Georg Ohm and Ohm's Law

Georg Simon Ohm

Georg Simon Ohm (1789 - 1854) first investigated the relationship between the current in an electric circuit element and the electrical potential difference, often called voltage, across the circuit element. Some sources list his birth year as 1787.

Ohm was born in Erlanger, Bavaria, now part of Germany. Though not formally educated himself, Ohm's father, a locksmith, encouraged his son to get the best possible education.

While working as a teacher Georg Ohm investigated electric currents and discovered the law now bearing his name. He published his work in an 1827 book, Die galvanische Kette, mathematisch bearbeitet (The Galvanic Circuit Investigated Mathematically), which was poorly regarded at the time. Modern physicists regard Ohm's work as important to our understanding of electric circuits.

Ohm's Law

For a circuit or circuit element, Ohm's law mathematically relates the current, the voltage, and the electrical resistance. The voltage is more properly called the electrical potential difference, but voltage is often used because it is less of a mouthful.

Ohm found that in a circuit the voltage and current are directly proportional to each other. That means if the current, I, in a circuit is plotted along the horizontal, or x, axis and the voltage, V, is plotted along the vertical, or y, axis, then the result will be a straight line. The slope of this line is the resistance, R, of the circuit.

The mathematical equation for Ohm's law is:

The voltage or electrical potential difference, V, is measured in volts. The electrical current, I, is measured in amperes, commonly called amps. The resistance, R, is measured in ohms, which are volts per amp.Ohm's Law is Not a Fundamental Law

Many laws of physics, such as Conservation of Energy, are fundamental laws that always apply without exception. Ohm's law is not one of these laws. It is an empirical law, found by experiment, that works pretty well most of the time. There are times however where Ohm's law does not work.

One example is an incandescent light bulb. The tungsten filament in the bulb does not follow Ohm's law. As the voltage in the wire filament increases it heats up. The resistance of a wire changes as its temperature changes. Hence the graph of the current and voltage in the wire will curve. The light bulb filament violates Ohm's law. Often if extreme currents are applied to wires, they heat up, change their resistances, and violate Ohm's law.

Ohm's Law and Short Circuits

When a short circuit occurs in an electrical appliance, most of the circuit for the appliance is bypassed. Hence the resistance becomes very low. The appliance may have a high electrical resistance, but the wire leading to the appliance does not.

By Ohm's law, the very low resistance in a short circuit causes a very high current. This high current blows the circuit breaker or fuse. If circuits did not have fuses or circuit breakers, high currents in the circuits could heat the wires to the point of starting a fire. Fuses and circuit breakers are therefore protective devices.

Ohms law is a useful relationship between the voltage, current, and resistance in an electrical circuit.

Further Reading

Hecht, E., Physics:Algebra/Trig 2nd ed., Brooks/Cole, 1998.

Wilson, J.D, Buffa, A.J., & Lou, B. College Physics 6th ed., Pearson, 2007.

Ohm, G.S. Die galvanische Kette, mathematisch bearbeitet (The Galvanic Circuit Investigated Mathematically), 1827, in German.