Ohm's Law

Ohm's Law

Ohm's Law is a fundamental and basic law of electronics which describes the relationship between voltage, current and resistance.

Before we jump into the Law, let's pay our respects to Georg Ohm.

Who was Georg Ohm?

Georg Simon Ohm

Georg Simon Ohm was born on 16 March 1789 in Erlangen Germany, he was a physicist and mathematician.

His parents were not formally educated, however, Ohm's father was a respected man who had educated himself to a high level and was able to give his sons an excellent education himself, good man. Georg did attend Erlangen Gymnasium from age 11 through to 15, however let's just say the education he received was rather basic in comparison to what his Dad was teaching him.

At the age of fifteen, in 1805, Georg Ohm began studying at Erlangen University, however, he left after three semesters after being too preoccupied with leisurely activities such as dancing, ice skating and playing billiards. His studies took the back seat.

Ohm's father noticed this and sent his son over to Switzerland for further education the following year where he accepted a position as a mathematics teacher in a school over in Gottstadt. Outside teaching Ohm also continued pursuing his private study of mathematics.

Ohm returned to school teaching in 1813, teaching mathematics and physics and writing a geometry textbook at a rather poor quality school in Bamberg for three years. In 1817 he accepted a position at a Jesuit gymnasium in Cologne in 1817 and it is here where he conducted physics experiments in the high quality laboratory. While here, he developed his incredibly important theory between the relationship of resistance, electric current and voltage. Yep, this is where the magic happened!!

In 1825, Ohm published his first paper which examined the decrease in the electromagnetic force produced by a wire as the length of the wire increased. It was not well received. Ohm published two more papers in 1826, and his famous book containing Ohm’s Law, in 1827 in which he detailed his complete theory of electricity.

In 1841, he received the Copley Medal of the Royal Society of London. In its notice of the award, the society praised Ohm's work.

The light which these investigations has thrown on the theory of current electricity is very considerable....Had the works of Ohm been earlier known, and their value recognised, the industry of experimentalists would have been better rewarded.

In 1852 was appointed to the chair of physics at the University of Munich, a position he held until his death.

So as you can see he was a remarkable person that came from VERY humble beginnings that is well worth studying even just for inspiration alone!

Statue of Ohm at the Technical University of Munich

Ohm's Law states that the current flowing through a resistor is proportional to the voltage across it and inversely proportional to the resistance.

This makes sense intuitively as a lower resistance means that it is easier for charges to flow through the material and a higher voltage means that the charges have more energy which increases the drift velocity and more tightly bound electrons start conducting.

The formula is given in all three forms below:

From the formula for resistance we can see that the definition of 1Ω of resistance is a material which permits 1A of current flow when a voltage of 1V is applied across it.

As opposed to memorizing one or all of the above formulas the triangle below can be memorized and used to derive them:

From this we can see that V = IR if we cover the V, I = V/R if we cover the I and R = V/I if we cover the R.

A resistor is an electrical component which resists the flow of current and can be represented by either of the symbols shown below:

Example:

Q. What is the voltage drop V1 across the resistor shown below?

Questions:

1. What is the current flowing through a 10kΩ resistor with a voltage drop of 1.5V across it?

Answer

0.15mA

2. A resistor has a voltage of 5V on the positive side and 2.7V on the negative side with 2.5mA of current flowing through it. What is its resistance?

Answer

920Ω
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