[Music] hi and welcome back to fre science lessons. co.uk by the end of this video you should be able to calculate the efficiency of energy transfer you should then be able to describe ways to increase the efficiency of an energy transfer but that's only for higher tier students I'm showing you here two different types of light bulbs on the left we've got an old style incandescent bulb these are being phased out but you can still find them in buildings on the right hand side we've got an energy efficient LED bulb these are now increasingly popular in homes around the UK so in this video we're going to look at what's meant by efficiency and how we can calculate the efficiency of an energy transfer efficiency tells us what fraction of the energy we put into an appliance is transferred to useful forms of energy we calculate efficiency using these equations efficiency equals the useful output energy transfer divided by the total input energy transfer or efficiency equals the useful power output divided by the total power input you're not given these equations in the exam so you do need to learn them take a look at this question an electric hob is used to heat water 15,000 Jew of thermal energy is transferred by the hob 12,000 Jew of thermal energy passes into the water calculate the efficiency of the energy transfer so the useful output energy transfer in this case is 12,000 Jews of thermal energy passing into the water the total input energy transfer is 15,000 Jew of thermal energy from the hob putting these numbers into the equation gives an efficiency of .8 now there are a couple of point about this that you need to learn firstly we've calculated efficiency as a decimal and that's fine you're expected to do that in your exam however we can also calculate efficiency as a percentage and to do that we multiply our decimal value by 100 like this now you could be asked to do that in your exam as well the second important point is that most students who struggle with calculating efficiency write the calculation upside down like this now you can tell if you've done this because you'll get an efficiency value of greater than one or greater than 100% and that's just not possible here's a question for you to try an electric current delivers a power of 60 wats to a light bulb 3 wats of electrical energy are transferred to light energy calculate the efficiency of the bulb okay pause the video now and try this yourself so here's the equation that we're going to use efficiency equals the useful power output divided by the total power input the useful power output is 3 wats and the total power input is 60 wats putting these numbers into the calculation gives us an efficiency of .05 or 5% so as you can see these bulbs are not efficient at all in fact most of the electrical energy that these bulbs receive is transferred into thermal energy which is simply wasted LED bulbs such as the ones we saw before are much more efficient as a much greater percentage of energy is transferred to light and much less to thermal energy now in the exam you could be asked how we can increase the efficiency of an energy transfer so we're going to go back to the pan of hot water that we saw before now as we saw this method of energy transfer wasn't very efficient we want thermal energy to pass from the hob into the water however a lot of the thermal energy is wasted we've got thermal energy passing into the air around the edges of the hob that are not in contact with the pan we've also got thermal energy passing into the air at the surface of the water we can reduce both of these by using a pan with a wider base and a lid like this both of these reduce unwanted transfer thermal energy now the other issue with using a pan to heat water is that a lot of thermal energy is actually used to heat the base of the pan itself so we can overcome that by placing the heating element inside the water and that's how an electr Kettle works like this now there is one final point to consider here if we go back to the pan we can see that metal is a good conductor of heat so thermal energy will pass through the sides on the lid of the pan into the air like this now plastic conducts heat less well than metal we say that it's got a lower thermal conductivity and we're going to look at thermal conductivity in a later video so looking at the cattle we can see that less thermal energy passes through the walls and the lid and into the air so again this makes the kettle a more efficient way of heating water than using a pan remember you'll find plenty more questions on calculating efficiency in my vision workbook and you can get that by clicking on the link above okay so hopefully now you should be able to calculate the efficiency of energy transfer you should then be able to describe ways to increase the efficiency of an energy transfer but that's only for highs students [Music]