Ohm's Law is derived from the device that measures electrical resistance. The device measures electrical resistance in ohms. 

Ohm's Law states: Resistance (R) = Voltage (V) / current (I)

The excepted electrical resistance is calculated by dividing the given voltage value by the given current value.

Resistance is represented by the symbol "R" and the unit "ohm". Shorthand is Ω. Each cable and each wire has a certain resistance value. Ohm's law allow understanding the electrical resistance of each conductor through which a current flows. 

Voltage on the other hand is represented by the symbol "U" and the unit "Volt". The shorthand notation "V" describes a different voltage charge potential between two poles. An outlet located between two contacts usually has a voltage of 120 V.

Current is represented by the symbol "I" and has the unit "ampere." Shorthand is "A". The current charge flows through electrical conductors such as a cable.

A brief sample calculation shows:

U = 120 V
I = 2 A

When using R = V / I, it follows that:

R = 120 V / 2 A = 60 Ω.

According to mathematical rules the formula can also be changed over to either 
Voltage (U = R * I) or 
Current (I = V / R). 

This can be extended to specify the concept of power (P) in watts, to describe the power output generated by applying voltage to a load of flowing current stream. The benefit arises only if a closed circuit exists to consume the current.

Power (P) = Voltage (V) * Current (I)

A sample calculation shows:

U = 120 V
I = 2 A

From this:

=> P = 120 V * 2 A = 240 W

A load connected to an American outlet (120 V), and through which a current of 2 A flows consumes 240 W.

This formula can be modified, for example, to calculate the current that flows through a 100 W bulb:

I = P / U

=> I = 100 W / 120 V = 0.8333 A

This corresponds to 833.33 mA (milliamps).