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This starts a series of videos in which I will introduce some of the basic quanities
involved in electric circuits to aid students in my introductory circuit modeling class.
This is a pilot attempt by an amatuer, so don't expect anything too impressive.
I'll start by introducing Electrical engineering. Electrical engineering deals with the manipulation
of electrical and magnetic quantities for the benefit of mankind
It is a very broad field dealing with everything from generating and delivering power to homes,
businesses, and factories to generating and transmitting the signals
used in wireless communications, satellite communication, radar
all the way down to manipulating materials at near atomic scales to create computing
and communication devices. Regardless of the realm or subdiscipline we
are working in it Starts with the basic quantity of charge
Charge is: (according to Merriam-Webster) an amount of electricity
Electricity is (also, according to Merriam-Webster): a form of energy that is carried through wires
and is used to operate machines, lights, etc. Still not all that descriptive....
Another definition is given by thefreedictionary.com, and I have to admit that I like this one a
bit better Not because it is more descriptive, I just like how it sounds
According to thefreedictionary.com: Charge is: the property of matter that is responsible
for electrical phenomena, existing in a positive or negative form.
From these definitions, we may not be closer to understanding what charge is, but that
is ok. We can leave that to the pure scientists.
From an engineering standpoint, knowing that it is there and how it behaves, we can use
it. Here are some basic properties of charge:
it comes in two different polarities. This means we have some charges we call positive
and others we call negative. Charge is measured in a unit called Coulombs
A coulomb is a large amount of charge The most commonly refered to particle of charge
is the electron (a tiny sub-atomic particle with a negative charge)
To get a coulomb of charge I would need more than six billion, billion charges or about
6.215 times ten to the eighteenth electrons
Another property is that charges can act on each other at a distance due to invisible
forces from electric fields Like charges repel each other
Opposite charges attract Because charge can act at a distance, I can,
for instance use a collection of positive charge to push a positive charge
or I can use a collection of negative charge to pull a positive charge
Let’s assume that we can collect these charges, because, well, we can.
Then I can use that collection of charge to move another charge.
A fixed positive charge can push another positive charge away.
The motion of charge is another fundamental quantity that is called current
Current is measured in coulombs per second or amperes
Amperes are the SI units for electricity
A little aside SI refers to the French form of saying the
international system of units The standard units are:
The meter for expressing length The kilogram for expressing mass
The second for expressing time The ampere is used for expressing electric
current Interestingly this is the only SI unit that
is composed of other units, that of the coulomb and the second since current
is a measure of charge per unit time. All the other units are base units.The kelvin
is used to express temperature The mole is used to express the amount of
substance and the candela is the unit of luminous intensity
These are the agreed upon units from which we build all other units
I may come back to these in a later video
As I was saying Electric current is the flow of charge
Since electrical quantities have a polarity,or direction, associated with them, I need to
define current with a direction, like a vector Positive charge flowing in a positive direction
is positive current Now by some strange series of coincidences
(Ben Franklin) it turns out that what we call positive charge is largely stationary in solids
As positive charge is most commonly found in the nucleii of atoms.
SO, most commonly when we’re referring to current we are referring to the movement of
the negative charge or electrons That’s OK, because much like in the case
of the motion of objects positive and negative only refer to direction.
What that means for us, is that negative charge moving in and negative direction is also a
positive current.
For our purposes of this series it really doesn’t matter what type of charge is moving.
The fact that we have current is the important thing.
Now if we have charge, and we have the movement of that charge in current.
There must be something that makes the charge move
The force that causes the motion of charge is the electromotive force or commonly called
voltage Voltage is a measure of how much electric
pushing power there is to move that charge The more voltage we have the more charge we
can move.
Using the illustration I used earlier if a little positive charge can push a little bit
of positive charge, then a lot of positive can push a lot of positive
charge So where does voltage come from, well, it
comes from charge This may seem a little bit circular, but we might expect that since i
started by saying that charge was responsible for electric phenomena
So it makes sense that it is at the center of everything, not only is it the thing that
is getting pushed around, it is also the thing doing the pushing.
So here we have our fundamental quantities of electronic phenomena
We have charge The movement of which is called current
And the thing that makes these moves is called voltage
Now how can we have a presentation electrical engineering that does not include at least
one little derivation?
So let’s see if we can put these pieces together in a meaningful way
We already have that the coulomb per second is the ampere
If we express this a little more mathematically we see the change in charge per unit time
is equal to current If we wanted to go even a little further current
is equal to the derivative of charge with respect to time
we also have that the volt is equal to a joules per coulomb
expressing this a little more mathematically we have that voltage is equal to the change
in energy per unit charge going to calculus this says voltage is the
derivative of energy with respect to charge so we put these together
the product of voltage times current gives us the derivative of charge with respect the
time times a derivative of the energy with respect to charge
the charges cancel and we have the derivative of the of energy with respect to time which
is called Power Power measures the amount of energy per unit
time Power is equal to voltage times current
The unit of power is the watt There will be many more details on this as
we go forward, but that is enough to get us started.
So what we covered today is that charge is the basic physical property responsible for
electric phenomena That the motion of this charge is called current
and is measured in amperes The quantity that causes this motion as a
voltage and it is measured in volts And just for fun at the end we threw a new
equation for Electric Power Electric Power is simply the product of the
voltage times current and it is measured in Watts
That’s all for today, go out and make it a great one.