Terminal Information

Basic Electronics: Ohm's Law

What is Ohm's Law?

Ohm's law describes how Voltage, Current, and Resistance all relate to one another in a circuit. It was discovered and later published in 1827 by German physicist Georg Ohm. It is the base on which many other principles of electronics stand. It states that the voltage is equal to the current times the resistance. But what does this mean? What are voltage, current, or resistance?

A Little Analogy

Often to describe how a circuit works and Ohm's law, the circuit is compared to a water hose. This analogy, while not perfect, has many parallels and does a pretty good job on helping us understand the basic idea of Ohm's Law and how to use it.

Current

In electronics, electrons are used as the power source. An electron is a negatively charged particle. Just like a magnet's negative pole is attracted to another magnet's positive pole, electrons are negatively charged and attracted to positive charges. Electrons in the negative side of a power source are attracted to the positive side of the source. The differently charged sides of the source are connected by a circuit allowing electrons to flow though the circuit from the negative side to the positive side.

This flow of electrons is called the Current. The current, in our hose analogy, is compared to the water that flows though the hose. When we refer to current in Ohm's law, we are describing the rate at which electrons are flowing though the circuit.

This flow rate of the current is measured in Amperes (usually referred to as Amps). One amp is roughly equivalent to 6.24 × 10¹⁸ electrons per second.

NOTE:

While physically current flows from the negative side of a power source to the positive side, convention in electrical engineering treats current as though it flows from the positive to negative. While using either method will work as long as consistency is used, we will be following convention and treating current as though it flows from positive to negative. This may seem confusing at first, but just do your best to keep it in the back of your mind and not focus too much on the doublethink.

Resistance

Resistance is the force that restricts or pushes against the current. You can think of this like squeezing or putting your thumb over the end of the hose. When you do this you can feel the water pushing against the resistance you've created.

The unit used to measure resistance is called the Ohm.

Voltage

Voltage is the force that pushes the current though the circuit. It can be compared to the water pressure in our hose that pushes the water though the hose. If there is no voltage, no current flows, just like with no pressure, no water flows.

In order to create a voltage in a circuit, there needs to be some resistance. In a hose without resistance, the pressure inside remains at just about the same pressure as the outside of the hose, and all the water quickly flows out the end. If you were to squeeze the hose you could feel how little pressure there was pushing back.

When you restrict the flow of water at the end of the hose, flow decreases, but pressure can begin to build up.

The unit used to measure voltage is the Volt.

Using Ohm's Law

Now that we have a better understanding of how voltage, current, and resistance relate, we can start to see how to use Ohm's law:

Ohm's Law

Using this equation you can calculate any one of the variables using the other two. Try out the voltage calculator below to get a better feel for how this relationship works:

Ohm's Law Calculator

Ohm's law shows how to calculate the voltage in a circuit. Using some simple algebra you can alter the equation to calculate either the resistance or the current:

Equation for Current:

Equation for Resistance:

Moving Forward

With a better understanding of Ohm's Law you can start to get the basic idea of how a circuit works. The continued practice of using it will help to solidify its principles. This website has tutorials for practice that are accessible to beginners and include instructions that requires no soldering. Check it out and continue learning!