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Physics - Wasted Energy and Efficiency
LED lighting consumes a fraction of the power of a filament lamp.

Physics - Wasted Energy and Efficiency

This Physics quiz is called 'Physics - Wasted Energy and Efficiency' and it has been written by teachers to help you if you are studying the subject at senior high school. Playing educational quizzes is one of the most efficienct ways to learn if you are in the 11th or 12th grade - aged 16 to 18.

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When energy is transferred only part of it is usefully transferred - the rest is ‘lost’. Only it isn't really lost. It is still there but in different forms and spread out into the surroundings, so we refer to it as 'wasted energy'. This wasted energy is often in the form of heat but could be in other forms like light and sound. One way or another, the energy that has been transferred to the surroundings makes them warmer. The wasted energy is increasingly spread out and is therefore a lot less useful.

What is the efficiency of a computer that consumes 25 J/s of electrical energy and wastes 9 J/s of heat energy?
It is important to remember that efficiency is a measure of the useful energy transfer. The way the question is worded could make you forget that the calculation gives you the percentage of energy that has been wasted and needs to subtracted from 100% to find the efficiency. Watch out for this sort of question in your exams
What is a Sankey diagram?
A diagram of the London Underground railroad system
A diagram drawn to scale (usually using squared paper) to represent the efficiency of an energy transfer
A circuit diagram for a machine that can be used for measuring the efficiency of energy transfers
A graph showing only the percentage of useful energy produced in an energy transfer
These crop up almost every year in the exams
A houseowner replaces their filament light bulbs with LED lighting. This is 85% efficient and requires only a 7 watt input. What is the useful power output of the new lights?
2.95 W
4.95 W
5.95 W
Not only does LED lighting consume a fraction of the power of a filament lamp, it is also much more efficient at transferring the electrical energy to useful light energy
When the outside temperature is cold, the air inside a car is warmed by some of the thermal energy from the engine. What effect does this have on the overall efficiency of the car engine?
It remains the same
It increases
It decreases
A Sankey diagram is needed to work it out
Since this is a useful energy transfer, it increases the overall efficiency by using some of the heat energy that would otherwise be wasted
A roof receives 1,000 W/m2 of energy from the Sun on a clear, sunny day. How many watts would be produced from a 5cm wide by 20cm long solar cell positioned on the roof if the efficiency of the solar cell is 14%?
1.4 W
1.6 W
14,000 W
14 kW
This question could look very complicated at first. The knack is to break down complicated questions into smaller parts which makes them more manageable. So, first you need to work out the area of the solar cell which is 0.1 m2. That means that it receives 100 W of the insolation (incoming solar energy). Finally, rearranging the equation for efficiency provides the answer:
(efficiency x input wattage) ÷ 100 = output wattage
Distributing electrical energy through the 400,000 V cables of the National Grid wastes some of the energy of the electricity (see below). What is the efficiency of this part of the system to the nearest whole number?

Total power generated: 62,161.5 MW
Step-up transformer losses: 157 MW
Step-down transformer losses: 142 MW
Losses caused by heating of the cables during transmission: 1.4%
The total energy transfer losses are 1.87% so to the nearest whole number that is 2% wastage, leaving 98% efficiency. Efficiency is worse in the lower voltage sections of the National Grid because the higher currents cause more energy loss as heat
If a filament light bulb consumes 150 J/s and produces 12 J/s of light energy, what is its efficiency?
Answer four would be what you got if you did the division the wrong way round. The majority of the electrical energy is wasted as heat
How is the efficiency of an energy transfer expressed?
As a percentage
As a proportion
As a ratio
In kilowatt hours
Efficiency is always a percentage worked out by dividing the output energy by the input energy and multipying by 100
As a train travels along its rails, its wheels create sparks. Apart from this light energy, what other forms of energy are wasted?
Sound only
Heat only
Kinetic only
Heat and sound
The key useful energy transfer is to produce kinetic energy. The noise of the train and friction waste energy in the form of heat and sound
Burning 2kg of wood in a particular woodburning stove releases 29 MJ of heat energy. If 25 MJ of that heat is transferred to the room, which of the following statements is true?
The fire is 100% efficient
The fire is between 90% and 100% efficient
The fire is between 85% and 90% efficient
The efficiency of the fire is less than 85%
The efficiency is 25 ÷ 29 x 100 = 86.2% efficient
Author:  Kev Woodward

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