Now let's talk about circuit elements in a little bit different way.
Let's talk about realistic voltage and
current sources and i-v curves associated with common elements.
Let's talk about open and
short circuits and switches, particularly wiring three-way switches.
An ideal voltage and current source has no resistance.
The voltage source just produces voltage, the current source just produces current.
But a more realistic non-ideal voltage source and current source has resistance.
A voltage source has a resistor in series and
a current source has a resistance in parallel, called the shunt resistance.
You know that most of your sources are realistic because they heat up when
you use them.
Your power drill, for example, gets hot when you use it very long,
and that's because of its internal resistance.
Here is a graph of the internal resistance of a battery.
Batteries get hot when we use them too, certainly my laptop battery does.
So you can see that this battery, this is an Energizer 1.5 volt AA battery.
As the temperature changes, so does its internal resistance.
Its internal resistance is just going to be, oops, I want it to go up, not down,
is going to just be this series resistance that's right here with a battery.
So let's take a look.
Now, if you were at room temperature,
the internal resistance of that battery would be about 0.15 ohms.
And if you were in Antarctica, which is where I did some measurements, and
let's say that you were about -10 degrees centigrade,
then your resistance would be about 0.3 ohms.
Let's see how that affects you.
Let's suppose that you just want to hook up this battery with its
internal resistance to a very small load.
I'm going to use a 1-ohm load for this example.
I would find the current that's in the circuit by taking the voltage and
dividing by the sum of the two resistors.
So in this case, if I had an ideal source, it would be 1.5 volts divided by my
1 ohm load, and so I would have 1.5 amps for the ideal battery.
But now let's say that I'm at room temperature.
In that case, my internal resistance is 0.15, so if I add that right in here to my
1 ohm, I'm going to have 1.3 amps at room temperature.
It's a little less than I had for my ideal battery.
Now let's go to my Antarctica battery, and
that's where I have 0.3 ohms connected on to my 1 ohm load, and
I have 1.15 Amps, than I really intended to have 1.5.
So what did that do?
This is why our batteries went dead so fast in the Antarctic cold.
Now let's talk about I-V curves.
Here are the I-V curves for the simple circuit that I just showed you.
You have a battery connected onto a resistor.
And what we would be plotting is voltage on the x axis and
current, or I, on the y axis.
The I-V curve for a resistor is always a straight line.
The resistor is a linear device.
If I had a 1 kiloohm resistor, for example,
connected onto a 1 volt battery, I would have 1 milliamp of current.
If I had a 2 kiloohm resistor, I'd have half a milliohm of current.
And if I had a really high resistance,
let's say infinity, I would have no current at all.
On the other hand, if I had a very tiny resistor, let's say 0 ohms,
I would have no voltage, but I'd have a very large current.
What does that mean?
Having a resistor of 0 is a short circuit.
That's where my current runs away with me, I have a huge current, and
my voltage would be 0.
An open circuit has a very high resistance, resistance of infinity, and
in that case, you have no current, but you have a very large voltage.
The I-V curves for an LED are non-linear.
The LED is a little light bulb that turns on when we put a voltage across it.
We're going to be using it in our first or second lab.
So you can see right here that there are different colors of LEDs and that
as the voltage increases for each one, see here's, I'm turning up the voltage,
turning up the voltage, and the current becomes very high.
This elbow right here, where the current goes from being almost 0
to becoming almost its final value, it's called the forward turn on voltage.
And right here for the red one,
you can see the forward turn on voltage for a red LED is about 1 volt.
The turn on voltage for a white LED would be a little bit higher.
Now let's go back to our idea of the open circuit and the short circuit.
If I have an open circuit, my current is 0.
My circuit may not work, but at least it's not gonna run away with me.
My short circuit, on the other hand, has a very small resistance but
extremely high current.
That's what can actually do you some damage.
A fuse or a circuit breaker is what's intended to keep the current from running
away with you.
If the current was trying to go to infinity, but
we had a 10-amp circuit breaker in line with that, the circuit breaker would
trip when the current got to 10 amps and it would shut the circuit off.
A circuit breaker is a type of switch.
There's several other types of switches.
The most common that we use when we do our designs are single pole,
single throw switches.
That's just a switch that you just turn on, turn off, turn on, turn off.
The other is a single pole double throw, where you turn on here or
you turn on there, turn on here, turn on there, and there are many others as well.
I put a link on the website, and I thought it was pretty interesting for you to see
all the different kinds of switches that you might be using throughout your career.
One very interesting switch circuit is the three-way light switch, that's where
you have a voltage source connected on to two single pull double throw switches.
These are very common along stairways or
hallways, because you want to able to turn the light on or
off from the top of the stairs as well as from the bottom of the stairs.
Now right here in the circuit, you'll notice that these two
switches have to be connected by a double set of wires going between them,
and then they're connected on to the lamp or the light bulb along a neutral line.
We often choose to use the light bulb in the neutral line
instead of the power line.
Because in the event that someone was taking the lightbulb out, and
let's say that they accidentally got their finger or
maybe a piece of or maybe a tool across here.
And somehow they managed to short themselves to a live line,
sorry to the ground.
If we were connected to the neutral, nothing should happen, but
if they were connected to a live line, it should.
So most often, the lamp is put on the neutral line for safety.
So in conclusion today, we've talked about voltage and current sources, open and
short circuits, and switches.
I'd like you to take a minute to try the three-way switch
in your MultiSim simulation.
It's a good chance to get to practice with MultiSim.
And it's fun to see how the three-way switch works.
Now, let's talk about where the picture was in the front.
This is the Sunflowers & Thunderheads picture from
Blackhawk Trail In Payson Canyon, Utah.
See all these yellow sunflowers along here?
It's absolutely amazing.
This is normally the week of the Fourth of July.