[RC Circuits]
Dan: What do you have the PASCO AC/DC Electronics Laboratory circuit board out for, JJ.
JJ: We're going to do an experiment with a circuit that has two resistors and a capacitor in it,
or an RC circuit.
Dan: I can sort of see a circuit here, but I'm not sure I'm understanding the pathways here.
Can you point out the details?
JJ: Yeah. I'm going to put this up for a second.
This is the diagram that we're used to seeing.
This is the diagram of the circuit that we're going to use in this experiment.
There's a 3 volt battery source here, a switch, a couple of resistors.
One is in series with the capacitor and one is parallel with the capacitor.
Let me explain how that shakes out on our circuit board here.
We've got two batteries connected in series.
This little wire here that connects the positive and the negative of the two batteries right here,
so we get three volts as a result.
Then we've got a little push-button switch here.
The top of this battery is connected to the push button switch,
and then this is connected to this resistor right here, which is our 33-ohm resistor.
Then we've got wires that connect across these two spring clips to a capacitor over here,
which is 470 microfarads.
Now, you may have noticed a little arrows on that.
Dan: It's directional. Not all capacitors are.
This one is.
JJ: Right. So we make sure that's set in the right direction.
Then there's a resistor connected between these two spring clips right here.
Then there's a couple of wires that are connected together here.
The bottom of this capacitor and the bottom of this resistor are connected together.
Dan: They're in parallel.
JJ: Yep, in parallel, and then that is in turn connected to the back of this battery here
so we have a nice closed circuit.
Dan: It does match that circuit diagram very well.
JJ: I'll just throw that up there again for a second, make sure we all see what's happening.
Dan: We have the wireless current sensor and the wireless voltage sensor.
I assume we want to measure that across each of the components in the circuit.
JJ: That's exactly right.
In part one in the lab, we start with the capacitor.
We're going to measure the voltage across the capacitor while also measuring the current...
Dan: Through it.
JJ: ...through it.
Why don't I give you that voltage sensor and
you connect it to the capacitor across it in parallel.
I'm going to take this current sensor...
Dan: Because the current doesn't really flow through the voltage sensor.
It has a really high resistance to prevent that.
JJ: It's going to measure the voltage...
Dan: So this goes across.
JJ: Exactly.
Then, to measure the current through the capacitor, I'm going to take this white wire right here,
and I'm going to take it out,
and I'm going to replace it with this current sensor in series.
The current's going to have to flow through the capacitor, through the current sensor
before it makes its way...
Dan: So the charge is going to flow through the current sensor instead of that wire.
It just takes the place of the wire.
JJ: That's right.
Dan: They work very differently and we have to be careful once we hook them up.
Then I also see you're putting the positive, the current will flow from positive to negative.
JJ: That's right.
Dan: The charges will flow from positive to negative.
JJ: That's right.
We're going to measure current as it flows through this ammeter, and then back.
Whoops, I'm sorry, this goes right there because that's where the wire used to be.
The wires used to be connected from here to here.
Dan: I have a voltage graph and a current graph and we're measuring at one kilohertz.
If this happens fast, we should be able to see any changes.
This switch would close the circuit.
Whenever you're ready, I'll hit record.
JJ: OK.
Dan: We have our data,
and you can see the voltage went real rapidly up to some steady value across the capacitor,
and the current just quickly went up to some maximum value
and then dropped to zero.
Then when you released that switch --
this kind of switch makes a connection when you press it --
and then when you let go, it disconnects.
The voltage went to zero, and then the current flowed again, very quickly, and then was gone.
We can take a closer look at it.
This is something you'll want to do when you're analyzing this data.
We're going to give you all this data.
I have put the selection box around that.
I want to blow it up, so I'm going to hit autoscale.
Now, we can see that there's a curve here.
It's a very characteristic curve of charging a capacitor, and then same with current .
Once a capacitor reaches full voltage, current stop flowing.
That's something you may not have known.
JJ: That's what we call steady-state.
When the current's not flowing, that's where you would make your steady-state measurements
from this data that's in the graph,
which we'll give to you to make the measurements on your own.
Dan: Then if we were to look here at the discharge, and again, you will be doing this.
I select that, I can select it so it's all purple there.
We can see the discharge, another sort of the inverse of the charging,
a similar mathematical function would describe that.
Then the current also drops to zero.
What they're going to need to do is also measure the voltage across the capacitor in steady-state
and the current across the capacitor --
that's kind of obvious, the charge flowing through the capacitor, it's current at steady state --
and record that in the data table in the lab write-up.
JJ: That's right.
That's the data for part one of the experiment.
Part two has us, instead of measuring the voltage and current across and through the capacitor,
we're going to move this to our 33-ohm resistor right here.
We'll measure the voltage across the resistor, and then through the resistor.
Just like you had on the capacitor, we'll connect those leads in parallel across that resistor.
Now, there's a blue wire right here that connects this end of the resistor to these two components.
We're just going to pull that blue...
Dan: And replace it.
JJ: Yep, and that's just a little snug so I'm going to put off for a split second.
Then replace that with our current sensor.
We're going to go from there to there.
JJ: Whoops.
Then we need to put the...
Dan: That jumper wire in there.
JJ: We're going to put this wire back.
Dan: Run number two will be the voltage across the 33-ohm resistor,
and then the charge flowing through it or he current of the 33-ohm resistor which is in series.
This is part two in the lab write-up.
Whenever you're ready.
JJ: All right, go ahead.
Dan: We can see something similar, but not exactly the same.
The voltage went up and then dropped down to some steady-state value and then dropped to zero.
Then the current also went up to some value but then didn't drop down to zero,
and then eventually did after we closed the switch.
So current is flowing through that resistor, even when the capacitor is fully charged.
You'll need to be able to explain that as part of the lab.
You also will measure the steady-state voltage and the steady-state current.
You need to understand this circuit.
You need to understand about charging and discharging but not mathematically.
You do need to be able to understand the current and voltage values
once it's a fully charged capacitor and discharged.
JJ: Exactly.
For the last part, part three, we're now going to move our measurement tools
from this 33-ohm resistor over here to this 100-ohm, right?
Dan: 100.
Little resistor, but a bigger resistance...
JJ: Yeah, physically smaller.
Dan: ...so it makes it harder for the current to get through.
Dan: We have choices of where to get the current through it, but probably the easiest is to...
JJ: Oops.
Dan: ...get it going in.
JJ: We got the voltage sensor hooked up to measure the voltage across the resistor,
but now we want to be able to measure the current through it.
The first thing I'm going to do is I'm going to replace this wire that we took out for part two,
this little blue wire here, and now I'm going to take out this red wire right here
and replace that section of wire with our current sensor in series.
Dan: So the only way the charge can flow is through the ammeter, or current sensor.
JJ: Right. I'll connect that here.
And I might also add that these spring clips have two little springs,
and those are electrically connected.
So you'll see here that this black alligator clip and this red alligator clip
are clipped to two different springs...
Dan: But it's the same.
JJ: But they're electrically connected.
I'm going to take this and put it here.
So now we're measuring voltage across this resistor,
and then also the current through it before it meets back up at this junction here.
So right here is where we're measuring current.
Dan: So let's see what that looks like.
And so different.
You'll have to be able to explain that as well.
Also reading the voltage here and the current here and trying to understand that.
So you will have this data file and all the tools in Capstone.
You'll have the student handout and this video to help you complete this lab on RC circuits.