WEBVTT
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A 60-watt incandescent light bulb is left on for 30 seconds.
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In that time, it is supplied with 1800 joules of energy, but it only converts 36 joules of energy to light.
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What is the efficiency of the bulb?
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So we’ve been asked to calculate the efficiency of something — in this case the light bulb.
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We’ve also been given values of the wattage of the bulb — 60 watts — and how long it was left on for — 30 seconds.
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We’re told that in the 30 seconds it was left on for, it’s supplied with 1800 joules of energy.
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But it only converts 36 joules of energy to light.
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Now, in order to find out the efficiency, we first need to know what efficiency is.
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Efficiency is defined as the useful energy output divided by the total energy input.
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So we can plug in the values we’ve been given for the light bulb in order to find out its efficiency.
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The useful energy output is the amount of energy converted to light, which happens to be 36 joules.
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The total energy input is 1800 joules.
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So we need to calculate 36 divided by 1800.
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And this turns out to be 0.02.
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But we can write this as a percentage as well simply by multiplying by 100 percent.
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And this way, we find that the efficiency of the light bulb is two percent.
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Now, there are pieces of information in the question that we haven’t used, for example, the fact that the bulb is 60 watts and it was left on for 30 seconds.
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These pieces of information are irrelevant for an efficiency calculation.
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Yes, it’s true that we can calculate the total energy input by using these two values: 60 watts and 30 seconds because the energy supplied to the bulb can be calculated by multiplying the power of the bulb by the time for which it’s left on.
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And this happens to be 60 watts multiplied by 30 seconds, which ends up being 1800 joules.
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But we’ve already been told this.
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So we don’t need the values of 60 watts and 30 seconds in this calculation at all.
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And this is something to be wary of.
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Some questions will give you values that you don’t need to use in the calculation.
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And this is done so that the question can test your understanding.
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It checks if you can think through the question and work out whether or not the answer that you’ve just calculated makes any sense.
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In this case, we’ve got an efficiency calculation.
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So it makes sense for the efficiency to be given as a percentage.
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So remember if you come across any pieces of information that you haven’t had to use in the calculation, don’t panic.
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Sometimes you don’t need those pieces of information.
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Think through your working out and make sure you can convince yourself that those extra pieces of information are not needed.