KS3 Physics - Conservation of Energy

Added: 2026-05-12

[00:00:07]In this video, we're learning about the conservation of energy.

[00:00:10]So, we'll cover the principles of the conservation of energy and then take a look at energy transfer diagrams, as well.

[00:00:20]If we begin with the principles of the conservation of energy, there are three important principles you need to know.

[00:00:26]The first is that energy can never be created or destroyed. It can only be transferred from one store to another.

[00:00:33]This means the total amount of energy always remains constant. The energy just moves between different energy stores and different objects.

[00:00:41]The second principle is that energy is only useful when it's transferred from one store to another.

[00:00:47]For example, the chemical energy in your muscles is only useful when it's converted to kinetic energy to make your body move.

[00:00:55]The third principle to understand is that energy input is equal to energy output.

[00:01:00]Or in other words, the energy put in is the same as the energy you get out.

[00:01:05]However, it is worth noting that the output will be a combination of useful energy, which is the stuff we want, but also wasted energy, which is the stuff we don't want.

[00:01:15]For example, when you turn on a light bulb, the input is electrical energy, and this will be transformed into light energy, which is useful energy output, but also heat energy, which is wasted energy.

[00:01:27]The most efficient devices are those that convert the highest proportion of the input energy into useful energy output and convert the smallest amount of input energy into wasted energy output.

[00:01:38]For instance, an LED bulb is more efficient than an old filament bulb because it wastes less energy as heat.

[00:01:49]So, now, let's take a look at energy transfer diagrams.

[00:01:52]These diagrams help us to visualize how energy is transferred from one store to another.

[00:01:58]Let's use the example of a modern LED light bulb and an old filament light bulb.

[00:02:03]Imagine the LED receives 100 joules of electrical energy.

[00:02:07]Not all of this energy is converted into useful light energy like we would want it to be.

[00:02:12]For example, only 75 joules might be converted into light energy, while the remaining 25 joules are wasted as heat energy.

[00:02:20]If we then compare this against the old filament light bulb, these bulbs are much less efficient.

[00:02:26]If the same 100 joules of electrical energy is supplied, only about 10 joules might be converted into light energy, with the remaining 90 joules being wasted as heat energy.

[00:02:36]In exams, it's quite common to get questions where they give you a diagram like this and then ask you to work out a missing number.

[00:02:43]For example, if we had a diagram where we have 150 joules of input energy and 120 joules of useful light output, you could be asked to find the missing waste heat output.

[00:02:55]To do this, you need to remember from earlier that energy input equals energy output.

[00:03:00]Or in other words, the energy input is equal to the useful energy output plus the waste energy output.

[00:03:07]So in our case, the 150 joules of input energy must equal 120 joules of useful light output plus the unknown waste heat output.

[00:03:17]So we can just rearrange and do 150 minus 120 to find that there must be 30 joules of heat output.

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