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# Efficiency

### Efficiency

In this lesson we will cover the concept of efficiency and calculate it for appliances that deals with energy.

In this lesson, we will look at what we mean by **efficiency** and how to calculate it.

Look at this lamp. The purpose of it is to provide light. When it is switched on, it heats up quite a lot. Do you think the lamp will be 'more' or 'less' efficient when it gets very hot?

Useful energy and wasted energy

The energy that devices (like lamps) transfer into the energy that they are supposed to create (like light energy) is called **useful energy**. Any energy that is transferred into a different kind of energy is referred to as **wasted energy** (like heat).

Efficiency tells us what **fraction** of the energy we put into an appliance is transferred to **useful** energy.

Lamp 1 converts almost all of the electrical energy into light energy. Lamp 2 converts just a little bit of energy into light energy. Which lamp is more efficient?

Is there such a thing as 100% efficient appliance, do you think? Answer yes or no.

In fact...

it is impossible to avoid wasting at least *some* kind of energy during energy transfer, so there is no such thing as a 100% efficient appliance!

What do we call the energy we give an appliance?

Can you guess which expression roughly gives the efficiency of an appliance?

Calculating efficiency

We calculate the efficiency of any energy transfer...

by dividing the useful output energy transferred by the total amount of input energy transferred: $Efficiency=\frac{Useful\:Output\:Energy}{Total\:Input\:Energy}$

To calculate the efficiency as a percentage...

multiply this equation by 100:$Efficiency=\frac{Useful\:Output\:Energy}{Total\:Input\:Energy}\times100$

Do you think efficiency can ever be larger than 100%? Answer 'yes' or 'no'.

If you forget which way round output and input energy go in the equation...

then remember that your answer must always be less than or equal to 1 (or 100%). So if your answer for efficiency is greater than 1, your equation is upside down!

A kettle is being used to heat up some water. $15,000 J$ of thermal energy is transferred by the kettle. $12000J$ is actually transferred into the water. Calculate the efficiency of the kettle.

What is the efficiency of a light bulb which requires $200,000J$ to work, but which only transfers $150,000J$ into light?

Power is measured in Watts, which is equal to **Joules per second**. Now, how is **power** (P) related to **energy** (E)?

Out of these options, which do you think are good ways to increase efficiency? Pick all the options you think are correct.

You can select multiple answers

To sum up!

Efficiency is the proportion of the total input energy given to a device that is transferred into useful output energy.

$efficiency=\frac{Useful\:Energy\:Output}{Total\:Energy\:Input}$

Efficiency can also be calculated from the power input and output of a device

$efficiency=\frac{Useful\:Power\:Output}{Total\:Power\:Input}$

To find efficiency as a percentage, simply multiply the decimal by 100

$efficiency \times 100 = \%$

Finally, you can increase the efficiency of a system by reducing energy wastage.

The more efficient a device is, the less energy it will use because the less energy it will waste.