hello physical sciences and mouse learners i'm miss martins and welcome to my youtube channel please don't forget to give the video a thumbs up don't forget to subscribe to my channel so you don't miss a lesson tell me what topic you would like to see next enjoy the lesson relevance today we're doing ohm's law i'll also be going through some more basic calculations with you that we have covered last year but we need to practice okay so ohm's law i have mentioned this before it's this relationship between voltage or potential difference current and resistance so this little triangle comes into play over here we're going to be speaking about the mathematical relationship between these three things here's a little photo again just to illustrate what these three things are and what they do so the voltage or potential difference gives the push to the current to the little charges in the circuit so you see he's kicking the amp so that's the current the current flows and the resistance resists the current so it kind of impedes the flow so yeah you can see he's tying a little rope around the current and this current guy is saying currently i'm not traveling very fast okay it's supposed to be funny okay ohm's law this is a definition that we can ask you so maybe i should just make that clear definition all right yeah and i want this in your books please because this is the number one new definition you are learning this year in grade 11. okay so let's read the potential difference across a conductor is directly proportional to the current in the conductor at a constant temperature so it's this v equals i times r thing so potential difference is directly proportional to current and you know what directly proportional means here we go with the mathematical relationships okay so it means that as the one goes up the other one goes up by the same amount in the same proportion the one goes down the other one will go down so as potential difference or voltage increases so will the current increase and that makes sense because remember the potential difference is the push that the charges need and when we push the charges that causes current so if we give more push okay that sounds weird if we provide the charges with more of a push so more power basically essentially then we'll have a stronger current okay here's the relationships again we can use this triangle to derive any one of the three equations it's the same thing it's just isolating a different variable each time and okay yeah so this is ohm's law as an equation over there and this is ohm's law as symbols so v voltage or potential difference is directly proportional that's a little fish sign to i which is current but the thing is it must be at a constant temperature if we change the temperature this relationship will not work so voltage is directly proportional to current at a constant temperature this is important and you need to have that in your book as well okay now we have a graph you can potentially receive a graph like this in your exams and you have to understand what's going on so over here we have voltage on the y-axis current on the x-axis and we have a straight line a graph that's a straight line going through the origin in other words it cuts here at zero zero so here it says the graph of best fit is a straight line through the origin that's important and what this means is if it's a straight line and it's going through the origin it means that the conductor is obeying ohm's law and ohm's law basically says that the resistance will remain constant provided that the temperature doesn't change so if you look here voltage divided by current will give you resistance and the resistance won't change so as i go up in voltage i'm going to go up in current by the same proportion so this ratio won't change so say for example it's 10 divided by 2 will give you 5. or 15 divided by three will give you five or 50 divided by 10 will give you five so you can see that the ratio of voltage and current will give you a constant r and that's why the graph is straight and it goes through the origin you can think of this line this the gradient of this line as being resistance because we know that we calculate gradients you should know this this is literally grade nine work we calculate gradient by saying the change in y over the change in x so what is y on this graph my y is my voltage so change in voltage over change in currents change in x that gives me my gradients here they give you the little triangle we know that that is used to illustrate gradients change in y over change in x or rise over run however you learn gradients or learned gradients that will give you the gradient of the line which is resistance and you can see the gradient stays constant okay another thing just to note again best fit straight line through the origin that means that voltage is directly proportional to the current so as voltage increases so does current by the same proportion here it says in the same proportion so we know that that is what that mathematical relationship means okay so let's just take a look at these formulae i'm saying here the greater the resistance the smaller the current so look at this middle formula here if i make r bigger so resistance bigger so a big number i is going to go smaller so the greater the resistance the smaller the current so in terms of the mathematical relationship it makes sense but also think of it logically if i'm providing a greater resistance so i'm resisting the flow of the current i'm going to have a smaller current so it's like going back to this picture of this little resistance guy if he provides more resistance there's going to be less current flow okay the greater the resistance the greater the potential difference across the resistor so what that means is if i choose to include a resistor in my series and it has a very high resistance i'm going to need to increase my potential difference because it's providing such a high resistance in order to get my current to flow i'm going to need a greater potential difference or greater push that makes sense so v voltage or potential difference is directly proportional to r so you can see that here with this formula if i increase r so make this number bigger v is going to go bigger okay now that just brings us to the last little section of this feat ohm's law and you know what ohm's law is so if it obeys ohm's law we're going to see a straight line through the origin when our plots voltage and currents against one another so it means that the resistance is constant the resistance does not change here we can see straight line through the origin constant resistance voltage divided by current will always give me the same r value so here so we can use this relationship or this equation and it says most metals and commercial resistors are close to being ohmic okay now nonomic does not obey ohm's law look at this voltage and current graph you do not see a straight line that goes through the origin you see a kind of a curved line okay so that means that the resistance is not constant if i go voltage divided by current i'm going to get different resistance values as i go along so it'll only work this equation will only work for specific values for specific sets of values but it doesn't always work the ratio doesn't always remain constant therefore resistance is not always constant so you just need to be aware of how a ohmic graph looks versus a non-omic here's another comparison so this is voltage or potential difference against current here's a ohmic resistor and a non-ohmic see it curves like that and again over here resist um resistance is constant here if it's a omic device resistance is not constant here if it's a non-ohmic device if you guys are paying attention you'll actually notice that in this example they've swapped the axes so here we've been doing if you look at all these graphs we've been doing voltage on the y but if you look here voltage is now on the x that's okay doesn't matter all that means is that the gradient in this case won't be r it won't be resistance it'll be one over resistance okay it's not it's not a massive train smash say you work up the gradient here and it's a half one over two then resistance will be two over one so the inverse if you work out the gradient for this line and you get 10 so the gradient is 10 resistance will be 1 over 10. that's just because you've swapped current and voltage don't worry too much if you're not understanding what i'm saying we should cover this in calculations same thing here current and voltage we've swapped axes here and that's why the curve is going like this okay look at the curve here it's going curving like upwards here it's curving downwards it's just because we swapped the axes okay so you need to look at the axis carefully if you get a question like this okay so that's ohm's law in the next little video i'm going to go over some more basic calculations covering work that we did in grade 10 so please watch that video you