P1aL8 Using Energy Efficiently
Key Words
Efficiency - fraction of the total energy that is useful energy
Energy input - amount of energy put in.
Energy output - how much energy is got out as useful.
Energy transfer diagram - picture of how energy is transferred and transformed into different kinds of energy.
Environment - the surroundings, including plants and animals.
Insulation - material that conducts heat badly.
Kinetic energy - Energy of a moving object.
Payback time - time needed to recover the costs of doing energy-saving work.
Wasted heat energy - energy lost as low grade heat to warm up the environment.
Grade E
An efficient machine is one that uses as little energy as possible to do a particular job. Little energy is wasted.
The equation for efficiency is:
Efficiency = useful energy output
total energy input
Often the efficiency is given as a percentage:
Efficiency = useful energy output × 100 %
total energy input
You cannot ever get more than 100 % efficient.
A diesel engine gives out 350 J of kinetic energy for every 1000 J of chemical energy put in. It is 35 % efficient. A petrol engine gives out 300 J of kinetic energy for each 1000 J put in. It is 30 % efficient.
It's essential to use less energy, as there are serious problems affecting the environment, such as global warming. Various small things can help:
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Switching off appliances that are not in use;
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Turning heating down;
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Walking or cycling for short journeys instead of using a car;
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Going by bus or train for longer journeys;
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Insulating our homes.
If everyone did these, large amounts of energy could be saved.
Grade C
We can link efficiency to Sankey Diagrams:
You can see that the efficiency is:
Efficiency = kinetic energy = 300 J = 0.3 = 30 %
chemical energy 1000 J
Efficiency can be expressed as a fraction, a decimal, or a percentage.
We can link efficiency to power as well.
Example
A crane uses 5000 W to lift a 200 kg mass 3 metres in 10 seconds. What is its efficiency?
1. Work out the potential energy the mass gains by being lifted:
Gravitational potential energy (J) = mass (kg) × acceleration due to gravity (m/s2) × vertical height (m)
= 200 kg × 10 m/s2 × 3 m = 6000 J
2. Work out the power that is used to lift the load:
Power (W) = energy (J) ÷ time (s)
= 6000 J ÷ 10 s = 600 W
3. Now work out the efficiency:
Efficiency = 600 W × 100 % = 12 %
5000 W
We can make our homes use less energy by:
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Installing double glazing;
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Insulating the loft;
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Using energy-saving light bulbs;
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Cavity wall insulation;
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Draught-proofing doors and windows.
Each of these will cost us money, but we will get the money back in the savings we make. This is the payback time:
Payback time = cost of insulation ÷ saving made per year.
Loft insulation may cost £250 to install, and saves £50 a year on heating bills. The payback time is 250 ÷ 50 = 5 years.
Grade A
No machine is ever 100 % efficient. If it were, it would be a perpetual motion machine. Some toys are sold as "perpetual motion machines" but actually have a small battery to power the machine (usually through a small electromagnet). If the battery runs out, the machine stops.
Electrical transformers are among the most efficient machines, about 95 %.
The Wider Picture
When we put energy saving measures into our homes, the argument about payback time is not the only thing we need to consider. Energy is used by the factories that make the products in the first place.
Also some draughts are needed, otherwise our homes would have a stale atmosphere and running gas appliances could be dangerous, because carbon monoxide could build up. Carbon monoxide is a highly toxic gas, which has no smell. Exposure to it can be rapidly fatal.
Energy saving bulbs might give out more light and less heat, but they are ugly and the light given out is like that in an office or classroom. Additionally they require much more energy to make, they are expensive, and contain toxic materials like heavy metals. Also an ordinary light bulb helps to heat the room, so less heat is needed from the central heating.