Electrical Terms and Definitions

1 Ohm’s Law and Watt’s Law

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This section provides a brief description of two of the most fundamental electrical relationships: , which describes current flow in electrical circuits, and , which describes how power is dissipated.

Ohm’s Law

Combining the elements of , , and , 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]1\text{ A}=\dfrac{1\text{ V}}{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 a tank with a 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]?=\dfrac{1\text{ V}}{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]0.5\text{ A}=\dfrac{1\text{ V}}{2\text{ ohms}}[/latex]

Watt’s Law

Combining the elements of , , and , named after James Watt, Watt’s Law is defined as the following formula:

[latex]\text{P}=\text{E} * \text{I}[/latex]


  • P = Power in watts
  • E = Voltage in volts
  • I = Current in amps

Electric 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 .

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 .

Current, another of our favourite electrical terms, measures charge flow over time in terms of the , 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 the available voltage is 10 volts, then the circuit dissipates power at a rate of 100W.

[latex]100\text{ W}=10\text{ V} * 10\text{ A}[/latex]

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Basic HVAC by Aaron Lee is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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