OHM'S LAW

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OHM'S LAW

After reading this section you will be able to do the following:
Identify Ohm's law and discuss why it is important.
Calculate the amount of electric current in a circuit using Ohm's law.

Probably the most important mathematical relationship between voltage, current and resistance in electricity is something called “Ohm’s Law”. A man named George Ohm published this formula in 1827 based on his experiments with electricity. This formula is used to calculate electrical values so that we can design circuits and use electricity in a useful manner. Ohm's Law is shown below.
OHM'S LAW

I = V/R,

I = current, V = voltage, and R = resistance

*Depending on what you are trying to solve we can rearrange it two other ways.

V = I x R

R = V/I

*All of these variations of Ohm’s Law are mathematically equal to one another.


Let’s look at what Ohm’s Law tells us. In the first version of the formula, I = V/R, Ohm's Law tells us that the electrical current in a circuit can be calculated by dividing the voltage by the resistance. In other words, the current is directly proportional to the voltage and inversely proportional to the resistance. So, an increase in the voltage will increase the current as long as the resistance is held constant. Alternately, if the resistance in a circuit is increased and the voltage does not change, the current will decrease.

The second version of the formula tells us that the voltage can be calculated if the current and the resistance in a circuit are known. It can be seen from the equation that if either the current or the resistance is increased in the circuit (while the other is unchanged), the voltage will also have to increase.

The third version of the formula tells us that we can calculate the resistance in a circuit if the voltage and current are known. If the current is held constant, an increase in voltage will result in an increase in resistance. Alternately, an increase in current while holding the voltage constant will result in a decrease in resistance. It should be noted that Ohm's law holds true for semiconductors, but for a wide variety of materials (such as metals) the resistance is fixed and does not depend on the amount of current or the amount of voltage.

This explains why electrical transmission lines are operated at very high voltage. It might seem based on Ohm's Law that the high voltage would result in an increase in resistance which would result in higher energy losses. But this is not the case because the resistance is fixed by the wire size and the material, so the current must decrease. Additionally, the energy losses due to heat generation in a circuit are directly related to the current. Another physical law known as Joule's First Law tells us that the rate of heat dissipation in a conductor is proportional to the square of the current.

As you can see, voltage, current, and resistance are mathematically, as well as, physically related to each other. We cannot deal with electricity without all three of these properties being considered.



(The symbol for an Ohm looks like a horseshoe and is pictured after the "100" in the diagram above.)

Review
Ohm's Law is used to describe the mathematical relationship between voltage, current, and resistance.





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