Topic Two: P1.2.1 Energy transfers & efficiency
Forms of What forms of are there? How can we illustrate changes?
Energy is the capacity to cause change no, no change Types of : Kinetic Light Thermal Sound These are all forms of on the move Gravitational potential Chemical potential Magnetic potential Mechanical potential Electrical These are all forms of stored
Energy can neither be created nor destroyed......but it can be transformed from one chemical potential heat light form to another...or it can be stored...or it can be dissipated ( allowed to spread out and become useless)
Electric bell: electrical kinetic sound
Solar cell and motor: light electrical kinetic Heat Sound
Spring launcher: chemical potential (in muscles) mechanical potential (in compressed spring) gravitational potential (in compressed spring) kinetic (in rising spring) sound heat kinetic (in falling spring)
Clockwork bug toy: chemical potential (in muscles) mechanical potential (in compressed spring) kinetic (in dancing toy) sound heat
Handheld fan: chemical potential (in cells) kinetic heat sound
Electric light: electrical heat light
Candle: chemical potential heat light
Sankey Diagrams In any transfer, some will always go into a form you don't want*. We call this waste input 100J of electrical 40J of light useful output *There's no helping this it's a law of the universe 60J of thermal waste
We use Sankey diagrams to show the transformations and how much changes form: 10 squares = 100J of electrical input I've used 1square = 10J but you can use any scale you like 6 squares = 60J of waste thermal 4 squares = 40J of useful light The length of the arrows is irrelevant Useful transfers are shown going to the right. Wasteful transfers are shown going up or down
Sankey diagrams are named after Captain Matthew Henry Phineas Riall Sankey, an Irish engineer who is considered to have been the first to use this type of diagram in 1898 in a publication on the efficiency of a steam engine
Question: A team of scientists test a brand new hybrid car. Question: How much is wasted as sound? 5% 100% input 40% useful kinetic waste sound 55% waste heat Task: Draw a Sankey Diagram of these transformations
100% (chemical potential, cpe) 55% thermal 5% sound 40% kinetic I chose a scale of 1 square = 10% 10squares = 100% Answer 5.5 squares = 55% 4 squares = 40% 0.5 squares = 5% Only the width of the arrow counts the length of the arrow is irrelevant The arrow heads are just decoration!
Question: A car burns petrol as it travels along a flat road. The diagram shows what happens to each 100 joules (J) of released by burning the petrol. 5 15 If 20 squares = 100J, 1 square = 5J 60 Hint: Work out the scale first Complete the diagram by adding the missing numbers. (Total 3 marks) They have to add up can neither be created nor destroyed
Efficiency What do we mean by efficiency? How efficient can a machine be?
Useful Useful Inefficient device Most transformed to waste Waste Efficient device Waste Most transformed to useful output.
We can calculate the efficiency: 100J filament lamp 90J 10J efficiency = useful out total in efficiency = useful out total in = 10J 100J = 0.1 100J low lamp 75J 25J efficiency = useful out total in = 75J 100J = 0.75
1(a) The diagram shows the transferred each second by a television set (i) What form of is transferred as waste by the television set? (1) Heat ( no useful function in a TV) (ii) What effect will the waste have on the air around the television set? (1) The air will get warmer
(iii) Use the following equation to calculate the efficiency of the television set: total input useful output efficiency = useful out total in efficiency = useful out total in = (119 + 1) = 0.6 200 (2)
(b) The diagrams show the transferred each second for three types of lamp. For each lamp the electrical input each second is 100 joules. (i) Which type of lamp is the most efficient? (1) Street lamp (ii) Give a reason for your choice The street lamp transforms the greatest proportion of the input (electrical) into useful output (1) (light) total: 6 marks
Efficiency homework The picture shows a solar powered aircraft. The aircraft has no pilot. (a) Use words from the box to complete the following sentence. electrical light heat sound Solar cells are designed to transform... light into... electrical (2 marks)
(b) On a summer day, 175 000 joules of are supplied to the aircraft s solar cells every second. The useful transferred by the solar cells is 35 000 joules every second. (i) Use the equation in the box to calculate the efficiency of the solar cells. efficiency = 35000J 175000J = 0.2 Show clearly how you work out your answer (2 marks)
(ii) Give one environmental advantage of using electric motors to drive the aircraft propellers rather than motors that burn a fuel.... This process does not involve burning fuels so there is no pollution/greenhouse gas production...(1 mark) (iii) Eventually, the designers want to produce an unmanned aircraft that can fly at twice the height of a passenger jet for up to six months. Suggest one possible use for an aircraft such as this.... Anything sensible! eg, mapping the ground, tracking migrating animals,...(1 mapping air currents, recording air pollution mark) (Total 8 marks)
(ii) What happens to the that is not usefully transferred by the solar cells? Lost as heat...(1 mark) (c) The aircraft propellers are driven by electric motors. As well as the solar cells, there are fuel cells that provide additional power to the electric motors. (i) Suggest one advantage of the aircraft having fuel cells as well as the solar cells. It can continue to fly at night/cloudy weather...(1 mark)
We also have to be able to calculate a "percentage efficiency" [ ] x 100% = useful output total input...and finally Some * is always wasted so an efficiency can never be 1 or greater, Example: 100J 90J 10J The %efficiency is 10% but the efficiency could be 0.99999999999999999999! *Law of the Universe
%efficiency = useful out total in x 100%