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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 a washing machine drum? (light / sound / heating (thermal) / movement (kinetic)) → movement (kinetic)Prompt: What is the device for?
  2. A wind-up toy car is wound up and let go. What happens to the energy? (thermal → nuclear / elastic potential → kinetic / kinetic → chemical / light → sound) → elastic potential → kineticPrompt: Winding it up squeezes a spring.
  3. Which device is mainly designed to give out light? (a toaster / headphones / an LED lamp / a washing machine drum) → an LED lampPrompt: 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 lamp is supplied with 100 J of energy. It transfers 75 J usefully. How much energy is wasted, in J? → 25 JPrompt: Energy is conserved: input = useful output + wasted output.
  2. Where does most wasted energy usually end up? (back in the battery / inside the plug / heating the surroundings / destroyed) → heating the surroundingsPrompt: Machines often feel warm after use.

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 kettle is supplied with 2000 J of energy. It transfers 1500 J usefully. How much energy is wasted, in J? → 500 JPrompt: Energy is conserved: input = useful output + wasted output.
  2. Conservation of energy: which statement is NOT true? (The total energy in a closed system stays the same. / A Sankey diagram shows useful and wasted energy. / Wasted energy is often transferred to the surroundings by heating. / A machine can give out more energy than it takes in.) → 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

    A ball is dropped and each bounce is lower. Where did the energy go?

    • A. It went into the ball's mass
    • B. It was destroyed
    • C. Gravity took it
    • D. It was transferred to the surroundings as heat and sound

  2. Question 2

    Conservation of energy: which statement is NOT true?

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

  3. Question 3

    A motor is supplied with 2000 J of energy. It transfers 500 J usefully. How much energy is wasted, in J?

    AnswerJ

  4. Question 4

    A kettle is supplied with 1200 J of energy. It transfers 120 J usefully. How much energy is wasted, in J?

    AnswerJ

  5. Question 5

    What does the law of conservation of energy say?

    • A. Energy cannot be created or destroyed, only transferred
    • B. Machines create energy
    • C. Energy is used up and disappears
    • D. Energy is always destroyed as heat

  6. Question 6

    A motor is supplied with 100 J of energy. It transfers 40 J usefully. How much energy is wasted, in J?

    AnswerJ

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Answer key: Conservation of energy and wasted energy

  1. 1. D. It was transferred to the surroundings as heat and sound

  2. 2. B. Energy is used up and disappears.

  3. 3. 1500 J

  4. 4. 1080 J

  5. 5. A. Energy cannot be created or destroyed, only transferred

  6. 6. 60 J