Welcome! In this video we will talk about the fundamentals of an electrical system.
In particular we will talk about certain circuit quantities. There are basically
FIVE major electrical quantities that we are interested in --- charge, current, voltage,
energy and power. Among these five quantities, however, current, voltage and
power are typically used for circuit analysis. So, let's quickly talk about
some of these parameters. Charge : well in a physical world there are two kinds of
charges positive and negative charges and charges are measured in a unit
called Coulomb. Charges are typically quantized and a collection of charge is
basically a multiple of 1.602 x 10^(-19) Coulomb
which happens to be the smallest amount of charge that is present in an electron.
Basically "Conservation of Charge"
says that electrical charge can neither be created nor destroyed.
Now when a group of charge moves from one point to another or a group of
charges is forced to move around , it gives rise to what is known as
electrical CURRENT. So, electrical current is basically the measure of the rate of
flow of electrical charge through a given point. Whenever we are talking
about current, we are always talking about a flow of charge per unit time
through an element. Here is an example of an element and the current is flowing
from direction 'a' to 'b'. The unit of current is Amperes or an Amp or we
simply abbreviate it as A. Current has both a polarity and a direction; so it has
both MAGNITUDE as well as the DIRECTION.
Since it has polarity, current can be, positive or negative. Basically what a
positive or a negative current means is that a current is flowing in a
particular direction and if it has a negative value then it basically means
that the current was actually supposed to be flowing the opposite
direction than what we had thought. So, basically we start out by picking an
arbitrary direction for current and if the sign of the current that is
calculated or measured comes out to be negative, then really the current is
flowing in the opposite direction. More on this when we talk about
Passive Sign Convention (PSC) later. Now, the next thing that we want to talk about in
terms of circuit quantities is called a VOLTAGE or the electrical potential. The
voltage is electrical potential between two points in this space and it is
basically the amount of energy that is required to move one Coulomb of
charge from point 'A' to point 'B'. The unit of voltage is joules per Coulomb
and it is written as volt or simply V. Whenever we calculate voltage,
since it is the amount of energy that's needed to move a charge from point A to
B, voltage is always written with respect to a particular reference point. So
in this particular example, we have nodes 'a' and 'b'. Node 'a' is at a particular
potential and 'b' is at another potential and the voltage is considered to be the
difference between these potentials. So we write this as V subscript ab (Vab)
which means Va minus Vb. So if I have an electrical element like this and
I am use a multimeter to measure the voltage, which we will be
talking about more. So if you take a multimeter to measure the voltage and
the multimeter let's say it reads negative 1.4 (-1.4). So the RED is the positive
part of the multimeter and the BLACK is going to the negative end of the
multimeter and we read negative 1.4. So what that basically means is the voltage
difference between node a and node b is minus 1.4 (-1.4) and that's the same as saying
that the voltage difference between notes b and a or in other words Vba
which is Vb minus Va is positive 1.4 (+1.4). Okay, so it is basically a reference. Since
a voltage is always defined as being between two points,
it is useful to typically define a common voltage reference point.
In an electrical circuit that common voltage point is called GROUND. Just like
when we measure the height of a mountain, typically the height of the mountain is
measured with respect to a sea-level. For example, the height of Mount Everest is
8,848 meters. But if you go to the base of Mount Everest and take a measuring
tape and measure to the top of the Mount Everest it is not 8,848 meters. That
height to the tip of the mountain is measured with respect to a recognized
reference point known as sea-level. So we can measure and compare the
height of two mountains, for example, Mount Everest which is in Nepal and
Mount Kilimanjaro in Africa. So comparing two mountains in two different locations
becomes easier because we have a common reference point for the height. We
started at sea-level. Similarly, comparing voltages between
different nodes becomes easier if we can reference it to a single point and we
call that particular reference point GROUND. The GROUND has a voltage of zero
volts. So in our prior example, when we said Va that basically meant we were
looking at the voltage at node 'a' with respect to a common node called ground
somewhere and when we said Vb we basically meant we were looking at
reference node b voltage with respect to a reference node
ground somewhere else. Now another important quantity is ENERGY and
POWER. Power is basically energy over time and it's given as a product of
voltage times current. If we expand voltage -- it was defined as dW over dq (dw/dq);
meaning amount of energy required to move charge from point A to point B and
current was defined as the amount of charge per unit time. So, that gives power
equals voltage times current.
so that is power and the unit of power is known WATTS.Nnow in a
electrical circuit there are certain sources that provide voltage or current.
Those sources are called ideal voltage sources. Circuit elements that maintain a
fixed voltage across its terminal regardless the amount of current flowing
in those terminals is known as ideal voltage source. An example of a voltage
source is a battery. Similarly a current source is shown here. It is a circle with
an arrow on it. Basically a current source is an electrical component that is
able to provide fixed current to a load
regardless of how much voltage is across those terminals. There are two types of
current and voltage sources. One is called Direct Current (DC). For example, direct
current can be thought of something that doesn't change. A volt produced by a
9.6 volt battery or a 1.5 volt battery or a car battery is an
example of a direct current; it doesn't change whereas the voltage that we get
in our electrical outlets fluctuates sinusoidally at 60 Hertz and it's
called alternating current. So when we when we draw and show voltage sources, for
DC voltage sources we will basically use a circle with a plus(+) minus(-) and write the
voltage value here. If we are explicitly showing batteries, this is the way to
show it. AC or alternating current or the voltage that's available in the power
outlets is generally shown as a circle with a sinusoidal source on it.
Besides sources, there are electrical components called LOADS.
Here is an example of some LOADS--- RESISTOR which is pictured here; a
CAPACITOR pictured here with electrical schematics right here shown as two
parallel plates as that is what a capacitor is; an INDUCTOR shown here in a picture and
here as a symbol. An inductor is basicallya coil wrapped around a core so that's what
the symbol is showing. Switches --- normal switches we'll be using are either
toggle switches which you set into high or low or a push-button switch.
We might also be using simple loads called light-emitting diodes which are
shown as such. So here is an example of a very simple circuit. A simple
circuit here has a source and a load and sometimes has a switch in between.
This circuit has a open switch right now so no current can flow through it. If I
however close this circuit by pushing on this switch; so if I push on the switch
and close the circuit the circuit is now complete. Current can flow from the top
voltage source and the energy can be dissipated in the load and the current
can come back. So a complete circuit requires a loop to travel and it always has
a source and a load.