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The most fundamental equation in
electrical circuits is called Ohm's Law. While doing
experiments on how well metals conducted electricity,
German physicist Georg Ohm discovered the law in 1827.
Ohm's Law is the equation V = I * R and is used in both
AC and DC circuits. Knowing two items in this equation
allows you to calculate the third.
Questions you may have include:
What do the parameters in the equation
mean?
What are AC and DC circuits?
How can you use Ohm's Law?
Parameters
Ohm's Law states that in a simple
electrical circuit, the voltage (V) equals the
electrical current (I) times the resistance (R).
V = I * R
Voltage is the electrical potential
energy and is measured in volts.
A good analogy is to think of a water
hose. There is water pressure or potential energy on the
other side of the faucet or outlet valve. Once you open
the faucet, the pressure causes the water to rush
through the hose.
The unit symbol for volts is V, as in
110V.
Current indicates the amount of
electrons passing through the wire and is measured in
amperes or amps for short. For some reason, they use I
to indicate current instead of a different letter. The
unit symbol for amps is A, as in 2.0A.
Electrical current is similar to the
rate of water flowing through a hose.
Electrical resistance can be thought of
as the "friction" on the movement of electrons in a
wire. Resistance is measured in ohms, and the unit
symbol for it is the Greek letter omega, Ω. Thus 3 ohms
is often written as 3 Ω.
Most devices in an electrical circuit
can be considered resistors, including light bulbs and
electric motors. Even the wire itself provides some
resistance. Just as you get some heat from friction,
electrical resistance also results in heat. That is why
the light bulb filament gets hot and glows.
Following the water hose analogy,
resistance is similar to the friction inside the hose.
But also, the resistance increases with a narrower hose,
just like a thin copper wire has more electrical
resistance than a thick wire.
Circuits
Ohm's Law applies to electrical
circuits. The equation V = I * R says that in a simple
electrical circuit with a source of resistance--such as
a resistor or a light bulb--the voltage equals the
current times the resistance. This is true in a DC
circuit and in an AC circuit.
Ohm's Law applies whether the current
flows in one direction or alternates.
A simple AC circuit is illustrated
below. A circle with the sine wave symbolizes an AC
generator with some given voltage.
The resistor could be a single light
bulb or an electrical motor. Or it could be a
combination of resistive devices, such as several bulbs.
Complex AC sub-circuits that include capacitors and
inductors, besides resistors, are not being discussed in
this lesson.
The power source for a DC circuit could
be a battery or DC generator. The (+) and (-) indicate
the direction of the current.
The resistor could represent a series of
devices. Or it could be resistors in parallel.
Using equation
The importance of Ohm's Law is that if
you know the value two of the variables in the equation,
you can then determine the third.
You can measure any of the parameters
with a voltmeter. Most voltmeters or multi-meters
measure voltage, current and resistance for both AC and
DC.
If you know current and resistance, you
can find voltage from V = I * R. For example, if the
current I = 0.2A and the resistance R = 1000 ohms, then
V = 0.2A * 1000 Ω = 200V
If you know voltage and resistance, you
can use algebra to change the equation to I = V / R to
find the current. For example, if V = 110V and R = 22000
ohms, then
I = 110V / 22000 Ω = 0.005A
If you know voltage and current, you can
use algebra to change the equation to R = V / I to find
the resistance. If V = 220V and I = 5A, then
R = 220V / 5A = 44 Ω
Ohm's Law is the equation V = I * R that
shows the relationship between voltage, current and
resistance in a simple electrical circuit. It applies
both the AC and DC circuits. |
