okay so we're going to talk about power a little bit more here and I know we've spent a lot of time in a lot of lesson time here doing examples and really talking about charge current voltage and power but these are fundamental concepts that we really miss make sure we understand just to make this course easier but also to make any of your future courses easier so let's talk about power again here and we're gonna have here I'm just going to have one device here and then I'm gonna make up several cases so we can talk about what's going on in each case I don't know why I drew it so big here but that's fine so we're going to compute the power and then also is device absorbing or delivering power so the first case we're going to do here is we're going to let v1 equal five volts and then I 1 is going to be equal to 2 milliamps now the nice thing here when doing this computation here again the power is an easy computation but one thing you always want to make sure when you do this power computation is you want to make sure that the arrow of the current is entering the positive terminal of your voltage because that's how our equation for power was defined so i1 is entering and hitting the positive terminal of this voltage v1 so we do know that power then is simply 5 times 2 milliamps which would be 5 volts times 2 milliamps which would simply just be 10 now we you can do the scientific and everything but you can know that volt times amps is just watts and so we wind up with 10 milliwatts as our our final power now again we don't write the positive but since it's positive we know that this is absorbing all right now let's look at another case here so for this one we're gonna let v1 equal negative 3 volts and I 1 equal 2 amps now you might say oh wait v1 is negative I have to do something with it no v1 being negative 3 volts is fine voltage can be positive or negative and it doesn't matter you might say well wait but you know the positive terminal can't be negative that way the way we define plus and minus is just how we measure the voltage it's not saying that it's a positive voltage or a negative voltage this plus and minus is only referring to if I were to hook up a voltmeter this is how I'd hook it up I put the positive terminal up here and I put the negative terminal down here and I'd see what the voltage reading is so these plus and minuses don't tell me that the voltage is positive or negative all they do is tell me how to measure it and in this case when we you know measured I'm just obviously making the numbers up but when we measured it we got negative 3 volts and then the current of course is 2 amps what we said so in this case the power would simply be negative 3 volts times 2 amps which would be negative 6 watts now negative power is fine now when you get a negative power though that means you're not absorbing power this means we are delivering power and this would make sense like if you looking at again going back to maybe your car as an example or actually instead of a car let's go back to your like a cell phone for instance so when you're using your cell phone driving it throughout the day you have a battery there and that battery is of course delivering power to your circuitry so if we were to measure the power across that battery it would be a negative power because it's in essence that battery is losing power it's getting drained with from power because it's using that power and giving that energy transferring that energy to circuit elements so that of course you can use your phone and then of course when the power of that battery goes out you know you recharge your phone all right now let's look at one more example here that's one that sometimes gets people a little messed up here so well well look at v1 this case will say let's just go ahead and give this a smaller number here so this is 20 millivolts and then what we're gonna do is we're gonna say I 2 is 1 milliamp now first off we have to be careful with how we compute this power I will write down something you might not want to put this in your notes I see a lot of people saying P is just equal to 20 millivolts times one milliamp and this is wrong and you might say well wait a minute it's just voltage times current but remember our equation for power equals current times voltage I must flow in to positive terminal of the voltage but if I look at what's going on here this eye it's not going into the positive terminal over the voltage in fact it's leaving the positive terminal voltage but that's not a big deal I one of course is going into the positive terminal and we can clearly just see here that well I 1 is simply going to be negative 1 milliamp because remember we can simply just flip the direction of the current you know and it becomes negative but we can always just flip the direction of the current to get the correct value for it that we need for our equations so in this case here power would be 20 millivolts times negative 1 milliamp now you need to be careful I'm assuming you can use calculators and scientific notation I this course is really not to you know remind you how to do things of that nature but be careful with your computations here and you can always use a calculator again to help but you would get minus 20 micro watts and I'm gonna just double check just to make sure here that that's what you should get but if I do that in my head correctly that is what we should have yes and that is indeed what you get and now as far as if this is delivering or absorbing power this because it's negative again because it's negative that means it's delivering power now one final comment I want to make here and this is actually pretty critical and then we'll start getting into some circuit elements and some circuit analysis here to be able to look at circuits the final comment here is that the in any circuit the power delivered must equal the power absorbed now let me give you a case that sometimes messes people up here and explain sometimes why we have to be careful with this let's suppose we have a voltage source delivering 60 watts of power to Alou and I'll even say we'll do an LED well actually instead of 60 watts let's do 10 watts of power to an LED light bulb the light bulb absorbs 9 watts of power have we broken the rule that power absorbed equals power delivered and the problem hasn't given us all the information they but the real answer is no you might say well wait where's what and my question is well where does the 1 watt of power go it has to go somewhere it's not going into the light bulb and I'm not going to answer that for you because that's going to be part of a question we have on either homework assignment or discussion board and what we will answer it as a class or you know I will provide the answer eventually but I'm not going to put it in this video I want you to think about this where does that 1 watt of power go because it has to go somewhere can't just magically just somehow disappear we have to have conservation of power but that does happen sometimes where you have a 10 watt being delivered to a light bulb but only 9 watts being absorbed by the light bulb all right so think about that and in the next videos we will be starting starting with Ohm's law sorry about that