reduce friction

  4. Question 4

    A motor is supplied with 100 J of energy. It transfers 90 J usefully. How much energy is wasted, in J?

    AnswerJ

  5. Question 5

    Where does most wasted energy usually end up?

    • A. back in the battery
    • B. inside the plug
    • C. heating the surroundings
    • D. destroyed

  6. Question 6

    A speaker is supplied with 100 J of energy. It transfers 25 J usefully. How much energy is wasted, in J?

    AnswerJ

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Answer key: Conservation of energy and wasted energy

  1. 1. A. Energy is used up and disappears.

  2. 2. 300 J

  3. 3. D. lubricate the parts to reduce friction

  4. 4. 10 J

  5. 5. C. heating the surroundings

  6. 6. 75 J