Ohm's Law
You
have discovered that voltage and resistance both
have an effect on the current flowing in a circuit.
Other factors being constant, an increase in voltage
results in an increase in current. On the other
hand, an increase in resistance results in a decrease
in current.
In 1826 Georg Ohm discovered this relationship
and developed a mathematical law of electric current.
This is now known as Ohm's Law.
The law
One way Ohm's Law can be stated is: "a
current flowing through a conductor is directly
proportional to the voltage, given the temperature
of the conductor remains constant".
Therefore, if the resistance is kept constant,
then doubling the voltage doubles the current.
Conversely, if the voltage is constant then doubling
the resistance halves the current. There is a
very simple relationship between all three variables
as you can see.
Using the symbols V for voltage, R for resistance
and I for current the formula for Ohm's Law is:
V = R × I.
Note: Ohm's Law is stated for a conductor at
a constant temperature. Usually as temperature
rises in a wire or resistor, resistance also rises.
Three relationships
By simple transformation of formulae, two
other equations can be written:
R = V/I and I = V/R.
Putting these into words Ohm's Law states that:
- Voltage equals resistance multiplied by current
- Resistance equals voltage divided by current
- Current equals voltage divided by resistance
A triangle
to make it easy to remember
The calculation triangle in the Ohm's Law animation
is divided into three sections with the letter
V representing voltage in the top section and
letters I for current and R for resistance in
the two bottom sections. Using your mouse click
on any one to find the formula relating to that
variable.
Ohm's Law
at work
An example calculation using Ohm's Law:
Connected in a circuit are a battery (the name
for more than one cell) of 9 volts, and a resistor
having a resistance of 3 ohms. Calculate the current
flowing in the circuit.
The current is needed so the required formula
is:
current = voltage divided by resistance
or I = V/R.
Substituting the numbers you find that I = 9 /
3 = 3. However, you need the unit for current.
Current is measured in amperes, so the answer
is 3 ampere (amps).
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