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You measure the efficiency of an energy transfer by using the formula: Efficiency = (Useful Energy Output / Total Energy Input) × 100%.

Efficiency is a way to see how well energy is being used in a process. To calculate it, you need to know two things: the useful energy output and the total energy input. The useful energy output is the energy that is actually used for the intended purpose, while the total energy input is the energy that is put into the system to start with.

For example, imagine you have a light bulb. The total energy input is the electrical energy it uses. The useful energy output is the light energy it produces. However, not all the electrical energy is converted into light; some is lost as heat. By measuring both the electrical energy input and the light energy output, you can use the efficiency formula to find out how efficient the light bulb is.

The formula is: Efficiency = (Useful Energy Output / Total Energy Input) × 100%. This gives you a percentage. If a device is 100% efficient, it means all the input energy is converted into useful output energy. In reality, no device is 100% efficient because some energy is always lost, usually as heat.

For instance, if a light bulb uses 100 joules of electrical energy and produces 60 joules of light energy, the efficiency would be (60 / 100) × 100% = 60%. This means 60% of the electrical energy is converted into light, and the remaining 40% is wasted, mostly as heat.

Understanding efficiency helps us to choose better devices and improve energy use, which is important for saving energy and reducing costs.

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