[Music] hi and welcome back to frees science lessons. co.uk by the end of this video you should be able to describe what's meant by power and then you should be able to calculate power using either energy or work done so far in this topic we've seen how energy can be transferred from one store to another we've also seen that we can use work done to calculate the energy transferred by a force moving an object so in this video we're going to look at Power here's the definition power is the rate at which energy is transferred or the rate at which work is done here are the equations for power you need to learn these as they won't be given in the exam the power in watts equals the energy transferred in Jews divided by the time in seconds we can also calculate power from work done in this case the power in watts equals the work done in in Jews divided by the time in seconds now the unit for power is the Watt and that has a symbol capital W one watt is an energy transfer or a work done of one Jewel per second I'm showing you here two beakers containing the same volume of water both of these are being heated by buns and burners and we're going to calculate the power of these energy transfers the left hand bunson burner is transferring a total of 250,000 Jew of thermal energy in 500 seconds and the right hand bunson burner is transferring a total of 180,000 Jew of thermal energy in 500 seconds the power is the energy transfer divided by the time so if we look at the left hand bunson burner we can see that we've transferred 250,000 Jew of thermal energy in 500 seconds putting these numbers into the equation gives us a power of 500 watts looking at the right hand bunson burner we're transferring 180 ,000 Jew of thermal energy in 500 seconds putting these numbers into the equation gives us a power of 360 wat so as you can see calculating power is relatively straightforward here's a question for you when a car stops 40,000 Jews of work is done by the brakes in a time of 5 Seconds calculate the power of the brakes pause the video now and try this yourself okay here's the equation that we need to use power equals work done and Jew divided by the time in seconds so the work done is 40,000 Jew and the time is 5 Seconds dividing 40,000 by 5 gives us a power of 8,000 wat and that's our final answer here's another question for you to try I'm showing you hear two Motors lifting two identical masses motor a transfers 500 Jew of electrical energy into gravitational potential energy in 40 seconds motor B also transfers 500 Jew of energy but this motor takes 60 seconds calculate the power of both Motors so pause the video and try this question yourself okay let's start with motor a the power is the energy transferred divided by the time in this case the energy transferred is 500 Jew and the time taken is 40 seconds putting these into the equation gives us a power of 12.5 wats looking at motor B again the energy is 500 Jew but now the time is 60 seconds putting these numbers into the equation gives us a power of 8.3 wat to one decimal place now you can find plenty more questions on calculating power in my vision workbook and you can get that by clicking on the link above okay so hopefully now you should be able to describe what's meant by power and then you should be able to calculate power using either energy or work done [Music]