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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 toaster? (light / sound / heating (thermal) / movement (kinetic)) → heating (thermal)Prompt: What is the device for?
  2. A wind-up toy car is wound up and let go. What happens to the energy? (elastic potential → kinetic / thermal → nuclear / light → sound / kinetic → chemical) → elastic potential → kineticPrompt: Winding it up squeezes a spring.
  3. Which device is mainly designed to give out movement (kinetic)? (a toaster / an electric fan / a torch (flashlight) / a loudspeaker) → an electric fanPrompt: 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 speaker is supplied with 200 J of energy. It transfers 120 J usefully. How much energy is wasted, in J? → 80 JPrompt: Energy is conserved: input = useful output + wasted output.
  2. A ball is dropped and each bounce is lower. Where did the energy go? (It was destroyed / It was transferred to the surroundings as heat and sound / It went into the ball's mass / Gravity took it) → It was transferred to the surroundings as heat and soundPrompt: Energy is never destroyed.

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. How can wasted energy in a machine with moving parts be reduced? (make it move faster / paint it a darker colour / remove the oil / lubricate the parts to reduce friction) → lubricate the parts to reduce frictionPrompt: What causes heating where parts rub?
  2. A lamp is supplied with 500 J of energy. It transfers 125 J usefully. How much energy is wasted, in J? → 375 JPrompt: Energy is conserved: input = useful output + wasted output.

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 kettle is supplied with 100 J of energy. It transfers 60 J usefully. How much energy is wasted, in J?

    AnswerJ

  2. Question 2

    Conservation of energy: which statement is NOT true?

    • A. The total energy in a closed system stays the same.
    • B. A Sankey diagram shows useful and wasted energy.
    • C. A machine can give out more energy than it takes in.
    • D. No real machine is 100% efficient.

  3. Question 3

    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

  4. Question 4

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

    AnswerJ

  5. Question 5

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

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

  6. Question 6

    A motor is supplied with 1000 J of energy. It transfers 100 J usefully. How much energy is wasted, in J?

    AnswerJ

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Answer key: Conservation of energy and wasted energy

  1. 1. 40 J

  2. 2. C. A machine can give out more energy than it takes in.

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

  4. 4. 75 J

  5. 5. C. lubricate the parts to reduce friction

  6. 6. 900 J