1. Energy Calculations Objective
17/02/2019
Energy Calculations
Objective
To demonstrate that forces transfer energy by doing work and that power is the rate of transfer of energy in a system.

2. Codebreaker Work out the key words from these combinations!
Example: 36E74Y = ENERGY
9288
9675
5685E
769E7
96R5 366E
545692T8 468R

3. Were you successful? energy joule kilowatt hour power watt work
5685E = JOULE
769E7 = POWER
96R5 366E = WORK DONE
545692T8 468R = KILOWATT HOUR
energy
joule
kilowatt hour
power
watt
work
work done

4. Work Done = Force x Distance
The work done is the amount of energy required to move an object by a certain amount. Worked Example If a box is pushed 10m along a floor with a force of 50N, calculate the work done? Work done = F x D = 50 x 10 = 500J Example If a box is pushed 5m along a floor with a force of 20N, calculate the work done? Work done = F x D = 20 x 5 = 100J
Work Done = Force x Distance
(J) (N) (m)
Discuss where the work is done, remind pupils about energy being transferred. If the object is lifted then it gains gravitational potential energy. If the object is pushed against a frictional force then the environment is heated as the energy dissipates. Use pages 328, 334 and 335 of the Student’s Book to assist with these explanations if needed.

5. Work Done = Force x Distance
The work done is the amount of energy required to move an object by a certain amount. Example A pupil jumps 1.2m in the high jump, they have a mass of 60kg and a weight of 600N, calculate the work done? Work done = F x D = 600 x 1.2 = 720J
Work Done = Force x Distance
(J) (N) (m)
Discuss where the work is done, remind pupils about energy being transferred. If the object is lifted then it gains gravitational potential energy. If the object is pushed against a frictional force then the environment is heated as the energy dissipates. Use pages 328, 334 and 335 of the Student’s Book to assist with these explanations if needed. Extension: Complete some further examples with more difficult calculations.

6. Power
The power is the rate of transfer of energy in a system. What is the rate? An amount in a given time. The power is the energy used in a given time. What is the unit of energy? What is the unit of time? Watt is the unit of power. Example A lightbulb uses 400J of energy in 10s. Calculate the power of the bulb. P = E ÷ T = 400J ÷ 10S = 40W
Power = Energy ÷ Time
(Watt) (Joule) (Sec.)

7. Work Done = Force x Distance
Calculate Your Power
You have one minute to lift a 1 kg mass (10 N) up off the desk to a height of 50 cm and lower it back down. You should do this as many times as possible within the minute. Calculate the total work done (number of times lifted × force × height). Calculate your power. Compare your power with other members of the class.
Work Done = Force x Distance
Power = Energy ÷ Time
Point out to the students they need to consider the number of times they lift the weight and the units of the height they lift it to and the time they do it for. The exercise should also cause their arm to warm up and you will be able to discuss the dissipation of energy with them.

8. Energy Power = Energy ÷ Time
We can rearrange the formula to find the energy: Example If a 40W lightbulb is on for 1 hour, how much energy does it use?
Energy = Power x Time
Talk through rearranging the formula to find the power if you are given the energy and time in a question. Use a triangle if it helps.

9. Homework Complete question 2 in your book.
ss_th2_phys_term 4_l8_sq_energy calculations_q2-3

10. Which lightbulb would you choose?
Talk through what each of the facts mean and then get pupils to have a think themselves, then discuss it with a partner and then share it with the class and see what reasons are given.