3 - Energy and Efficiency
29 June 2020 22:28
Spec points
4.4 know and use the relationship between efficiency, useful energy output and total
energy output:
Text book pages - p137
Aim: to be able to calculate efficiency
1.
Retrieval practice
1. List the stores and transfers
2. The principle of conservation of energy states..........
3. State the unit for Force, Distance and Work Done
4. State the formula linking Work Done, Force and Distance
Starter
Complete the Sankey Diagrams:
1. A cheetah accelerates, transferring 100J of chemical energy to 60J of kinetic
energy. 40J is wasted as thermal.
2. A lift has an input energy of 500J. The useful energy transferred to gravitational
potential energy is 400J. The rest is wasted as thermal.
Teacher Notes
Efficiency is a measure of the ratio between input energy and useful output energy.
Maximum value = ????
e.g.
A light bulb gets 100J of electrical energy input. It produces 20J of light, and 80J are
wasted as thermal. Calculate the efficiency of the bulb.
Practical / Demo
Core questions
Board it up
Optional questions
Draw a Sankey Diagram and Calculate the Efficiency:
Worked solutions
Simulation
Energy Skate Park: Basics
Quiz
Plenary
Create a short video explaining the key concepts that we have met so far…
• The energy stores
• The energy transfers (and examples)
• Conservation of energy
• Sankey diagrams
• Efficiency
Extension ideas
Energy efficiency questions
1. Rachel uses 24 kJ of energy drying her hair with a hairdryer. The hairdryer is
45% efficient. How much energy is wasted as sound? [3] [grade 5]
2. At the top of a rollercoaster Julia has a gravitational potential energy store of 900
J. Halfway down the ride, 375 J of this energy has been transferred into a kinetic
energy store, giving Julia a thrilling rollercoaster ride. a) How efficient is the
rollercoaster? [2] b) How much energy will have been wasted in total by the time
Julia reaches the bottom of the ride? [2] [grade 6]
3. The daily recommended consumption of calories for the average adult woman is
2,000 kcal, equivalent to 8,400 kJ of energy gained from food. In a typical day,
1,680 kJ of this energy is used by the brain and 5,040 kJ is used in physical
activity and body processes. The rest is transferred externally as thermal
energy. a) How efficient is the human body? [3] b) Is the thermal energy ‘wasted’
energy? Explain your answer. [2] [grade 7]
4. Discuss whether it is possible practically to have a machine that is 100% efficient.
Justify your answer, including reference to the principle of conservation of energy.
[4] [grade 7]
5. Both uranium and coal are used in power stations to generate electricity. 700,000
MJ of energy is released per kilogram of uranium, and 25 MJ per kilogram of coal.
1. If using uranium to generate electricity is 35% efficient, and using coal is 40%
efficient, how much of each fuel by itself would be needed to generate 490,000
MJ of electrical energy? [3]
2. How might the results of this calculation influence which power stations are
built to supply electricity? [2] [grade 8]
NB Markscheme below!
1 24 kJ = 24,000 1
Useful transfer = Eff x Input store = 0.45 x 24000 = 10,800 J 1
Wasted = 24000 – 10800 = 13,200 J 1
(can also be calculated as 55% of 24,000)
2 1. Halfway down, transfer = 900 / 2 = 450 J 1
Eff = 375 / 450 = 0.83 or 83% 1
2. 2 x (450 – 375) = 150 J
2
3 1. Useful transfers = 1680 + 5040 = 6,720 kJ 1
Eff = 6720 / 8400 1
Eff = 0.8 or 80% 1
2. The thermal energy is a product of cellular respiration. The human body needs to be maintained at around 37 C to properly
function, and without the thermal energy released by respiration, it would be difficult to maintain this temperature. Therefore, 2
in a sense, the thermal energy is not a wasted transfer. connected
points
4 Possible answer:
The principle of conservation of energy states that energy cannot be created or destroyed, only transferred between different energy 4
stores. connected
points
Any machine, process or interaction between objects involves some transfer of energy between stores’
In any practical machine there would always be some friction or resistance opposing a transfer of energy.
This would result in a wasted energy transfer, and therefore the machine could not be 100% efficient.
5 1. Uranium: Input energy transfer = Useful transfer / efficiency
Input = 490000/0.35 = 1,400,000 MJ 1
Mass of uranium needed: 1400000 / 700000 = 2 kg
1
Coal:
Input = 490000/0.45 = 1,225,000 MJ
Mass of uranium needed: 1225000 / 25 = 49,000 kg
2. Less uranium is needed to generate similar amount of electricity, so fewer power plants would be needed, and less 1
infrastructure would be required to support them. This could potentially mean that power stations could be situated further
away from cities. 2
connected
points
Seneca
Efficiency:
https://app.senecalearning.com/classroom/course/8d64c500-1ba7-11e8-9
294-c3528be8998b/section/91f82a16-7b7c-4d05-
a395-2a99b53bb087/session
GCSE Physics Online - Efficiency:
Efficiency - GCSE Physics
FreeScienceLessons - Efficiency:
GCSE Science Revision Physics "Efficiency
