Basic Ohm's Law
Here we'll attempt to explain Ohm's law
basics!
Ohm's law
can be very difficult to understand by anyone who has never had any
basic understanding or training in basic electricity. We'll assume
that you have some knowledge of basic electricity. We'll explain it in
terms of water flow! DON'T GET WET!
What is Ohm's
Law:
Ohm's Law is made from
3 mathematical equations that shows the relationship
between electric voltage,
current and
resistance.
What is voltage? An
anology would be a huge water tank filled
with thousands of gallons of water high on a hill.
The difference
between the pressure of water in the tank and the water that comes out of
a pipe connected at the bottom leading to a faucet is determined by the
size of the pipe and the size of the outlet of the faucet. This difference
of pressure between the two can be thought of as potential Voltage.
What is current? An analogy would be the
amount of flow determined by the pressure (voltage) of the water thru
the pipes leading to a faucet. The term
current refers to the quantity, volume or intensity of electrical flow, as
opposed to voltage, which refers to the force or "pressure" causing the
current flow.
What is resistance? An analogy
would be the size of the water pipes and the size of the faucet. The
larger the pipe and the faucet (less resistance), the more water thatcomes
out! The smaller the pipe and faucet, (more resistance), the less water
that comes out! This can be thought of as resistance to the flow of the
water current.
All three of these: voltage, current and resistance
directly interact in Ohm's law.
Change any two of them and you effect
the third.
Info: Ohm's Law was named after Bavarian
mathematician and physicist Georg Ohm.
Ohm's Law can be
stated as mathematical equations, all derived from
the
same principle.
In the following equations,
V is voltage measured in volts (the size of the
water tank),
I is current measured
in amperes
(related to the pressure
(Voltage) of water thru the pipes and
faucet) and
R is resistance measured
in ohms as related to the size of the pipes and faucet:
V = I x
R (Voltage = Current multiplied by
Resistance)
R =
V / I (Resistance = Voltage divided by
Current)
I = V / R (Current =
Voltage Divided by Resistance)
Knowing any two of the values of a circuit,
one can determine (calculate) the third, using Ohm's
Law.
For example, to find the Voltage in a
circuit:
If the circuit has a current of 2 amperes, and a
resistance of 1 ohm, (< these are the two "knowns"), then
according to Ohms Law and the formulas above, voltage equals current
multiplied by resistance:
(V = 2 amperes x 1 ohm = 2
volts).
To find the current in the same circuit above
assuming we did not know it but we know the voltage and
resistance:
I = 2 volts divided by the resistance 1 ohm = 2
amperes.
In this third example we know the current (2 amperes) and
the voltage (2 volts)....what is the resistance?
Substituting the
formula:
R = Volts divided by the current (2 volts
divided by 2 amperes = 1 ohm
Sometimes it's very helpful to
associate these formulas Visually. The Ohms Law "wheels" and graphics
below can be a very useful tool to jog your memory and help you to
understand their relationship.
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The wheel above is
divided into three sections:
Volts
V (on top of the dividing line)
Amps (amperes) I (lower left
below the dividing line)
Resistance R (lower right below the dividing
line)
X represents the (multiply by sign)
Memorize this wheel
To use, just
cover the unknown quantity you need with your minds eye and what is left
is the formula to find the unknown.
Example:
To find the
current of a circuit (I), just cover the I or Amps section in your mines
eye and what remains is the V volts above the dividing line and the R
ohms (resistance) below it. Now substitute the known values. Just
divided the known volts by the known resistance.
Your answer will be
the current in the circuit.
The same procedure is used to find the
volts or resistance of a circuit!
Here is
another example:
You know the
current and the resistance in a circuit but you want to find out the
voltage.
Just
cover the voltage section with your minds eye...what's left is the I X R
sections. Just multiply the I value times the R value to get your answer!
Practice with the wheel and you'll be surprised at how well it works to
help you remember the formulas without trying!
This Ohm's
Law Triangle graphic is also helpful to learn the formulas.
Just cover
the unknown value and follow the graphic as in the yellow wheel examples
above.
You'll
have to insert the X between the I and R in the graphic and imagine the
horizontal divide line but the principal is just the
same.
In the above
Ohm's law wheel you'll notice that is has an added section (P) for Power
and the letter E* has been used instead of the letter V for
voltage.
This wheel is used in the exact same fashion as the other
wheels and graphics above.
You will also notice in the blue/green areas
there are only two known values with the unknown value in the yellow
sections. The red bars separate the four units of
interest.
An
example of the use of this wheel is:
Let's say that you know the power
and the current in a circuit and want to know the voltage.
Find your
unknown value in the yellow areas (V or E* in this wheel) and just
look outward and pick the values that you do know. These would be the P
and the I. Substitute your values in the formula, (P divided by I) do the
math and you have your answer!
Info: Typically, Ohm's Law is only applied to
DC circuits and not AC
circuits.
*
The letter "E" is sometimes used in representations of Ohm's Law
for voltage instead of the "V" as in the wheel
above.
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