Hi. It’s Mr. Andersen and this AP Physics essentials video 74. It is on electric circuits.
And when my father was younger he used to work on the telegraph. And the telegraph is
a simple electric circuit with a switch. And so you can run electricity through it and
you can make a click sound that you can hear on the other side. But that simple electric
circuit is no different than all the circuits that we are going to find in an iPhone like
this. And so a circuit, to understand what is going on, the two big things you should
get are going to be the voltage and the current. So the voltage is going to be the potential
difference in different parts of that circuit. And then the current is going to be the flow.
So it is really the movement of electrons through the circuit. You should also be able
to understand how different elements in a circuit behave. And those four elements in
AP Physics you should understand are the electromotive force like the battery, also the resistors,
the capacitors and finally the switches. You also should understand how these elements
are hooked together. Are they hooked together in a series? Or are the hooked together in
a parallel circuit? And so this is what the circuit elements are going to look like. We
have got the electromotive force. The nice thing about AP Physics is you do not have
to deal with AC power. We are just going to be dealing with DC power. But you should be
able to recognize what does a resistor look like, capacitor, switch and then finally a
wire is just going to be a straight line. Now to understand what is going on in a circuit
like this you have to an understanding of how circuits work. And if you know anything
about a switch you should know that nothing is going to happen because we have this switch
down here open. But a lot of people do not understand really what is going on at the
gut level in an electric circuit. And that is why analogies are really really helpful.
And so in this analogy we are going to use water moving through pipes to represent the
electric circuit. And so in this analogy the battery is going to be represented by a pump.
So that is going to be our emf, electromotive force. And so what we have is voltage. Voltage
is going to be the potential difference. So if you were to stick your hand here and compare
that to the pressure right here we are going to have more force here or more potential
difference across that. But we are just measuring how much potential energy there is. We are
not measuring the actual movement of that water in this case. And that is going to be
our current or I in an electric circuit. So how much water is actually moving is going
to be the current. And those are different things and they are going to behave differently
in an electric circuit. So if we look at the other elements a resistor is going to be anything
that slows down the movement of that water. And then a capacitor is going to be something,
it is almost like a dam. A dam you could think of where we have plastic rubber sheet that
goes across it. And so what it does is it stops current so it can build up charge on
that capacitor. And then a switch is anything where we can simply just turn the whole thing
off. So water can not flow. And so if you have this analogy of how each of these elements
work it really helps you to solve simple problems. And so let’s use a PHET simulation to look
at a simple circuit. So what we have right here is a battery. And if I hook up a voltmeter
you can see that we are going to have potential difference of 9 volts across that battery.
If we put an ammeter in here to measure the current you can see that there is not going
to be any current. Why is that? Well you can see that we have essentially a switch up here.
So it is not closed. And so when I close it you can see there is a huge amount of current.
What we really would have done at this point is we are shorting out that batter because
all of that current is quickly moving through. And so let me kind of split that junction.
And now let’s put one of those first elements in. We are going to put a resistor in here.
Remember that is going to slow the movement of that current. And let’s say that this
is an ohm resistor. So could you figure out, before we connect that resistor into the circuit,
could you figure out what is going to be the current in the circuit? Well remember Ohm’s
Law is pretty simple. All we do is take the voltage, and that is going to be equal to
the current times the resistance. And so what is our voltage? You could see it is 9 volts.
What is going to be our current? We do not know. That is an unknown. And what is our
resistance? It is 10 ohms. And so if I put a 9 here and a 10 here, dividing both sides
by 10 I should get a current of 0.9. So let me connect that and make sure that we are
right. Like that. So it is 0.9 amps. And so that is a pretty simple simulation. Now let’s
figure out what happens when we get rid of that resistor. So I am going to remove that.
And then we are going to add a capacitor to it. And so what I want you to do is when we
connect this capacitor and remember using our analogy a capacitor is going to be like
a dam, so what I want you to do is as I connect this capacitor together watch the amps. Watch
the amps right down here. And so as I connect that you could see that it briefly goes up.
So there is a build up of current. But then it stops. And that is because we are building
up charge on each of those plates inside that capacitor. And so in electric circuits you
are going to have circuits that are either in series. In a series circuit one element
just comes after another. So one after another after another. And then in a parallel circuit
we are actually going to have a number of different circuits that are connected together.
And so you should be able to understand what happens if I add a switch right here in a
series circuit. Well think back to that flow of water. Since the water can not get through
here, it is not going to be able to through the whole thing. And so there is going to
be no current in this whole circuit. What if I move it over here? What if I move it
down here? It doesn’t matter. In series connection if you ever have a switch in the
middle the whole thing is going to stop. And that is just like those old Christmas tree
lights where one bulb would go out and all of them would go out. So in a modern Christmas
tree light string they are not going to be connected in series they will be connected
in parallel. And so watch what happens if we add a switch right here. Well there is
not going to be flow through this side. In other words this side of that parallel circuit.
But we are still going to have current through here and we are still going to have current
through here. What if we were to add it there? Well we are not going to get return on these
two, but we are still going to have flow through here. Or if we were to add it just to that
one element we are going to have flow or current through these two over here. So understanding
how a series is different from a parallel circuit and it can get more complex than that.
And so now I have connected a number of different resistors in series. And what we are going
to look at is their voltage. In other words that potential difference and then the current.
And so you can see that I have connected it to a 9-volt battery. And now I am going to
move that voltmeter, and you can see that the voltage across those resistors right there
or that one resistor is 3 volts. If I move it over to this one it is going to be 3 volts.
If I do both of them you can see it is going to be 6 volts. And so really what we are doing
is we are dividing that voltage across all three of those resistors. But now let’s
look at these three 10 ohm resistors. Could you predict what is going to be the current
through the circuit? So just like we did before Ohm’s law is going to be helpful. And so
what is our voltage? Our voltage is going to be 9 volts. What is going to be our resistance?
Well let’s make this easy. Since we have three 10 ohm resistors what if it is 30 ohms.
What is we can simply add those up? So if we put a 9 here and a 30 here, let’s watch
what we get. We are going to get 0.3 amps. And so Ohm’s law works in this case. And
so when you are connecting a bunch of resistors in series you simply add the resistance of
all of those resistors and that is going to give you the over all resistance. It does
not matter where I put that ammeter, it still is going to be 0.3 amps. And that is going
back to the analogy of water, all the water is flowing through that circuit. So now I
have connected a bunch of resistors in parallel. And let’s look at what happens to the voltage
across each of those elements. So if I put the voltmeter here it is 9 volts instead of
3 volts. If I move it down to the next element it is still going to be 9 volts. Or down here
it is going to be 9 volts. And so it does not matter. In a parallel circuit it is going
to be the same voltage or same potential across all of those. Now let’s look at the current.
So we are going to use an ammeter. Before remember with those three 10 ohm resistors
it was 0.3 amps. But let’s move this ammeter around and see what we get for current. So
here it is 2.7 amps. If I move it down here it is going to be 1.8 amps. If I move it down
here it is 0.9 amps. And so I am getting different current in different parts of this electric
circuit. And so that is a little bit confusing. And so if we go through circuit arrangement,
if we are looking at resistors, resistors connected in series, the resistance is simply
going to be the summation of all of the resistance of all of those resistors. And so since we
had three 10 ohm resistors, we just add those up. So we would have 30 ohms of resistance
in that whole circuit. Then we can use Ohm’s law to solve the problem. So if we were to
measure the resistors in parallel it is going to be a little more complex. Where 1 over
the resistance in parallel is equal to the summation of each of those elements. And so
1 over the resistance in parallel is 1 over 10 ohms plus 1 over 10 ohms plus 1 over 10
ohms. And so that is 3 over 10. And I could solve my resistance in parallel as 3.33 ohms.
And that is why when we have all three of those together we get 3.33 ohms. And when
I figure out my current on a 9 volt battery it is going to be 2.7 amps. And so did you
learn to make predictions of the changes? So as we move the elements around how is that
going to affect a circuit? And then could you plan to collect data. And so again I am
using a PHET simulation. I will put a link in the video description down below.
But I hope that was helpful.