Ohm's Law and Power

Ohm's Law and Power

Ohms Law

The relationship between Voltage, Current and Resistance in any DC electrical circuit was firstly discovered by the German physicist Georg Ohm. Ohm found that, at a constant temperature, the electrical current flowing through a fixed linear resistance is directly proportional to the voltage applied across it, and also inversely proportional to the resistance. This relationship between the Voltage, Current and Resistance forms the bases of Ohms Law.

Ohms Law Triangle

Here the three quantities of V, I and R have been superimposed into a triangle (affectionately called the Ohms Law Triangle) giving voltage at the top with current and resistance at the bottom. This arrangement represents the actual position of each quantity in the Ohms law formulas.

and transposing the above Ohms Law equation gives us the following combinations of the same equation:

Then by using Ohms Law we can see that a voltage of 1V applied to a resistor of 1Ω will cause a current of 1A to flow and the greater the resistance, the less current will flow for any applied voltage.

Electrical Power in Circuits

Electrical Power, ( P ) in a circuit is the amount of energy that is absorbed or produced within the circuit. A source of energy such as a voltage will produce or deliver power while the connected load absorbs it. Light bulbs and heaters for example, absorb power and convert it into heat or light and the higher their value or rating in watts the more power they will consume.

The quantity symbol for power is P and is the product of voltage multiplied by the current with the unit of measurement being the Watt ( W ).

The Power Triangle

Again, the three quantities have been superimposed into a triangle this time called the Power Triangle with power at the top and current and voltage at the bottom. Again, this arrangement represents the actual position of each quantity in the Ohms law power formulas.

and again, transposing the basic Ohms Law equation above for power gives us the following combinations of the same equation to find the various individual quantities:

So we can see that there are three possible formulas for calculating electrical power in a circuit. If the calculated power is positive, (+P) in value for any formula the component absorbs the power, that is it is consuming or using power. But if the calculated power is negative, (-P) in value the component produces or generates power, in other words it is a source of electrical power such as batteries and generators.

Ohms Law Pie Chart

To help us understand the the relationship between the various values a little further, we can take all of Ohm’s Law equations from above for finding Voltage, Current, Resistance and Power and condense them into a simple Ohms Law pie chart for use in AC and DC circuits and calculations as shown.

Ohms Law Matrix Table

As well as using the Ohm’s Law Pie Chart shown above, we can also put the individual Ohm’s Law equations into a simple matrix table as shown for easy reference when calculating an unknown value.