ICSE Class 9 Answered

Why do we use ohm's law in a parallel circuit and in a series circuit? Why do we need Ohm's law? How did the Scientist Ohm has proved his own law?

Asked by Sanjay | 09 Apr, 2017, 07:22: AM

Expert Answer

How did the Scientist Ohm has proved his own law:

Before Georg Ohm, Henry Cavendish experimented with jars and glass tubes of varying diameter and length filled with salt solution.
Cavendish wrote that the "velocity" (current) varied directly as the "degree of electrification" (voltage).
He did not communicate his results to other scientists at the time, and his results were unknown.
Ohm did his work on resistance in the years 1825 and 1826, and published his results in 1827 as the book "The galvanic circuit investigated mathematically".
For experiments, he initially used voltaic piles, but later used a thermocouple as this provided a more stable voltage source in terms of internal resistance and constant voltage. He used a galvanometer to measure current and knew that the voltage between the thermocouple terminals was proportional to the junction temperature.
He then added test wires of varying length, diameter, and material to complete the circuit.
This is how he came to the result which is now known as Ohm's Law.

Ohm's law in Georg Ohm's lab book

Need for Ohm's Law:

Ohm's Law is extremely significant in understanding and determining the ratings of the electrical circuit components for the desired connection.
It explains us the relationship between the voltage and current with change in resistance, temperature, length and cross-sectional area of the conductor.
Domestic and special purpose industrial wirings can be done efficiently once the interrelationships between these physical quantities are estimated accurately with the help of Ohm's Law.
All the circuits in the world are designed with respect to the relationships set by Ohm's Law.

Why do we use ohm's law in a parallel circuit and in a series circuit:

Parallel circuits and series circuits are the arrangements of connecting resistances either continuously end to end like in series connection or connecting resistances by their same ends like it is in parallel connection.
Ohm's Law helps in determining the variation in the current and voltage across the circuit in these arrangements of the resistance.
In the series connection, the resultant resistance is the algebraic sum of the individual resistance.
Whereas, in parallel connection, the resultant resistance is the sum of reciprocals of the individual resistances.
That is the reason if the desired output is to have minimum resistance and maximum flow of current, the connections are done in parallel.
And, if the desired output is to have maximum resistance and thus minimum flow of current, the connections are done in series.