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# Question 2ab: 2015 AP Physics 1 free response

## Video transcript

some students want to know what gets used up in an incandescent lightbulb when it is in series with a resistor current energy or both they come up with the following two questions in one second to do fewer electrons leave the bulb then enter the bulb and does the electric potential energy of electrons change while inside the bulb so this first question is really a measure of current because current is how much charge per second is going through a particular part of our circuit so charge per second is what you could also think of electrons per second so currents going to measure the first one and the electric potential energy of the electrons well that's going to be voltage does the voltage change is there a voltage drop when we go from one side of the bulb to another the students have an adjustable power source insulated wire lightbulbs resistors switches volt meters and meters in other standard lab equipment assume the power supply in volt meters are marked in intense of a volt increments and a meters are marked in hundredths of an amp increments all right describe an experimental procedure that could be used to answer questions one and two above in your description state the measurements you would make and how you would use equipment to make them include a neat labeled diagram of your setup all right so let's do Part A right over here so I have so Part A so we're going to have a power source and I think they said it was a variable power source did they say that they say yeah the students have the students have an adjustable power source insulated wire lightbulbs resistor switches and volt meters and they wanted to know an incandescent bulb light bulb when it is in series with a resistor so an incandescent light bulb when it is in series with the resistor so let's make an attempt at drawing this so just the circuit the circuit only that they care about we could do our adjustable power source so let me draw that so you could do that like this make a couple I'll do a couple here so like this so that would be a power source where this is the positive end this is the negative end and then let me make my circuit before any measurement tools then we'll add the measurement tools so I'm going to make it in series with a resistor so let's put a resistor a resistor here of some resistance and then let's keep going with our circuit so we're going to keep going with our circuit and I'll let's put our incandescent light bulb and the symbol for an incandescent light bulb there's actually several I'll use this one where I do a little bump here and then I'll continue in August you're going to put a circle around that bump so it would be just like I'm almost there I'm trying to it neatly because they're telling us to do it with a neat diagram all right let me draw it a little bit better than that so it would look something like this and then let me put a circle around this so putting a circle around this this is our incandescent light bulb this is our power source and to show it's a variable power source I can put an arrow across it like this that shows us that it is a variable power source so this is a circuit that I've just set up but I want to use some ammeters and voltmeters in order to measure what's happening as as our electrons are going through the lightbulb and the standard convention is to show current going from the positive terminal to the negative terminal but we know and if you don't know I encourage you to watch the Khan Academy videos on it what's actually happening is you have electrons traveling from the negative terminal to the positive one but in general if we just wanted to talk about the current you would denote it the current would the convention is that the puck the current goes from the positive direction to the negative direction you could view it as there the positive gaps of electrons or however you want to but the electrons are actually moving in the other direction so the first question is do you have a different number of electrons moving per second before entering the lightbulb then when you come out of the lightbulb well the way you can measure that is by measuring the current on either side of the lightbulb and the way we can measure the current on either side of the lightbulb is we can insert we can insert a meters on either side and ammeters have to be inserted in series so giving them some space so I can insert my and meters so that is one ammeter it's going to measure current right through that part so I'll put a there and then that is another ammeter right over there and meter so these are going to measure current on either side of our light bulb so current measures current on either side on either or I could say measuring current entering and exiting light bulb current entering and exiting and exiting the light bulb entering the light bulb or I'll just save me by saying and exiting the bulb and we also care about the voltage drop so we could put a voltmeter and the voltmeter can be put in parallel with a light bulb so let me draw the voltmeter here so I'm trying to draw it neatly so this is the voltmeter and it's going to measure the voltage drop from one side from one side to another to connect this right over K so this measures the voltage drop measures measures voltage measures voltage drop so I've done my diagram we'll see what else 22 do so they say describe an experimental procedure that could be used to answer questions 1 & 2 above in your description state the measurements you would make and how you would use equipment to make them include a neat label diagram of your setup ok so I guess my description I could say I'd put put 2 ammeters m meters in series in series with bulb 1 1 before bulb bulb right bulb 1 after one after if current same on both then same number of electrons entering and exiting bulb if the current current is the same the same on either side either side then then electrons per second entering and exiting will be the same entering and exiting the same if the currents are different if the currents are different then the rate of electrons is different then rate of electrons passing are different are different alright so that's the first part the first statement to try to go for this first statement for statement one in one second do fewer electrons leave the bulb then enter the bulb if the ammeters are measuring the same current well then you have the same number of electrons per second entering and leaving the bulb if the ammeter measures different currents well then you get different numbers of electrons all right now for statement 2 so let me write this this is statement 1 statement statement 1 that is my procedure right over there now for statement two statement 1 test I guess I could say and then statement to test statement 2 test I think you guys get the idea by this point but I'm just writing it out because you would have to if you were taking this ap test I would say put voltmeter I can lowercase voltmeter put voltmeter in parallel with let me write it out with bulb if measures voltage drop voltage drop or I could say if and only if so well I'll say if measured voltage drop then how do they actually phrase the statement see does the electric potential energy of electrons change then electric potential energy of electron changes electric they call see electric potential energy electron changes then electric potential energy of electrons I don't normally write this much changes otherwise does not if no voltage drop then no change in potential energy if no voltage drop voltage drop then no change in potential energy electric potential energy we could write potential energy and there you go that is part a where I've set up my neat diagram I measuring the current entering and exiting the light bulb and actually few view it from the electrons point of view this one is measuring electrons going in that direction at this is electrons entering electrons exiting but either way the electrons per second would affect current same current well then you have the same electrons per second a different current then you have different electrons per second exiting bulb and this measures the electric potential energy across the light bulb and if that is if you see a if you actually measure a voltage here then then that means that the potential energy is is changing from one side of the bulb to another and so I'll stop there and then I will I will well actually I think I answered part B to explain how data from the experiment you described can be used to answer question one above explain how the data from the experiment you described can be used to answer question two above so this is really Parts A and B so actually let me write this down so this is a plus B right over here we have the diagram and if you're taking the test depending on how your time pressure is you might want to label this more but if you're running out of time then you might not have time to describe it in as much detail
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