Ohm's Law
The law Three relationships A triangle to make it easy to remember
Ohm's Law at work

Georg OhmYou 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
Click for larger image 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:

  1. Voltage equals resistance multiplied by current
  2. Resistance equals voltage divided by current
  3. 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 triangle

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).

Copyright owned by the State of Victoria (Department of Education and Early Childhood Development). Used with Permission.

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Other symbols from electronics
Electricity in the home
  Question 1
Question 2
Question 3
Question 4
Voltage Electric current
Resistance Amp