Ohms
Law
An electric circuit is defined by an emf or voltage source, a current
path, and a resistance or load. The following drawing is called a
schematic of an electric circuit.
Drawing of simple circuit
A complete circuit must have an unbroken path so current can flow
from the battery, through the resistance or load, and back into the
battery. If the circuit becomes broken, or open, the current cannot
flow. There is a device that has been designed to open and close an
electric circuit. This device is called a switch, and can be compared
to a gate in a stream that can be closed to stop the flow of water.
Drawing of switch, schematic
symbol
The values of the current, the voltage, and the
resistance in a circuit are totally dependent on each other. The
relationship between them is commonly known as Ohms Law. It can
be stated as follows: The current that can flow in a circuit will
increase if the emf increases, and it will decrease if the resistance
increases.
The equation of Ohms Law is:
I (amps) = E
(volts) / R (ohms)
The current is equal to the voltage divided by the resistance.
The above equation will calculate the value of the current in amps
when the values of the voltage and resistance are known. This equation
can also be re-arranged so that any of the three values may be calculated
if the other two are known.
E (volts) = I
(amps) X R (ohms)
The voltage is equal to the current times the resistance.
R (ohms) = E (volts)
/ I (amps)
The resistance is equal to the voltage divided by the current.Diagram
below is an easy way to remember the Ohm's Law equation. If you
cover up any one of the three variables you want to solve for, the
equation remains.
For example, if you cover up the E,
I X R remain. The equation
for E is I
X R (current multiplied by resistance).
If you cover up the I ,
E / R remain. The equation
for I is E
/ R (voltage divided by resistance).
If you cover up the R, E
/ I remain. The equation for R
is E / I (voltage divided
by current).
All three forms of this equation are used extensively by someone
designing an electric or electronic circuit. The values of the voltage,
resistance, and current are usually measured in units that are fractions
of a single volt, ohm or amp. These values of voltage, resistance,
and current are usually seen with these prefixes attached to them:
micro - one-millionth (abbreviated
())
milli - one-thousandth (abbreviated
m)
kilo - one thousand (abbreviated
K)
mega - one million (abbreviated
M)
In order to use the values of voltage, resistance and current in
the Ohm's Law equation, they must first be converted into the fractional
part of a single volt, ohm, or amp.
Example: 40 mA or 40 milli-amps would be converted into 0.040 amps.
500 (V or 500 micro-volts would be converted to 0.005 volts.
1M( or 1 mega-ohm would be 1,000,000 ohms.
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