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: Ohm's law, which describes current flow in electrical circuits, and Watt's law, which describes how power is dissipated.
Ohm’s Law
Combining the elements of voltage, current, and resistance, George Ohm developed the following formula:
[latex]\text{I}=\dfrac{\text{E}}{\text{R}}[/latex]
Where:
- 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 voltage, current, and power, named after James Watt, Watt’s Law is defined as the following formula:
[latex]\text{P}=\text{E} * \text{I}[/latex]
Where:
- P = Power in watts
- E = Voltage in volts
- I = Current in amps
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 voltage (E).
Current, another of our favourite electrical 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 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]
Text Attributions
- This chapter was adapted from “Ohm’s Law and Watt’s Law” in Basic Motor Control by Chad Flinn and Aaron Lee, which is under a CC BY 4.0 Licence. Adapted by Aaron Lee.
Current = Voltage divided by Resistance (or I=E/R).
Power = Voltage times Current (or W=EI)
The difference in electric potential between two points, which is defined as the work needed per unit of charge to move a test charge between the two points. It is measured in volts (V).
The rate of flow of an electric charge, measured in amperes (or amps). When one coulomb of charge moves past one point in once second, current is said to flow at a rate of one ampere. Current flows from negative potential to a positive potential through a load.
The opposition to the flow of current in an electric circuit, measured in ohms (Ω).
The rate at which work is done. It is measured in watts (W), or joules per second (J/s).
The unit used to measure power in an electric circuit, equivalent to one joule per second, or the power dissipated when one volt pushes one amp through a circuit.
The unit used to measure electrical current. It is equal to a flow of one coulomb per second. It may also be called "amp."