Conservation of Energy and Power


  • Conservation of energy principle: energy can be transferred usefully, stored or dissipated (spread out) but can never be created or destroyed.
  • When energy is transferred between stores, not all of the energy+ is transferredusefully to the store you want it to go to. Some energy dissipated = wasted energy = not being stored in a useful way (usually thermal energy stores)
  • Mobile phone: energy transferred from the chemical energy store in battery (useful). Some energy is dissipated as thermal energy (wasted )
  • Power = rate of energy transfer OR rate of doing work. It is measured in watts
  • 1 watt = 1 joule of energy transferred per second
  • P= e/t
  • Power (w) = energy transferred (j) / time (s)
  • P=W/t
  • Power (w) = work done (j) / time (s)
  • Power (w) = work done (j) / time (s)
  • Powerful machine = transfers lots of energy in a short amount of time
  • Two cars which are identical except they both have different engine powers. Both cars race the same distance to a finish line. The car with the more powerful engine will reach the finish line faster than the other car.
  • How long have you…? (present perfect 4) Exercises




    Use the information given to create questions beginning with ‘How long…?’

    • It is snowing.

    • The weather is shining.

    • John and Kate are married.

    • My brother has gone on holiday.

    • My aunty and uncle live in Canada.

    • My sister is a teacher.

    • I work at the chemist.

    • I’ve known Susan since I was a baby.

    • Jonathan is learning to speak to Mandarin.




    Fill in the missing gaps in the sentences using the present perfect (=has/have + past participle)

    • I have known Kate a long time.

    • My brother has been playing the piano he was 11 years old.

    • My mum and dad on holiday to America. They

    gone since last Sunday.

    • I Ben and Emma since primary school.

    We to the same primary and secondary school.

    • My sister learning to speak French because she
    is going to France next month.



    Use the appropriate words in the missing gaps.

    have you lived       lives      to

    has been       works        have known

    I have bought       have worked.

    • My sister in Spain. She is studying Spanish

    with her friend Rosie. I Rosie since I was 5.

    • Spencer like with Jamie. They have worked

    together for 2 years.

    • How long in Australia?

    • a new dress for prom.

    • He to Thorpe park.



    Challenge: Create 4 of your own questions beginning with ‘How long……?’








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    “Every accomplishment starts 
    With the decision to try.” Gail Dever

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