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Science lesson plan · 8th grade (Year 9 in England) · 45 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 · 5 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 hairdryer's heating coil? (light / sound / heating (thermal) / movement (kinetic)) → heating (thermal)Prompt: What is the device for?
  2. Which energy is wasted by an old filament light bulb? (thermal energy (heat) / nuclear energy / light / chemical energy) → thermal energy (heat)Prompt: Touch an old bulb that has been on (carefully!).
  3. Which device is mainly designed to give out heating (thermal)? (a toaster / an LED lamp / a torch (flashlight) / headphones) → a toasterPrompt: Think about what you use each device for.

Hook · 5 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 · 10 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 · 10 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 motor is supplied with 2000 J of energy. It transfers 400 J usefully. How much energy is wasted, in J? → 1600 JPrompt: Energy is conserved: input = useful output + wasted output.
  2. Where does most wasted energy usually end up? (inside the plug / back in the battery / destroyed / heating the surroundings) → heating the surroundingsPrompt: Machines often feel warm after use.

You do: independent practice · 10 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 8 questions are on the student sheet below, with answers in the key.

Exit ticket · 5 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 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.
  2. Where does most wasted energy usually end up? (back in the battery / destroyed / inside the plug / heating the surroundings) → heating the surroundingsPrompt: Machines often feel warm after use.

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/ 8

  1. Question 1

    Conservation of energy: which statement is NOT true?

    • A. The total energy in a closed system stays the same.
    • B. No real machine is 100% efficient.
    • C. A machine can give out more energy than it takes in.
    • D. Wasted energy is often transferred to the surroundings by heating.

  2. Question 2

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

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

  3. Question 3

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

    AnswerJ

  4. Question 4

    Where does most wasted energy usually end up?

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

  5. Question 5

    A lamp is supplied with 2000 J of energy. It transfers 1800 J usefully. How much energy is wasted, in J?

    AnswerJ

  6. Question 6

    What does the law of conservation of energy say?

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

  7. Question 7

    A lamp is supplied with 500 J of energy. It transfers 50 J usefully. How much energy is wasted, in J?

    AnswerJ

  8. Question 8

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

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

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Answer key: Conservation of energy and wasted energy

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

  2. 2. B. lubricate the parts to reduce friction

  3. 3. 60 J

  4. 4. A. heating the surroundings

  5. 5. 200 J

  6. 6. C. Energy cannot be created or destroyed, only transferred

  7. 7. 450 J

  8. 8. C. It was transferred to the surroundings as heat and sound