—
Mr. Andersen describes the relationship between voltage, current and resistance in an electric circuit. Ohm’s Law is introduced through a circuit simulation.
—
—
Transcript Provided by YouTube:
00:03
Hi. It’s Mr. Andersen and today I’m going to be talking about voltage, current
00:09
and resistance. All of these things are found in electric circuits. It’s hard to see electricity
00:14
move and so lot’s of times we don’t have a gut instinct for what’s really going on. And
00:19
so I always like to start with an analogy. The analogy here is going to be the analogy
00:23
of water. And so if we have a bunch of water at the top of a water tower it has potential
00:28
energy. And potential energy in electricity is going to be called voltage. And we measure
00:34
that in volts. As that water flows down to a toilet or a sink, the current, and the symbol
00:41
for that is I, the current is going to be how much of that water is actually flowing.
00:48
And so the water’s going to flow down and eventually through the sinks and the toilet,
00:52
it’s hopefully going to be cleaned and then eventually pumped back up to the top of that
00:57
water tower again. And so what’s resistance? We call that R. Resistance is simply anything
01:04
that resists the flow of that electricity and so anything like smaller pipes or maybe
01:12
clogging inside the pipes that slow down the passage of the water is going to offer resistance.
01:17
Likewise anything in a wire or anything that slows down electricity is going to offer resistance.
01:22
And so voltage is measured in volts. Current is measured in amps. And resistance is measured
01:29
in ohms. And each of those terms comes from a famous scientist that figured somethings
01:34
out about electricity.
01:36
Now today I am going to use what’s used the circuit construction kit. If you want to play
01:41
around with this you could go to this website. It’s called phet.colorado.edu and they’ve
01:45
put together a wonderful simulation that shows you how electricity actually works. And so
01:51
it’s a circuit construction kit. The one that I’m using is a DC, that means direct current
01:56
construction kit. And so what you can do is you can add a battery. So let’s add a battery
02:02
and if you control click on it or right click on it, you can actually show the value. And
02:07
so this is a 9-volt battery. So that means it has that much potential energy. And so
02:14
the batteries that you put in a fire detector would be an example of a 9-volt. Let’s add
02:19
a wire to that and another wire and let’s measure the current that flows through it.
02:25
And so I’m going to put an ammeter out here. Grab that. And this is going to measure the
02:30
amps which is going to be the current that flows through it. Let’s also grab another
02:34
wire and let’s kind of make this a complete circuit. And so if I grab a wire here, another
02:41
wire here and then we connect it back up okay. And so if we add a wire over here as well.
02:47
Now if you know anything about electricity you know that something bad is about to happen.
02:51
I’m about to short this battery. So what happens to a battery when it’s shorted out? Well you
02:58
can see that the electrons are just cruising around. You see the ammeter is off the, it’s
03:04
pegged all the way to the top. It also says it’s reduced the animation to less than 1%
03:09
of its normal speed. And so in the lab that’s bad because you’ll get sparks and you’ll also
03:15
could explode your battery. So let’s not do that. I’m going to split this junction right
03:19
here and actually let me split it right here. So that’s bad. That’s not a healthy circuit.
03:26
And so now let’s add the third thing. So again
03:29
what we have is we have voltage, that’s the potential energy. We have current, that’s
03:34
the flow of the electrons, but now let’s add a resistor here. So if we add a resistor to
03:40
our circuit and then connect it up, and let it run, then we have a healthier looking circuit.
03:46
And so what we now have is potential energy that’s pushing the electrons is this direction.
03:50
It’s going through an ammeter which is measuring the amps as it moves through it and now we
03:55
have a resistor, and that’s something that’s slowing down the passage of those electrons.
03:59
And let’s actually show that value. Okay, so what do we have? Let’s get back to our
04:04
terms again. We’ve got voltage or V. And we can see that that’s a 9-volt battery. We have
04:12
amperage and so that’s going to be current and we use I to explain what current is. Or
04:17
that’s the symbol for current. We measure that in amps. And then the last thing we have
04:22
is resistance. And resistance is going to be measured in ohms. And so we can actually
04:28
look at these values and you should be able to figure out what’s called ohm’s law. So
04:33
we have 9 volts. We have 10 ohms of resistance and we have .9 amps. And so if you take 9.
04:43
9 equals 10, sorry about the bad writing, times .9. In other words we could start just
04:59
with the numbers itself. And so Ohm’s Law is simply this. V equals I times R. Whoa,
05:10
that’s an ugly looking R. So V equals I times R. In other words the voltage equals the current
05:18
times the resistance. And so what should happen, if we increase the voltage. Well if we increase
05:25
the voltage and the resistance is set, so it’s not going to change, if we increase the
05:30
voltage what should happen to the current? Well let’s try it.
05:34
So if I do that, if I increase the voltage,
05:40
excuse me, so let’s change the voltage, let’s now change it to a higher value, what’s going
05:49
to happen to the amps or the current? The current is going to increase. The resistance
05:53
stays the same and so as I increase the voltage, what’s going to happen to the current? The
05:58
current is going to increase. And the cool thing about this animation is watch. If we
06:01
increase the voltage even yet, it shows the electrons or tries to represent the electrons
06:06
and their speed. And so the more volts that we add to our battery the more current we
06:11
get moving through it. Now likewise, what happens if we change the amount of resistance?
06:19
So now let’s change the resistance. So it’s 10 ohms. Let’s increase it. Let’s say we increase
06:25
it. Let’s say we make it 51 ohms, what happens to it? Well as we increase the resistance
06:32
then the current is going to decrease. And so there’s an inverse relationship between
06:37
the resistance and the current. And then there’s a direct relationship between the voltage
06:42
and the current. And so that’s simply going to be Ohm’s Law. And so you can solve problems.
06:49
In other words if I were to close this up, let’s close this up, not show you what it
06:54
is. If I didn’t know what the voltage was but I knew knew that the current was .94 amps
07:01
and I knew that the resistance was 51.25 ohms we should be able to figure out the voltage.
07:08
And so in a simple circuit, you can figure out based on current, based on resistance,
07:14
and volt, you can figure out one of the other ones.
07:16
The best way that I remember this, and so
07:19
you may use a different kind of a mnemonic to remember this, is I have this pyramid,
07:24
where I have voltage at the top, I have current on one side and resistance on the other and
07:29
so I’ll I do is simply us my hand and I cover up the one that I want to find. And so let’s
07:34
say my unknown is voltage. I could simply cover up the V and that’s going to be the
07:43
current times the resistance. And so if I don’t know the voltage, so if I cover that
07:47
up, voltage is simply going to be the current times the resistance. If I don’t know the
07:54
current, and so let’s cover that up, let’s say I don’t know the current, current is going
07:59
to be the voltage divided by the resistance. And likewise, if I don’t know the resistance,
08:04
so if I cover that up, resistance is going to be the voltage divided by the current.
08:09
And so this whole thing is called Ohm’s Law. And this is bad writing. I’m not too good
08:15
at the mouse. So this would be Ohm’s Law and it just shows us the relationship between
08:21
voltage, current and resistance. And all of these are named after famous scientists that
08:26
worked on electricity. This would be Volta, so the volts come from Volta. This is the
08:34
resistance and so this is Ohms. And then current is Amp. Ampere is his name.
08:40
And so that’s Ohm’s law. And the easiest way
08:43
to figure out how each of these things work is to actually use the circuit construction
08:48
kit. You can also use things like this. We can use a voltmeter. So we could put a voltmeter
08:55
right here. And you can actually put the voltmeter in different places and it will measure the
08:59
voltage. And so that voltage would be 48.12. And so you can use an ammeter. And then let
09:06
me get rid of that for a second. If you really want to have fun then you add something called
09:11
a light bulb. And so if we add a light bulb to this. Again we have to have electricity
09:18
flow in one direction and in the other, we can actually generate light. And so if I change
09:23
the resistance, let’s change the resistance, so let’s get a variable resistor, I can decrease
09:30
the amount of resistance and we get greater current. So we have electrons that are flowing
09:34
at a greater rate and we also have more light and we have more heat and that’s a simple
09:40
circuit. I’ll talk more about what is a parallel circuit, what is a series circuit a little
09:44
bit later. But I hope that’s a great start.
—
Previously published on YouTube.
—
Photo credit: Screenshot from video.