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# Question 2ab: 2015 AP Physics 1 free response

## Want to join the conversation?

• Is it necessary to add another resistor in the circuit in addition to the bulb, as drawn in the diagram, or can we have a circuit with just the bulb? If I've thought it out correctly, we should be able to end up with the same result regardless. Or is there a mistake in my understanding?
(3 votes)
• Hello AThont,

Lets remove the resistor and see what happens. We now have a series circuit consisting of a battery, ammeter #1, bulb, and ammeter #2. Let's assume our power supply is adjusted correctly so as NOT to damage the light bulb. WRT the current, nothing changes. Since this is a series circuit the current is the same in both ammeters. Actually we can generalize this statement and state that "current is the same at every point in a series circuit."

For voltage you could:

1) disconnect the light bulb

2) measure the voltage of the power supply (will be whatever you set it to)

3) measure the voltage on the light bulb (will be zero)

4) connect the light bulb to the power supply and measure the voltage (will be whatever you set it to). The power supply and the bulb now have the same voltage.

We can now make a statement about the power supply and bulb forming a loop. The power supply is the “source” (like a pump) that increases the electrical potential energy and the bulb is a “sink.” It reduces the electric potential energy. For good measure describe how the law of conservation of energy fits into this picture.

And yes, you could continue for the remainder of the lab to determine if the light bulb was nonohmic. Just measure the voltage across and the current through the light bulb as the power supply is varied. Plot the results and make your determination...

Please take a few minutes to look up Kirchoff’s Current Law (KCL) and Kirchoff’s Voltage Law (KVL) as these are really what we are talking about.

Keep up the good work!

Regards,

APD
(5 votes)
• Why can't we just put voltmeters on either side of the bulb to see if there is a drop in electric potential energy?
(1 vote)
• Voltmeters measure potential DIFFERENCE between the points where the voltmeter is connected.
(2 votes)
• One question regarding the format of the test and how it is graded. Sometimes like Sal I end up answering part B in my answer to part A. Can I just do what Sal did and put A+B or do I have to write B as its own separate answer. Thanks!
(1 vote)

## Video transcript

- [Voiceover] 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, do fewer electrons leave the bulb than enter the bulb? 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. You could also think of electrons per second. So current's 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, voltmeters, ammeters, and other standard lab equipment. Assume the power supply and voltmeters are marked in tenths of a volt increments and ammeters are marked in hundredths of an amp increments. 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 the equipment to make them. Include a neat, labeled diagram of your setup. All right, so let's do part A right over here. So part A, so we're going to have a power source. I think they said it was a variable power source. Did they say that? Yeah, the students have an adjustable power source, insulated wire, lightbulbs, resistors, switches, and voltmeters. They want to know an incandescent bulb, lightbulb, when it is in series with a resistor. So an incandescent lightbulb when it is in series with a 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. 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 here of some resistance. Then let's keep going with our circuit. So we're going to keep going with our circuit. And now let's put our incandescent lightbulb. The symbol for an incandescent lightbulb, there's actually several, I'll use one where I do a little bump here and then I'll continue and I'm actually gonna put a circle around that bump. So it would be just like I'm almost there. I'm trying to do 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. 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 lightbulb. This is our power source. 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 wanna use some ammeters and voltmeters in order to measure what's happening as our electrons are going through the lightbulb. 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 convention is that the current goes from the positive direction to the negative direction. You could view it as they're 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. The way we can measure the current on either side of the lightbulb is we can insert ammeters on either side. Ammeters have to be inserted in series. So I'm clearing up some space so I can insert my ammeters. 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, ammeter. So these are going to measure current on either side of our lightbulb. So current measures current on either side. Or I could say measuring current entering and exiting lightbulb. Current entering and exiting the lightbulb. Entering the lightbulb, or maybe I'll say exiting the bulb. We also care about the voltage drop. So we could put a voltmeter, and the voltmeter can be put in parallel with the lightbulb. So let me draw the voltmeter here. So I'm trying to draw it neatly. So this is the voltmeter. It's going to measure the voltage drop from one side to another. Just connect this there. So this measures the voltage drop. Measures voltage. Measures voltage drop. So I've drawn my diagram. Let's see, what else do I need to do? So they say 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 the equipment to make them. Include a neat, labeled diagram of your setup. Okay, so I guess my description, I could say I'd put two ammeters in series with bulb. One before bulb. One after. If current same on both, then same number of electrons entering and exiting bulb. If the current is the same on either side, then electrons per second entering and exiting will be the same. If the currents are different, then the rate of electrons is different, then rate of electrons passing are different. All right, 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 than 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've got different numbers of electrons. All right, now for statement two, so let me write this, this is statement one. That is my procedure right over there. Now for statement two, statement one test I guess I could say, and then statement two 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 could lowercase voltmeter, put voltmeter in parallel with, let me write it out, with bulb. If measures voltage drop, or I could say if and only if, I'll say if measured voltage drop, then, how did they actually phrase the statement? Let's see, does the electric potential energy of electrons change? Then electric potential energy of electron changes. Did they say electric potential energy of electron changes? Then electric potential energy of electrons, I don't normally write this much, changes. Otherwise, it does not. If no voltage drop, then no change in potential energy. If no voltage drop, then no change in potential energy, electric potential energy you could write. And there you go. That is part A where I've set up my neat diagram. I'm measuring the current entering and exiting the lightbulb. And actually, if you view it from the electrons point of view, this one is measuring the electrons going that direction, 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. Different current, then you have different electrons per second entering and exiting the bulb. This measures the electric potential energy across the lightbulb. If you actually measure a voltage here, then that means that the potential energy is changing from one side of the bulb to another. And so I'll stop there. Then I will, well, actually, I think I answered part B too. 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 was 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. 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.