okay so to start this video off here we're going to go through an example relating to the current that we talked about in the previous video so this example here at time T equals zero the charge and a wire is two columns at T equals 300 seconds the charge in the wire is minus four coulombs so the charge has changed it says for us to compute what is the current in the wire and then assuming positive current flow is to the right what direction is the current flowing so the first part here for this is a very straightforward computation because all we have here is remember for this course we're defining our current as the change in charge over the change in time and so this would be minus four coulombs minus two well here let me go ahead and write the formula down here this is Q 2 minus Q 1 over T 2 minus T 1 so this again would be minus 4 minus 2 over 300 minus 0 which gets us -6 over 300 which I mean I could do this in my head but I always like to make sure I don't make silly arithmetic mistakes this is gonna be just give me a 2 here and if you do this in your calculator a lot of your calculators can already put it in engineering notation for us and so we get no minus 20 the engineering notation would be milli so we get minus 20 milliamps okay so now what we're saying is for this is essentially kind of for the first part for the second part let me draw a wire here now what the problem says is it's defining that this direction is positive current flow so if the current here is negative 20 milliamps that means the current is actually flowing in this direction here because if positive current flows to the right then negative current flow is going to be to the left so the current flow is to the left and that's the thing that's nice about current is it's basically one Direction's positive the other directions negative so you know if positive current flows to the right then negative current flows to the left so since we got a negative number we get that it's flowing to the left clearly if it had gotten a positive number I'd be flowing to the right now just to make sure we understand this here with one more quick example let's suppose I have a wire here and I have a current here I'll call I 1 and then I have a current going here called I 2 and I'm gonna tell you that I 1 is equal to 6 amps which is a side note is a lot of current what is the value of I 2 now the key thing to remember is again because of conservation of energy I can't suddenly have this current the the magnitude or the value of this current being any different so in other words I choose to us to be six it's just a matter of whether it's positive or negative so we know that I - we know that I - still has to be and I'll leave myself six here now I just have to decide is it positive or negative which one is it and again this is not supposed to be difficult it's going in the opposite direction of i1 and since I one was positive we know that I two is negative six amps and it's important to recognize because this is just a very important concept to recognize that you know as long as you just draw in the opposite direction all it does to the current is make it negative because we'll use this in analysis techniques and it's also useful just like so if you get a negative number as an answer you could say okay well yeah it's negative but if I flip it the other direction it would be positive all right so let's talk a little bit about voltage here so voltage there's a lot of ways that people try to explain something here as far as what voltage is and voltage obviously has plus and minus signs as polarity and things of that nature but I like to kind of first get us an idea of voltage first off voltage is always measured with respect to another terminal now one common one that we measure it with respect to his ground but we don't always measure voltage with respect to ground but that is one that we typically in ground as a sidenote ground is zero volts we just define ground to be zero volts alright so volts is always measured with respect to something but what is voltage I like to think of voltage and this isn't necessarily the best way or the only way to think of it I like to think of voltage and this is even what it's kind of defined as as potential and I'll put in parentheses here I almost think of it as kind of potential energy and and the way I like to think about this and you'll have to forgive my terrible sketching skills here is that I have a water tower here and out of this water tower there is a pipe and you know this pipe is going to you know feed our water system here so this is a pipe here and so what I have here and of course this is my ground here pipe goes underneath ground kind of thing so what I have here is of course some water in this thing here now because this is above ground and if you've ever used a rain barrel you will know exactly what I'm getting at here is that because this is above ground it's gonna exert some pressure on the water in this pipe and it's gonna kind of force this pipe to move and so let's say here you know we are kind of trying to measure we'll say this is my positive and this is my minus and then in terms of this we might measure this as far as like the height you know maybe and I'm just throwing a number out here maybe this is a hundred meters you know so that is an obviously a voltage but it's the idea that well because that water is a hundred meters up in the air it's got a certain amount of potential energy it's got a certain amount of way it can force the water down but we should understand that if I increased the height of that so let's say 200 meters I'll have more pressure being able to force more current down through this so that's the way I like to think of voltages it has some kind of ability to store potential energy for us and so we can use it kind of forcing if you want to think of it this water you can kind of think of its current going through the pipe so current going through the wire okay so that's how I like to think about voltage now when we think about voltage here again if we have some kind of terminal here so this is some kind of either device or circuit when we measure voltage there's always two terminals that we measure voltage with and we have plus minus and I'll say V so voltage is always measured with respect to two terminals now we'll get both a positive or a negative voltage so we can get both positive or negative voltage in it honestly does matter how you connect your device up if you've ever measured for instance the voltage of your car battery or if you've ever measured the voltage of just any battery you typically you know your voltmeter typically has a red and a black wire and you you know if you're measuring the voltage you typically put the red one I'm positive and the black one are negative you know if you're measuring a voltage you don't have to do that of course but you that's what you typically do but if we look at a device here and I'm going to again put two terminals here plus minus and I'm going to say V one and then I'm gonna do plus minus V 2 now here if I say that V 1 is equal to 10 volts then V 2 well the value is going to be the same the only thing that's different is that this is negative 10 volts for V 2 now the other thing that is useful to remember is that when we look at these here because we're also going to have current now that we have to think about so you have plus minus v1 and again we'll just let v1 be ten volts we typically think of the current flowing into the positive terminal it's not that you can't think of it flowing into the negative terminal but that's how we normally think about it is that current is flowing into the positive terminal okay now I know this is kind of a little vague with the voltage and everything of this nature but most of the time will be either measuring or just computing the voltage there is a much more technical scientific definition for voltage that we are just not going to cover in this course because it doesn't really serve us any useful information I think just having this kind of visual picture of potential energy for voltage is good enough and it kind of gives you a good intuition on what voltage is which to kind of even further that idea of potential energy let's even go back with one more example here to kind of illustrate this if we're talking about static shock and again apologize for my sloppy drawing here let's see a doorknob here and then you've got a finger and you've got some static charge on that finger well actually the electric shock does not happen when you hit that doorknob it actually happens right before it and there is basically well let me draw this like a lightning bolt and it's not a lightning bolt but I'll just kind of think of it as a lightning bolt here of static shock that or electricity that goes from your finger to the doorknob but it's actually occurs before you touch the doorknob and when you do that the voltage of that static shock there is actually well over 10 and volts and so that's why because it's such high voltage and that's why they say danger high voltage is that when you have high voltage like that you know air is not normally conducts electricity but because there's such a high voltage difference potential difference it's saying well hey I don't care about the fact that air doesn't conduct electricity I'm gonna ionize that air and conduct electricity through it so we have something being very high voltage alright last thing we want to talk about here is power now power fortunately for us as just a simple definition here so power is defined as P is equal to voltage times current so power is equal to voltage times current and we remember that the units for this are watts so power is basically a measure of energy transfer in other words if you do all the unit cancellation here which we're not going to do here as you would have energy over time is actually what a watt is the amount of energy you're delivering over time so that's the what a watt is it's how much energy is being either delivered or absorbed by it now this is very important for this V times I computation here let's just do an example to illustrate this here real quickly so when it's doing that V times I first off it's thinking that if I've got a device here and I have plus minus some voltage one here that the current i1 is flowing into the positive terminal here so v1 is flowing and the current is flowing into the positive terminal of this voltage here so let me give you some numbers here I 1 is equal to 2 amps and V 1 is equal to 3 volts so the power would simply be 3 volts times 2 amps which would be 6 watts now we don't obviously normally write the positive sign there but I want to emphasize that's positive because this is positive this means the device is absorbing / using power alright we're going to talk a little bit more about this in one more lesson here and with some examples here