- Good morning.
Today we are going to solve
free response question number two
from the 2015 AP Physics 1 exam.
Bo, could you please read through question two?
♫ Flipping Physics ♫
- 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:
One, in one second, do fewer electrons
leave the bulb than enter the bulb?
And two, does the electric potential energy of the electrons
change while inside the bulb?
- It is important that you realize while you should know
the answers to those two questions,
they're not actually asking you right now,
to answer those two questions.
What they're going to ask you to do
is to design an experiment which could find the answers
to those two questions.
Let's start with the circuit described in the problem,
a lightbulb in a series circuit
with a resistor and a power supply.
Bobby, could you please?
- Okay.
We start with the battery, and label it delta V sub T
for terminal voltage.
In a single loop, we include the lightbulb and the resistor.
I think that's it.
- Right.
I don't think there's actually a standard symbol
for lightbulb, so this is the symbol I use.
You could even draw a little picture of a lightbulb,
but I would just make sure to label it lightbulb.
Bo, could you please read through part A?
- The students have an adjustable power source,
insulated wire, lightbulbs, resistors, switches,
voltmeters, ammeters, and other standard lab equipment.
Assume that the power supply and voltmeters are marked
in 0.1 volt increments, and the ammeters are marked
in 0.01 amp increments.
Part A.
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.
- Let's now add to our circuit diagram,
the items we would need to add to our circuit
in order to answer the two questions.
Billy?
- The number of electrons that enter and leave the lightbulb
in one second can be measured using the current.
So, we need ammeters because they measure current.
Ammeters go in series with the circuit element
you are trying to measure the current through.
So place two ammeters in the circuit.
One before, and one after the lightbulb.
Voltmeters are used to measure
electric potential difference,
and they are placed in parallel across circuit elements.
So, place a voltmeter in parallel across the lightbulb.
- Now we've drawn the circuit diagram,
however we haven't described the experimental procedure.
Bo, could you please do that?
- We could say, place ammeters in series with the lightbulb
both before and after the lightbulb.
Use the ammeters to measure the current
both before and after the lightbulb.
Place a voltmeter in parallel with the lightbulb,
and use the voltmeter to measure
the electric potential difference across the lightbulb.
- And Bo, please read part B-I.
- Explain how data from the experiment you described
can be used to answer question one above.
- Bobby?
- Okay.
We need to write, if the current in both ammeters
is the same, then the number of electrons which flow
into the lightbulb will be the same
as the number of electrons which flow out of the lightbulb.
If the two currents are not the same,
then the number of electrons would not be the same.
- And part B, double I is, explain how the data
from the experiment you described can be used to answer
question two above.
Billy?
- We need to write, electric potential difference
equals the change in electric potential energy
per unit charge.
Therefore, if the electric potential difference
across the lightbulb is nonzero,
then the electric potential energy of the electrons
will change while inside the lightbulb.
- Yes.
Bo, could you please continue to read,
and read through part C, double I?
- A lightbulb is nonohmic if it's resistance changes
as a function of current.
Your setup from part A is to be used or modified
to determine whether the lightbulb is nonohmic.
Part C-I.
How, if at all, does the setup need to be modified?
Part C, double I.
What additional data, if any, would need to be collected?
- We are testing to see if the lightbulb is ohmic.
In other words, to see if it follows Ohm's law.
Ohm's law states that the electric potential difference
across an object is equal to the current through that object
times the resistance of that object.
And, if an object follows Ohm's law,
it is considered to be ohmic.
If this equation does not hold true for an object,
then the object is considered to be nonohmic.
And it turns out we actually don't need to make any changes
to our circuit in order to determine whether the lightbulb
is ohmic, or nonohmic.
What about part C, double I, Bo?
Do we need any additional data?
- Actually, we do need additional data.
We need more data points.
So, we can say, we need to adjust
the electric potential difference across the power supply
and take multiple measurements of the current
through the battery and the electric potential difference
across the battery.
- And Bo, please read part D.
- Part D.
How would you analyze the data to determine
whether the bulb is nonohmic?
Include a discussion of how the uncertainties
in the voltmeters and ammeters would affect your argument
for concluding whether the resistor is nonohmic.
- Bobby?
- We can write because Ohm's law
is electric potential difference equals current
times resistance, therefore resistance equals
electric potential difference divided by current,
and slope equals rise over run,
or change in y over change in x,
we can create a graph with the electric potential difference
measurements across the lightbulb on the y-axis,
and the current measurements through the lightbulb
on the x-axis.
If the resistance of the lightbulb is ohmic,
then we should be able to draw a best fit line
which approximates all the data well.
If the resistance of the lightbulb is nonohmic,
then we should not be able to draw a best fit line
which approximates all the data.
- Be careful, Bobby.
It specifically states you need to include a discussion
of how the uncertainties affect your argument.
- Okay.
Add the best fit line does not have to go perfectly
through all the measured data.
It only needs to approximate the data.
This is because of the uncertainties in the measured data.
Does that work?
- Absolutely.
Now, notice how the answers to this free response question
are completely different than the answers
to free response question number one.
This problem, the answers were completely short-answer,
and designing an experiment.
You can pretty much count on the fact that
every AP Physics 1 exam will have at least one
free response question which is short answer,
where you need to design an experiment.
In addition to that, notice how this is the second problem
in a row that is completely devoid of numbers.
So please, I beg you.
Let go of your numbers dependancy.
Thank you very much for learning with me today.
I enjoyed learning with you.