Welcome back. We're continuing on with our lesson on
physical resistors and our lab demos. In this particular case, we want to be
looking at real circuits. Previously in this module, we had looked
at abstract concepts and equations. In these two lessons we're trying to look
at physical resistors and their use in circuits.
So the particular lesson objectives are to demonstrate series and parallel
resistance and measure voltage and current using the voltage divider law and
Ohm's Law. Before we show the demo, let's go back
and review a couple of concepts. In the last lesson we learned about
protoboards. Recall that protoboards are used to make
building circuits easier because a lot of the connections are made for you.
So every group of five is connected together.
A wire in this point, in this hole right here is going to be electrically
connected to a wire in that hole there. So, these holes are connected together.
And along the side, side rails, everything along the side rail are, is
connected. So, everything shown in yellow will be
connected. The other two concepts we wanted to
remind you of, resistors in series. Remember that resistors in series add,
and resistors in parallel follow this form right here.
So let's start our lab demo. In this experiment, we will look at
resistors in series and in parallel. One way to connect resistors in series is
to touch the resistors end-to-end. Like this.
Its a little bit hard to, to keep them together that way, and so we usually like
to use the protoboard when we insert these in protoboard.
So if I insert the resistor in the protoboard.
Recall that protoboards, every row of five along the insides of of here.
Every group in a five is connected together.
So, as long as I put the end of this one in the same row as that one they are
connected together. That leaves my hands free to take the
measurements. If I look at these resistors by
themselves, they're supposed to be 1,000 ohm resistors.
Let's actually measure them, 994, this one is 991.
Okay, so they're pretty close to 1,000. They're within the tolerance of these
resistors. If I, if I put them in series, that means
I should sum them up. So this should be about 2000 ohms.
And they do. They sum up to be that.
Let me go ahead and replace one of these 1000 ohm resistors with, the 2000 ohm
resistor and try the same thing. We measure that 2000 ohm resistor.
It is well, close to 2000 ohms. This sum should be about 3000.
Very close, so within tolerance, it's, it's the 3000 ohms.
To connect resistors in parallel, you but both ends in the same rows.
So, let me pull out the 2000, and I want to put the another 1,000 in parallel
with this resistor. So both ends are in the same rows.
Now if you remember what resistor's in parallel are like.
What we'd find is that two resistors that are the same that are in parallel with
one another. The total resistance is actually half of
either one. So if this started out as two 1,000 ohm
resistors and I've put them in parallel, the, they should end up with about 5,000
or 500 and they are very close. And again, it doesn't really matter if
I'm touching it here. Were here they are the same spot
electrically. I get 496 ohms.
If I replace one of these resistors with the 2k resistor, then I've got a 1k
resistor in parallel with the 2k. The total resistance there using our
formula for resistors in parallel should be 666.
Very close, very close. Now let's hook together a circuit with
the voltage source in two resistors in series.
Here's my battery, it's going to be my voltage source.
Let's check the voltage on here. This is, it should be around 400, 4.5,
because I've got three, 1.5 volt batteries.
So, I get 4.63. So, it's a little bit high, but within
range of what we would expect. I want to put that in this protoboard.
But I'm going to add a resistor over here, a wire over here, so that I can
connect up this battery a little bit easier.
Okay. And then when I connect this battery up,
I want to put these, these connections into these same rows.
Okay. So now if I double check this, I should
get that same 4.65. 4.63 volts.
So to connect it up, I put the positive terminal in the same row as that
resistor, and I'm going to add another 1000-ohm resistor, so I have two 1000-ohm
resistors in series with one another. You take this negative terminal, put it
in the same, negatively, put it in the same row as this resistor.
And I want to look at the voltage divider law.
If you look at the schematic of the voltage divider law then the ratio the
voltage on the output is really proportional to the voltage of the
resistance across your what you're measuring.
So in this case if r2 is a larger resistor than the voltage should drop
across r2 would be larger than that across r1.
So larger the resistor the larger the percent voltage drop you have.
When you have two resistors that are in series with one another, that are equal,
that means that each resistor gets half of the voltage.
So look at this case we've got back to the circuit, we've got two resistors that
are equal, normally equal to one another, with intolerance range.
So they should each get half the voltage, and they do, 2.3 2 is about half, and
measured this way, 2.31 is half. Notice that if I switch the terminals,
the polarity on here, so I'm measuring positive to negative here.
If I switched it and did negative to positive, I should negated.
See, I get a negative voltage there. If I took out the 1000 ohm resistor, and
put in 2000 ohm resistor, then the 2000 ohm resistor is bigger, so this is 1 3rd
of the total voltage, and this one is 2 3rds of the total voltage.
So this should get 2 3rds of the current 2 3rds of the, the voltage.
2 3rds of the resistance should get 2 3rds of the voltage.
And it does. That's about 2 3rds and then this one is,
about 1 3rd. Now, I'm going to take out this 2k
resistor. Leave the 1k, and I'm going to put a 10
ohm resistor in there. So here I've got a thousand ohms, and
here 10 ohms. We can double check that.
Actually double check that resistance here.
9.9 10 ohms very close. So what happens in the in the voltage
divider. 10 ohms is about 1% of the total.
The total between this two is 1,010 ohms. So this resistor here would take about
99% of the voltage, and this only about 1% of the voltage.
So let's look at that. So this should be about 99% of the
voltage. Oh, we'll go back to voltage setting.
But 99% of the voltage, and this about 1% of the voltage.
And that's right. Seeing that, that fact that if I put a
very small resistor in series with a large resistor.
I'm able to use that I'd, that fact to be able to help me to determine the current.
For example, this one, if I want to figure out what the current is through
this loop, I would take this voltage, which is 4 point or, 45 millivolts, and
divide it by 10. I'm using Ohm's Law, divided by 10 ohms,
and I would get 4.5 milliamps. So that's the current that is running
through this circuit right here. The reason that I might, and I often use
this method to tell me what the current is, to record current.
I just put, take my regular circuit, and I put a very small resistor in series
with it. And I used a small resistor because I
don't want to upset the rest of the circuit.
I want to have minimal effect on the rest of the circuit so I put a very small
resistor in there. It takes very small voltage across it and
I read the voltage off of that and divide by the resistance and that gives me
current. So there are actually ways that commonly
we use for measuring current. The one on the right I just mentioned,
where I take R2, and make it a very small resistor in comparison to R1, and I
measure the resistance, the, the voltage across R2, and then use Ohm's Law to find
the current. The other way that I can do current
measurement is to take the resistor out, and this is on the left.
I take the resistor out, and put in ammeter in series with my circuit.
Going back to the circuit. Take it back to, I want to put, I had to,
I have to put this ammeter in series. So that is reading right there 4.5
milliamps, same thing as what I got before.
So in summary, we've looked at resistors in series, resistors in parallel.
The voltage divider lot and measuring current in a circuit.
Thank you. In summary, we've connected physical
resistors in parallel and in series, and we've measured voltages and currents in a
circuit, applying the voltage divider law, and Ohm's law.
In our next lesson, we will use superposition to handle multiple sources.
And I do want to remind you to visit the forum to go ahead and ask questions if
you have any doubts about any of the material, or if you're curious.
And to please try to answer questions that other people have post.
I would really like you to be engaged in this course.