What is Ohm’s Law?

Ohm’s law is electrical engineering’s bread and butter equation created by Georg Simon Ohm. It relates what we know as voltage (volts), current (amps), and resistance (ohms or Ω). The equation is written as follows:

V = I x R

Where…

V = Voltage measured in Volts (V)

I = Current measured in Amperes or Amps (A)

R = Resistance measured in Ohms (Ω)

The following image is a good representation of what Ohm’s law physically means. Often times people understand it best by applying Ohm’s law to real world applications.

Resistance restricts electrical current.

Resistance opposes the movement of free electrons (current) in electrical systems.

Resistance is measured in ohms. This is the symbol Ω.

The electrical symbol for a resistor is as follows:

Let’s start thinking about resistance in terms of Ohm’s Law. Ohm’s law is the bread and butter equation of electrical engineering.

The resistances in Ohms restricts the flow of free electrons (current measured in amps). Voltage is the electro magnetic force that pushes the electrons or flow of current along its path. I.e. the pipe

Let’s take a look at water flow to understand Ohm’s law.

Take it from these three photos.

(LEFT) Pressure Washer

(MIDDLE) Water out of Pipe

(RIGHT) Hoover Dam

Electricity is similar to the flow of water in “pipes.” Pipes being any mode of transportation of water. A pressure washer is a long narrow pipe that allows water to flow. Same with the pipes of the Hoover Dam. The pipes are much larger in diameter, therefore much more water can pass through.

(LEFT) When thinking about the pressure washer, the narrowness of pressure washer gun is thought of as a large resistance. You would think that a large resistance would mean a smaller current. This is accurate, but because the device creates a large amount of pressure, or “voltage” behind the water supply, the water exits very fast. You can think of it this way, when the pressure washer is off, the voltage is very low, little water exits the nozzle. When you turn on the pressure washer, the voltage rises and so does the current for the same resistance (which the pressure washer gun is not varying in resistance — it is the same pressure washer gun!).

(MIDDLE) Water exiting a pipe. A lot of water is able to flow from the pipe due to its larger diameter compared to that of the pressure washer. However, the “voltage” or pushing force in the pipe is non-existent. The water seems to be flowing by the force of gravity alone, there is no compressor creating a large pressure or “voltage” behind the pipe. Therefore, the water trickles out, but at a large rate because the pipe is larger in diameter which means the resistance is much smaller than that of the pressure washer.

(RIGHT) Finally, the Hoover Dam releasing water is as if you combine the characteristics of the pressure washer and the pipe. The dam has very large pipes meaning lots of water can pass through. The “resistance” of these pipes is super small, due to the large diameter. The force behind the water is the compounding effect of gravity feeding the water from the top of the dam to the bottom. There is an immense pressure or “voltage” behind the water allowing it to flow very fast out of the pipe signaling a very large current.

A large voltage on top of a small resistance, means you have a large current. Take it from Ohm’s Law:

I = V / R = a large (#) divided by a (small #) = large current

What happens when the Hoover Dam closes the pipes? The resistance increases to infinity or a very large number.

I = V / R = a large (#) divided by a (very very very very large #) = no current

We can tell there is no current, because there is no water exiting the dam. We have increased the resistance so much to the point where no water will flow, and therefore no current flows.