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The previous video introduced charge, current, and voltage. It also introduced the equation
for electrical power. If you are not familiar with those, you should watch the previous
video. In this video, we will continue with resistance
and Ohm’s Law. Back during the time when people were first
figuring things out about electricity (This was during the time period when it was still
common to become famous for coming up with a three variable equation), a person named
Georg Ohm discovered, while experimenting with thermocouples (devices in which two dissimilar
metals are placed in contact with each other producing an electric potential, this incidentally
is why you experience discomfort if you have fillings and accidentally bite on a piece
of aluminum foil. The dissimilar metals produce an electric potential.)…
Anyway, when he experimented with thermocouples, varying the type and dimensions of conductors,
he discovered there was a linear relationship between the electric potential and the current.
That is the ratio of voltage to current was equal to some fixed quantity.
This is the quantity we call resistance. In honor of Mr. Ohm, the unit of resistance was
named after him. Ohm is symbolized by the Greek letter capital omega.
In base SI units, the ohm is kilogram meters squared divided by amps squared times seconds
cubed. Not that that will help you very much at this point, but I thought I would throw
it in anyway. Now this was a great discovery, though it
seems common-place now. It was a huge step in the demystification of electricity. And
since electricity is invisible, that is kind of a big deal.
When we want to represent a resistor on paper, we use a zig-zaggy line, so ohm’s law, expressed
pictorially would look like this: And we would represent the relationship between
the voltage and current as v(t)=R times i(t). If we wanted to take a look at this with numbers,
we might start with a 100 ohm resistor. Adding a 1V DC source (DC stands for direct
current, which simply means that the source always provides 1V), we can rearrange Ohms
law to solve for current. Substituting the values of 1 volt and 100
ohms, we arrive at a current of 0.01 A, which could also be written as 10mA.
This brings up at least two further issues that I will have to talk about, the first
is, ‘How did I choose what direction to draw the current?’ the second is ‘What
is this prefix of milli?’, though many of you may already be familiar with this one.
Let’s start with the direction of the current. In the first video, I told you that charge
comes in positive and negative varieties. Though the decision to assign positive and
negative was largely arbitrary, (again Ben Franklin) as long as everyone agrees how to
use positive and negative, the system works. Though there is a physical argument for the
direction of the current, I am going to appeal to something called the ‘passive sign convention’.
As the name implies, this is the convention. Everyone everywhere agrees to this convention,
and that allows us to communicate about circuits effectively.
The passive sign convention defines the relationship between the polarity (that is the positive
and negative, or direction) of a voltage and the direction of the current associated with
that voltage. A voltage is a difference in electrical potential between two points. The
positive sign is assigned to the more positive potential energy.
The voltage source in this circuit supplies power. As the current travels through the
circuit, it encounters a resistor. A resistor is an element that USES energy, or dissipates
power. Usually, the electrical energy is converted to thermal energy, or heat, in the resistor
(though in extreme cases we can get light out for a brief amount of time). That being
said, electrical energy will be lost in the resistor. So, we will go from a higher electric
potential to a lower electric potential as we cross the resistor. So, according to the
passive sign convention, since the resistor dissipates power, the current enters the the
terminal associated with positive sign of the voltage, and leaves the terminal associated
with the negative sign of the voltage. So, when current enters the positive terminal
of the device that uses energy, that device dissipates or absorbs positive power. From
our power equation in the last video P=VI, the power dissipated by the resistor is P=1V*0.01A=+0.01W.
So, to summarize the passive sign convention, it can be stated that for any circuit element
that uses power, current flows from the positive terminal of the voltage to the negative terminal.
In this case, the element will absorb positive power.
Conversely, if an element is supplying power, current will flow from the negative terminal
to the positive terminal. In this case the element is absorbing negative power, which
is a more complicated way of saying that it is supplying power.
All right, now back to the milli prefix. In the last video, we covered the SI units
with the units being the meter, kilogram, second, ampere, Kelvin, and candela.
Along with the units, we have and prefixes that help is to communicate. In engineering,
we use a subset of the SI prefixes and we only use prefixes associated with factors
of 1000. The engineering prefixes are summarized in this table.
Most commonly we use the prefixes of giga down to pico, which is ten to the positive
nine, down to 10 to the negative twelve, although especially in computing, Tera, for ten to
the positive twelve, is becoming fairly common. As it applies to the example shown earlier,
0.01 Amps written in scientific notation would be ten times ten to the negative three amps.
The prefix that is used for ten to the negative three is milli, so this can be written as
10mA. Some common ranges of quantities of used in
this series on electrical engineering would be:
voltages generally will fall in the microvolt to kilovolt range,
currents will usually be from the microamp to the amp range,
common resistances will be from the ohm to the megaohm range,
inductances (we will talk about later) will generally be in the millihenry to microhenry
range, and capacitors will generally be in the microfarad
range down to the picofarad range. So, in today’s video I introduced the concept
of resistance. We learned about Ohm’s Law, the passive sign convention, and we briefly
covered engineering prefixes. Until next time, go out and make it a great one.
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