Ohm's Law and Watt's Law: Voltage, Current, Resistance, and Power

What is Ohm's Law and Watt's Law?:

Ohm's Law defines one of most fundamental relationships in electronics. That is the relationship between Voltage, Current, and Resistance. Watt's Law defines another of the most fundamental relationships in electronics. That is the relationship between power and the quantities defined by Ohm's law. We can not go very much farther into electronics until these concepts are understood.

 

Volts:

The unit that is used for measuring the parameter of voltage is the volt. The symbol that is used to represent the volt is the letter "V". Both upper case and lower case is used depending on the situation. The symbol for the parameter of voltage is also the letter "V".  If an electrical circuit were a garden hose, voltage would be similar to the pressure in the hose.  The unit V is equal to the amount of energy, in Joules, required to move one coulomb of electrons between two points. Voltage is sometimes referred to as "potential" because it has the potential to move those electrons.

 

Ampere or Amps:

The unit that is used for measuring the parameter of current is the ampere. Ampere is often shortened to amp. The symbol used to represent the amp is the letter "A". Both upper and lower case is used depending on the situation. The symbol used to represent the parameter of current is the letter "I". If an electric circuit were a garden hose, current would be similar to the flow rate of the water in the hose. The unit A is equal to the number of coulombs flowing through the circuit in one second.

 

Ohms:

The unit that is used for measuring the parameter of resistance is the ohm. The symbol used to represent the ohm is Ω. The symbol used to represent the parameter of resistance is the letter "R". If an electric circuit were a garden hose, resistance would be any valve or other restriction in the hose. The unit Ω is equal to the resistance that exists when 1 A is flowing between two points with 1 V of voltage between those two points.  This forms the basis for the forms of Ohm's law given in the next section.

 

The Forms of Ohm's Law:

This picture provides the three different forms of ohm's law for calculating voltage, current and resistance.

Power:

The unit that is most often used for measuring power in electronics is the watt. The symbol used to represent the watt is the capital letter "W". At its core, power is the rate at which work is done. In fact, one watt is equal to one joule per second.  From the definitions given for Volts and Amps given above, we can say that one watt is also equal to one volt multiplied by one amp because the volt is the measure of joules per coulomb and the amp is the measure of coulombs per second.  Coulombs cancel out and we are left with joules per second.

 

The Forms of Watt's Law:

This picture provides the different equations for finding power according to Watt's Law.

 

Combined Relationships of Ohm's Law and Watt's Law Wall Chart:

By combining Ohm's Law and Watt's Law we only need to know two of the quantities to determine the other two. These quantities being voltage (V) in volts, current (I) in amps, resistance (R) in ohms, and power (P) in watts. All of the relationships between these quantities are given in the chart below.

This chart provides all of the equations needed to calculate voltage, current, resistance and power when only two values are known.

 

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