Terms and Definitions

1 Ohm’s Law and Watt’s Law

This section provides a brief description of two of the most fundamental electrical relationships: Ohm's law, which describes current flow, and Watt's law, which describes how power is dissipated.

Click play on the following audio player to listen along as you read this section.

Ohm’s Law

Combining the elements of voltage, current, and resistance, George Ohm developed the following formula:



  • E = Voltage in volts
  • I = Current in amps
  • R = Resistance in ohms

This is called Ohm’s law.

Let’s say, for example, that we have a circuit with the potential of 1 volt, a current of 1 amp, and resistance of 1 ohm. Using Ohm’s law we can say:

[latex]1A=\frac{1V}{1\text{ ohm}}[/latex]

Let’s say this represents a tank with a wide hose. The amount of water in the tank is defined as 1 volt, and the “narrowness” (resistance to flow) of the hose is defined as 1 ohm. Using Ohm’s law, this gives us a flow (current) of 1 amp.

Using this analogy, let’s now look at the tank with the narrow hose. Because the hose is narrower, its resistance to flow is higher. Let’s define this resistance as 2 ohms. The amount of water in the tank is the same as the other tank, so, using Ohm’s law, our equation for the tank with the narrow hose is:

[latex]?=\frac{1V}{2\text{ ohms}}[/latex]

But what is the current? Because the resistance is greater and the voltage is the same, this gives us a current value of 0.5 amps:

[latex]0.5A=\frac{1V}{2\text{ ohms}}[/latex]

Watt’s Law

Electric power is the rate at which energy is transferred. It’s measured in terms of joules per second (J/s). One joule of work done every second means that power is dissipated at a rate equal to one watt (W).

Given the few basic electricity terms we know, how could we calculate power in a circuit?

Well, we have a standard measurement involving electromotive force, also know as volts (E).

Current, another of our favourite electricity terms, measures charge flow over time in terms of the ampere (A), which equals 1 coulomb per second (C/s). Put the two together, and what do we get? Power!

To calculate the power of any particular component in a circuit, multiply the voltage drop across it by the current running through it.

For instance, if current flows at a rate of 10 amps while voltage is 10 volts, then the circuit dissipates power at a rate of 100W.



Icon for the Creative Commons Attribution 4.0 International License

Basic Motor Control Copyright © 2020 by Aaron Lee and Chad Flinn is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

Share This Book