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P1 Conservation and dissipation of energy –Aiming for Grade 4

Aims

Using liquid and coins, you will model processes where energy is transferred from one form to another.Youwill practise calculations involving gravitational potential energy, kinetic energy, elastic potential energy, work done, power, and efficiency.You will consider situations where energy is wasted, and how that situation links to the idea of dissipation.

Learning objectives

After completing this activity, you should be able to:

  • name different types of energy store
  • do calculations involving gravitational potential energy, kinetic energy, elastic potential energy, work done, power, and efficiency
  • describe the difference between efficient and inefficient devices in terms of dissipation.

Safety

Do not drink the liquid. Ensure elastic bands do not cause injury.

Equipment

Part 1

  • Four250ml beakers
  • Coloured liquid
  • Pieces of card, about 8cm  3cm to use as labels
  • Elastic bands to secure labels to beakers
  • Coins or counters

Part 2

  • Bouncy balls
  • Tray of sand
  • Metre ruler
  • Digital balance (to measure mass of balls)

Part 3

  • Calculator

Worked example 1

Calculate the change in gravitational potential energy of a mountaineer who has climbed a mountain that is 3000m high. The mass of the mountaineer is 70kg. g10N/kg.

Step 1: Write down what you know.

change in heighth 3000m

massm 70kg

g 10N/kg

Step 2: Use the equation for Ep to find gravitational potential energy.

Epmgh

 70kg  10N/kg  3000m

 2100000J

Worked example 2

Calculate the kinetic energy of a sprinter of mass 70kg who is running at 10m/s.

Step 1: Write down what you know.

massm 70kg

speed v 10m/s

Step 2: Use the equation for Ek to find kinetic energy.

Ekmv2

 70  (10)2

 3500J

Task

Part 1: Modelling energy transfer and dissipation

Modelling is a very useful way to understand what is happening in a process where you cannot see the detail of what is happening. You could model a ball falling to the ground like this:

  • Step 1: Identify the store that has more energy at the start point – that is,before the ball is dropped.

At the start there is more energy in the gravitational store.

  • Step 2: Identify the store that has more energy at the end – that is, just before the ball hits the ground.

At the end there is more energy in the kinetic store.

  • Label beakers with the stores: gravitational and kinetic. Pour the liquid into the one at the start. Then transfer the ‘energy’ to the store at the end.
  • Think which other stores might fill up as well. There is air resistance, so you could pour a little of the ‘energy’ into a third beaker labelled ‘thermal store of the surroundings’too.

ALook at the list of processes/device in the table below. Complete the first three rows of the table. In each box write the type of energy store and where that energy is stored, for example‘chemical energy storedin the battery’.Pick two other devices or processes and complete the final two rows of the table.

Store or stores at the start / Store or stores at the end
battery powered radio
candle
cyclist cycling downhill

BFor each of the processes use the beakers and liquids to demonstrate what happens in the process.

CAnother way of modelling energy transfer is to use coins or counters to represent small ‘units of energy’.Work out a way to model energy transfers in a process or device by just using the coins or counters.

Part 2: Calculating energy

ACollect one of the bouncy balls and measure the mass. You are going to lift it to various heights. Start with a height of 1m and work out the gravitational potential energy (Ep) at that height. g  10N/kg.

BDrop it and measure the height of the bounce. Calculate Ep at that height.

CUse your two answers forEp to find the difference in Ep.

DUse what you know about the conservation of energy to complete the final column.

ERecord your results in the table:

Height dropped from in m / Ep at dropheight in J / Height bounced to in m / Ep at bounceheight in J / Difference in Epin J / Energy transferred to the surroundings
in J
1.00
0.50
0.25

Part 3: Power and efficiency

AHere are some electrical items with the energy that they transfer per second. Complete the table, and add the unit of power.

Item / Job it does / Energy transferred / Time / Power
in ___
kettle / 3000J / 2 seconds
light bulb / 6000J / 1 minute
radio / 600J / 30 seconds
oven / 10kJ / 1 second

Questions

Part 1: Modelling energy transfer and dissipation

1a List the different types of energy store.

(2 marks)

bCircle the correct answer to complete these sentences for the modelling in Part 1 of the Task:

iIn the torch the energy is transferred by an electric current/a force.(1 mark)

iiWhen the cyclist moves downhill energy is transferred by an electric current/a force.(1 mark)

2Describe how you used the coins to model energy.

(3 marks)

3For all of the processes that you modelled, some energy ends up in the surroundings.

aName two processes that transfer energy to the surroundings.

(2 marks)

bExplain why energy transferred to the surroundings is ‘dissipated’.

(1mark)

Part 2: Calculating energy

4In Part 2 of the Task you did work when you lifted the ball.

aDescribe what we mean by ‘work’ in science.

(1 mark)

bCalculate the work you did lifting the ball 1m. You need to calculate the weight of the ball from the mass. g 10N/kg. Weight  mass g.

(2 marks)

cExplain how you knew how to fill in the final column of the table.

(3 marks)

dSuggest and explain what happens when you drop a ball into sand instead of onto the floor.

(2 marks)

eA ball with a mass of 0.05kg is dropped by a student and reaches a speed of 4m/s just before it hits the ground. Calculate the kinetic energy.

(2 marks)

5A student drops a spring onto the ground and the spring compresses. The spring constant of the spring is 100N/m. You will need to use the equation:

elastic potential energy Ee (J) spring constant k(N/m)  extension2e2 (m2). Remember that the extension should be in metres.

Complete the table.(2 marks)

Height dropped from in m / Compression of spring in cm / Ee (J)
1.00 / 2.0
0.50 / 1.3
0.25 / 1.0

Part 3: Power and efficiency

6Write down two equations that you can use to calculate power.

(2 marks)

7In each of the following situations write down and explain which student is more powerful.You do not need to do any calculations.

aStudent A takes 25 seconds to lift 10 books onto a shelf. Student B takes 15 seconds to lift the same books onto the shelf.

(2 marks)

bStudent A transfers 10kJ swimming for 5 minutes, and Student B transfers 8kJ swimming for the same amount of time.

(2 marks)

8Explain the difference between an efficient and an inefficient appliance.

(1 mark)

9For every 100J of energy contained in the chemical store of petrol used by a car, only 20J is transferred to a kinetic store.About 50J is transferred by heating to the surroundings, and the remainder is transferred by sound.

aCalculate how much energy is transferred as sound.

(1 mark)

bCalculate how much energy is wasted in total.

(1 mark)

cCalculate the efficiency of the car.

(3 marks)

dA different car engine transfers 750J to a kinetic store from the 1000J supplied in fuel. Is this car more or less efficient? State your answer and explain why.

(3 marks)

© Oxford University Press 2016

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