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Ohm’s Law
Ohm’s Law is the mathematical formula that
explains the relationship of Voltage, Amperage
and Resistance.
Voltage can be explained as the potential
difference between to points in a circuit and is
represented by the symbol (V). Voltage is also
represented by the symbol (E) and can be used
interchangeably. (E) was originally used to
describe (emf) or Electro Magnetic Force and
was used to describe things such as batteries
but over time the 2 have become
interchangeable.
Amperage in a circuit can also be referred to as
Current just like a dog can be called a canine
these terms are also interchangeable. Current is
the amount of electrons that pass a particular
point in a circuit at the same time. In fact 1
Ampere is equal to 6.242 × 1018 electrons
passing a single point in 1 second. In other
words the higher the current the higher the
amount of electrons that would be flowing in the
circuit.
When you are using mathematical formulas to
decipher components of ohms law Amperage is
symbolized by the Capital letter (I).
Now you are wondering why use the letter (I)
seeing as Current starts with a (C). The reason
is back in the 1700’s most of the scientists that
were working on experiments dealing with
electricity were either English or French. In
France Amperage was referred to as the
intensite current (the intensity of the current).
So the symbol (I) came from the word intensite.
Resistance can be explained as the amount of
resistance voltage has to overcome in a circuit
for current to flow. The byproduct of resistance
is heat we’ll talk about that later.
Let’s now look at why copper is used as a carrier
for electricity. To find out what makes it so
special we have to understand what are some of
coppers special features.
First it can be drawn into long thin wires
(ductility)
Second it can be worked or bent into many
different shapes without breaking (malleability)
Other metals that have the above capabilities
are gold, silver, platinum and aluminum
As you can guess all the other metals except
aluminum are much more expensive and would
not be cost effective. Aluminum is not a choice
conductor because when electricity passes
through aluminum the aluminum tends to
vibrates.
The other feature of copper that makes it a
choice conductor for electricity is its atomic
structure. Copper has 29 electrons 28 of which
are held tightly by the atoms nucleus and one
that is not held very tightly so even slight
pressure will cause this one to be in motion.
It happens and no I can’t prove it.
Just to give you an idea of how many free
electrons there would be in a piece of copper
1 cubic inch of copper would have 1.4 × 1024 free
electrons.
Back to Ohm Law
OHM’S LAW
Ohms Law is fundamentally based on the
following formula:
V=IxR
This formula can be written different ways
depending on what you are trying to find as in:
V = IR
I = V/R
R = V/I
Let’s start by looking at a simple series circuit
like the circuits you were building with your
resistors last week.
A series circuit can be defined as a circuit that
has only one path. From the negative terminal of
the power supply, out through the circuit then
back to the positive terminal of the power
supply.
In other words all devices are connected
together like a group of people holding hands in
a circle.
It can be said that a change in voltage will have
a proportional change in current if the resistance
stays the same.
Let’s look at the next 2 circuits and see how this
works. Using the Formula
V=IxR
R=
V = 10V
V= 10V
I = 2A
R= ?
V = IR
10 = 2 x R can be simplified to equal
10/2 = R
Therefore the resistance of the unknown resistor
would be 5 ohms
Now let’s see the relationship of Voltage and
Current if the resistance in the circuit stays the
same
I=
V = 100V
R = 5 ohms
V = 100V
R = 5 ohms
I=?
V/R = I
100/5 = I
20 = I
Therefore I = 20A
So we can see if resistance stays the same in a
series circuit Voltage and Current change
proportional to each other.
If on the other hand we keep the Voltage the
same and change the resistance. You will
notice that as the resistance increases the
Amperage decreases.
This works the same as a water tap. As you
close the tap (put up more resistance to the
water) less water will flow. Likewise if you open
the tap more (put less resistance in the path of
the water) more water will flow out of the tap.
I=
V= 100V
V = 100V
R = 10 ohms
I =?
V/R = I
100/10 = I
R= 10 ohms
10 = I
I = 10A
Therefore as long as the Voltage stays constant
an increase in Resistance will result in a
decrease in Current.
Now let’s look at what happens if we have more
than one resistor in a circuit
R1 = 5 ohms
R2 = 10 ohms
V = 30 Volts
R3 = 15 ohms
Before we can tackle the above problem we
must learn another Formula that works with
resistors in a series circuit
RTotal = R1 + R2 + R3 + . . .
Now using the above formula lets look at how it
would work:
RTotal = ?
R1 = 5 ohms
R2 = 10 ohms
R3 = 15 ohms
RTotal = 5 + 10 +15
RTotal = 30 ohms
Now we have to figure out what the Current
would be:
I = V/R
I=?
V = 30V
R = RTotal = 30 ohms
I = 30/30
I=1
Therefore the current in the circuit would be 1A
Now we need to know a little more about
Current in a series circuit.
As current is flowing through a circuit it makes
sense that if you want to measure current you
have to put a meter in line with the circuit like
adding another person into the group of people
holding hands in a circle.
It is because of this that it can be assumed that
the current is the same at all points in a series
circuit.
Now we have to look at how resistance affects
voltage in a series circuit.
It is said that Voltage drops over resistors in a
series circuit. Also if you remember Voltage is
the potential difference between 2 points in a
circuit. It is because of this that when you are
measuring Voltage you measure between 2
points. So looking at the above circuit diagram if
we were to place our meter leads at point 1 and
2 we would be measuring the potential
difference over R1
To find this Voltage we would use the formula:
V = IR
V=?
I = 1A we know this from the previous equation
R = 5 ohms
V=1x5
V = 5V
So it can be said that the Voltage drop over R1 is
equal to 5 Volts.
If we repeat this step measuring between points
2 and 3 then also between points 3 and 4 we will
notice the Voltage drops will be:
Between 2 and 3 (R2) = 10 Volts
Between 3 and 4 (R3) = 15 Volts
You will notice that the voltage drops throughout
the series circuit will add up to equal the total
available Voltage
VTotal = V1 + V2 + V3 + . . .