[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 resistance you should then be able to calculate resistance in a circuit now over the last few videos we've seen that electricity involves a transfer of energy in an electric circuit the cell contains chemical energy which is transferred to electrical energy and is carried by the current and remember that the current is simply a flow of electrons around the circuit when the electrons pass through a component such as a lamp the electrical energy is transferred to other forms of energy such as light and thermal energy we use potential difference to work out the energy transfers involved the cell has a potential difference of 9 volts so the current is carrying N9 jewles of electrical energy per Kum of charge the potential difference across the lamp is also 9 volts telling us that all of the electrical energy carried by the current is transferred by the lamp into other forms of energy so the question we're looking at is why does this energy transfer take place what about components causes them to transfer energy and the answer is resistance as we've said an electric current is a flow of electrons through a conductor such as a metal wire as they move electrons collide with atoms in the metal electrical energy is transferred into other forms of energy for example thermal the resistance simply tells us the potential difference required to drive a current through a component in other words how much energy is required to push a coolum of charge through I'm showing you here two circuits containing different lumps a current of 1 ampere is moving through both circuits in lump a the current is being driven by a potential difference of only one volt this tells us that only a small amount of energy is required to drive the current through this lump so lump a is a low resistance lump in lump B the current is being driven by 10 volts this means that a relatively large amount of energy is needed to drive the current through Lum B therefore lump B has a higher resistance than lump a it simply takes more energy to drive the current through lump B so can you see that the idea of resistance is actually fairly straightforward now you could be asked to calculate the resistance of a component it's easy to do this we simply use this equation the resistance equals the potential difference in volts divided by the current in ampir the unit for resistance is the ohm and you could be asked that here's the symbol for the ohm now you're not given this equation in the exam so you need to learn it now many students prefer to use a triangle for equations so here's a triangle for this equation so we're going to work out the resistance of the two lamps that we saw before lamp a has a current of 1 ampere and a potential difference of 1 volt putting these into the equation gives a resistance of 1 ohm lamb B also has a current of 1 ampere but this has a potential difference of 10 volts putting these numbers into the equation gives us a resistance for lamp B of 10 ohms now in the exam you could be asked to work out the potential difference needed to produce a certain and current to do that we need to rearrange the equation for resistance like this the potential difference in volts equals the current in amperes multiplied by the resistance in ohms here's a typical question calculate the potential difference across the lamp we can see that the current is 0.5 amp and the resistance of the lamp is 50 ohms so to calculate the potential difference we multiply the current by the resistance .5 multiplied 50 gives a potential difference of 25 Vols across the lamp remember you'll find plenty of questions on calculating resistance in my vision workbook which you can get by clicking on the link above okay so hopefully now you should be able to describe what's meant by resistance you should then be able to calculate resistance in a [Music] circuit