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P1 I) Energy Efficiency
P1 I) Energy Efficiency
Energy can never be created or destroyed. Instead it is transferred usefully, stored or dissipated (wasted).
A useful device will transfer energy from one energy store to another energy store that is useful. However, not all of the inputted energy will be transferred to the useful energy stores because some of the inputted energy will be transferred to energy stores that are not useful (the energy is dissipated/ wasted). Usually, the energy that is dissipated is transferred to the thermal energy stores of the object (the object gets hot). For example, a battery powered fan transfers energy from the chemical energy stores of the battery to the kinetic energy stores of the blades on the fan (useful energy store) and to the thermal energy store of the fan (wasted energy store – the fan getting hot is not useful).
The efficiency of a device tells us the proportion of inputted energy that is transferred to useful energy stores. The formula for working out efficiency is shown below:
A useful device will transfer energy from one energy store to another energy store that is useful. However, not all of the inputted energy will be transferred to the useful energy stores because some of the inputted energy will be transferred to energy stores that are not useful (the energy is dissipated/ wasted). Usually, the energy that is dissipated is transferred to the thermal energy stores of the object (the object gets hot). For example, a battery powered fan transfers energy from the chemical energy stores of the battery to the kinetic energy stores of the blades on the fan (useful energy store) and to the thermal energy store of the fan (wasted energy store – the fan getting hot is not useful).
The efficiency of a device tells us the proportion of inputted energy that is transferred to useful energy stores. The formula for working out efficiency is shown below:
There is a very similar formula to work out efficiency if we have the total power input and useful power output. This formula is shown below:
Both of the formulas above will give the efficiency as a decimal. If we wanted to have the efficiency as a percentage, we can multiply our decimal by 100.
The closer that the percentage for efficiency is to 100%, the more efficient the device is at transferring the inputted energy into useful outputted energy. The closer that the percentage for efficiency is to 0%, the less efficient the device is at transferring the inputted energy into useful outputted energy.
All devices dissipate some energy, which means that all devices will have an energy efficiency that is less than 100%. The only exception is electric heaters, which are usually 100% efficient because all of the inputted energy is transferred to thermal energy stores (which is the useful energy store for electric heaters).
The closer that the percentage for efficiency is to 100%, the more efficient the device is at transferring the inputted energy into useful outputted energy. The closer that the percentage for efficiency is to 0%, the less efficient the device is at transferring the inputted energy into useful outputted energy.
All devices dissipate some energy, which means that all devices will have an energy efficiency that is less than 100%. The only exception is electric heaters, which are usually 100% efficient because all of the inputted energy is transferred to thermal energy stores (which is the useful energy store for electric heaters).
Example 1
An 800 watt microwave has a useful power output of 500 watts. Find the efficiency of the microwave. Give your answer as a percentage.
The values in the question are to do with power. Therefore, we work out efficiency by using the formula with power – this formula is shown below:
An 800 watt microwave has a useful power output of 500 watts. Find the efficiency of the microwave. Give your answer as a percentage.
The values in the question are to do with power. Therefore, we work out efficiency by using the formula with power – this formula is shown below:
The question tells us that the useful power output is 500 watts and the total power input is 800 watts. We sub these values into the formula to find the efficiency.
We want to have the efficiency as a percentage, so we multiply our decimal by 100.
The efficiency of the microwave is 62.5%.
Example 2
I mix some dough in an electric mixer. The mixer is mixing the dough for 3 minutes and whilst the mixer is on, the total energy that the mixer transfers is 72,000 joules. The mixer has an efficiency of 65%. Find the amount of energy that is transferred to the useful energy output.
The question is asking about energy rather than power. Therefore, we use the efficiency formula with energy – this formula is shown below:
I mix some dough in an electric mixer. The mixer is mixing the dough for 3 minutes and whilst the mixer is on, the total energy that the mixer transfers is 72,000 joules. The mixer has an efficiency of 65%. Find the amount of energy that is transferred to the useful energy output.
The question is asking about energy rather than power. Therefore, we use the efficiency formula with energy – this formula is shown below:
We want to find the useful energy output, so I am going to rearrange this formula to make useful energy output the subject. We do this by multiplying both sides of the equation by “total energy input”. This results in the equation becoming:
The efficiency needs to go into this formula as a decimal rather than a percentage. The question tells us that the efficiency of the mixer is 65%. We convert percentages to decimals by dividing by 100, which means that 65% as a decimal is 0.65 (65% ÷ 100 = 0.65). The question tells us that the total energy input is 72,000 joules. We can now find the useful energy output by subbing in the efficiency as 0.65 and the total energy input as 72,000.
The useful energy output is 46,800 joules.