funding for audible pain course television was made possible in part by the College Board almost nobody can reason with you Mr we's Cunningham McHugh the great founder himself and all other funding were made possible by viewers like you we forced you no God no God please no no hello and welcome back to another episode of the apct TV the EM Edition I'm your host Mr M and today you'll be watching season 1 electric circuits episode 5 EMF my BFF so the objectives for today's video is to be able to Define EMF in an ideal versus non ideal battery and to be able to find EMF graphically using a VI curve so EMF or what we call Electro motive force or EMF for short is the ideal potential difference across a battery all right so if you measured the voltage across a battery if it's ideal we call it the EMF it's the ideal voltage it's the voltage the battery should give you so if you have a 9vt battery it should give you 9 volts of EMF 9 volts potential difference it is not a force the word Force has nothing to do with EMF it's mostly just a misnomer so we simpol this using Epsilon or a curvy e and we measure it in volts all right and so like I said with an ideal battery like if you measure the voltage of battery it should be what it tells you in an ideal battery there's no internal resistance meaning there's nothing in the battery actually stopping uh the flow of current current can flow freely with absolutely zero resistance because as much as we'd like to pretend we do not live in a perfect world so there is in a real world situation an internal resistance in the battery there's some impedance to the flow of current but in an ideal scenario you have a battery connected to a resistor and you have current flowing and there is no resistance coming from the battery so if you take a voltmeter and you put it on points A and B across the battery and measure the terminal voltage or the actual measured voltage across the battery it would be the difference in those two points and that would just be the EMF if you put a lead on a and b you would just be measuring the EMF or the ideal amount of voltage from the battery as there's no internal resistance in the ideal scenario so terminal voltage what you measure would be the EMF you know that's the pleasant surprise you get when you measure a 9vt battery and you actually get 9 volts out of it if we use Loop rule cuz you can never get enough Loop rule with Kira in any circuit if you go around your Loop you have the sum of the volts is zero is the EMF for Epsilon minus the voltage across the resistor so in an ideal scenario the EMF is equal to the voltage across the resistor which is i r you could just do current times resistance and you would have the EMF or the ideal voltage which would also be your terminal voltage in this case now looking at a real scenario meaning a nonideal scenario because we don't live in a Ideal World is you have a battery with current running through it and there is some internal resistance there's a little bit it's not big that's why we use little r for internal resistance but there is some impedance to flow of current so when you take those same leads to Across the battery A and B all right you're measuring the terminal voltage again but you have to account for you're measuring the EMF minus the potential drop taken away by the potential by the internal resistance or I * little r so now what you're actually getting from the battery is the EMF what you should be getting minus what the interal resistance is taken away in terms of potential dropper I * little r and I want to clarify with this diagram that internal resistance is not a resistor that you put into the circuit it's an inherent property of the battery you put a battery in a circuit the circuit flows or the current flows and the battery has internal resistance so we always draw internal resistance right next to the battery because it's technically in the battery it's not part of the circuit it's not part of anything else sometimes you might even see it boxed just to signify again it's part of the battery it's not an external part like the resistor that you put into the circuit it's inherent to the battery and again what's happening is you have your ideal potential difference across the battery and now this internal resistance is taking some away so when you measure your ideal 9vt battery you might only measure 8 volts your terminal voltage your actual voltage might only be 8 volts instead of the ideal 9 because of internal resistance so one way to get um EMF for ideal voltage is to Loop rule this scenario and if you do Loop rule you get the EMF minus the current time little r as you travel your Loop minus big I Big R so you get EMF in a real scenario with internal resistance is the current times the total resistance or Big R plus little r they do add in series because internal resistance is always in series with the battery all right repeat that internal resistance is always in series with the battery so as promised we will now look at the key takeaways and how to graph find EMF graphically so two key takeaways is that if the voltage you're actually measuring in a real scenario with internal resistance is EMF minus I * little r if you increase the current of the circuit you decrease the voltage you're measuring all right I'll say that again if you increase the current of the circuit you decrease the actual voltage you're measuring across the battery you're actually getting across the battery because you're increasing the potential difference created by the internal resistance so bigger current less terminal voltage or less measured voltage on the battery the second key takeways similar is that only the only way to make the terminal voltage ideal or make it your EMF is to have no current whatsoever so if you have no current the minus IR term becomes zero and all of a sudden you are measuring your terminal voltage is the same as your ideal uh voltage your EMF all right so more current less terminal voltage and no current means you actually have an ideal battery or no um effects of internal resistance so graphically speaking finding EMF we can make a VI curve so we can plot what we're actually measuring across on in terms of voltage on the Y AIS so we could put terminal voltage on the Y we could put current in the circuit on the X and you'll get a downward uh negative linear slope so get a negative linear slope for your graph here and we interpret this by looking at our terminal voltage equation meaning we know V talir plus EMF I just switched EMF and ir and if you know the equation yal MX plus b you can interpret everything you need to know from this meaning terminal voltage is on your y AIS so y equals current is on your x axis so y = mx and that thing multiplied by X is your slope so actually our slope of this VI curve is internal resistance I should really say it's the negative internal resistance so in this VI curve you will get a negative slope and the SL equals R so if the slope was -10 ohms little r would be 10 ohms all right but that doesn't tell us how to get EMF well finish the equation y = mx + b we also know that VT = I plus b or plus EMF and B represents the Y intercept so the Y intercept here the top is your EMF which makes sense cuz that's when you have no current so your terminal voltage should be your EMF when you have no current so it makes perfectly good sense that when we have no current our terminal voltage we're measuring across the battery is the same as the EMF or the ideal voltage we should be getting but all you had to do here was look at y = mx plus b and look at VT equal uh EMF minus I and compare the two equations see oh VT is on the Y AIS or V is on the Y AIS I is on the x axis so my slope my M value must be R plus b which is plus EMF my Y intercept so if you ever have a a terminal voltage vers current graph the Y intercept will give you the EMF the slope will give you the negative internal resistance so if you have any comments or questions you can leave a comment or email me otherwise have a fantastic night [Music]