In 1827, physicist and mathematician Georg Simon Ohm formulated what we now call Ohm’s law. It is a fundamental relationship for ohmic resistors between voltage  V , current  I  and resistance  R  under the stated operating conditions. 

Because electronics is highly international, familiar quantities have many names. For example, voltage V is called “spænding” in Danish and “voltage” in English. Different formula symbols can also represent voltage in calculations. The table compares Danish and English terms and the symbols and letters used in calculations.

OHM’S LAW BASED ON TWO STATEMENTS

• For an ohmic resistor under stable operating conditions, R is independent of the applied voltage V.
• For the ohmic resistor, voltage V and current I have a linear relationship while operating conditions keep R constant.

Using Ohm’s law and P = V × I, two independent known quantities can determine the others under the stated assumptions.

OHM’S LAW WHEEL

To find two unknown quantities, you can use the Ohm’s law wheel. Many versions use different symbols or letters to represent the four quantities. They all do the same thing; choose whichever wheel you find easiest to use.

The wheel has four quadrants, each showing ways to calculate the central quantity. Let us look at a few examples.

The upper-left quadrant calculates power P, measured in watts (W), using three methods:

If voltage and current are known, power P can be calculated. Given a voltage V of 5 V and current I of 1.2 A, the dissipated power is 6 W, calculated with the following formula:

Many Ohm’s law calculators are available online:
Ohm’s Law Calculator

Some also help you learn and understand along the way:
https://phet.colorado.edu/sims/html/ohms-law/latest/ohms-law_da.html

VOLTAGE V

Voltage is the difference in electric potential energy per unit charge between two points: 1 V = 1 J/C. Pressure is only an analogy. In an ohmic circuit, voltage and resistance determine current through I = V/R.

V = volt. 1 mV = 0.001 V, and 1 µV = 0.000001 V.

CURRENT I (AMPERES, A)

Electric current I is charge per unit time. 1 ampere equals 1 coulomb per second, approximately 6.24 × 10¹⁸ elementary charges per second. It is not the speed of electron motion.

As noted in the table, symbols describe the quantity

A = ampere. 1 mA = 0.001 A, and 1 µA = 0.000001 A. The symbol for current is I; A is the unit.

RESISTANCE R

Resistance describes how difficult it is for current to pass through a conductor at a given voltage. Higher resistance allows less current. The image below shows a common resistor used in almost every kind of breadboard and stripboard project.

POWER P (WATTS, W)

Electrical energy is converted to heat in the resistor (Joule heating). The drawing is an illustration, not a physical description of current sweating. The dissipated power P is the energy dissipated per unit time, calculated as the product of current I and voltage drop U.

Always check the resistor’s rated power and temperature limits in its datasheet. Through-hole and SMD resistors come in many power ratings; 0.25 W is a common example, not a universal maximum. Allow an appropriate safety margin.

EXAMPLE

In this example, we are given a voltage V of 5 V and a resistance of 400 Ω. We can now use the Ohm’s law wheel or an online calculator to find the current and dissipated power. Let us start by calculating the current:

Since we now know three of the four circuit quantities, we can use any of the three power formulas. Here, we will use one that includes the current just calculated.

The calculated power is dissipated in the resistor: 0.0625 W in this example. A 0.25 W rated resistor may be suitable under its datasheet temperature and operating conditions, but 0.25 W is not a universal limit. Check the chosen component’s rated power and safety margin.

BUILD CIRCUITS ONLINE

Here, you can build small circuits and see the relationship between four quantities: voltage V, current I, resistance R and power P:

Circuit Construction Kit: DC (colorado.edu)

Check example: U = 5 V and R = 400 Ω give I = U/R = 0.0125 A = 12.5 mA and P = U × I = 0.0625 W. Here, U = R × I applies to an ohmic resistor under the stated operating conditions.