iii.1. Introduction to Ohm’s Law

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iii.1. Introduction to Ohm’s Law

Ohm’s Law is a fundamental principle in electrical engineering and physics, named after the German physicist Georg Simon Ohm. Ohm’s Law is essential for understanding and calculating the relationships between voltage, current, and resistance in electrical circuits, providing a foundational framework for the analysis and design of electronic systems.

Ohm’s Law Simplified: Understanding Electrical Concepts and Resistance

About Ohm’s Law: In 1827, a German physicist Georg Simon Ohm (1787–1854) found out that the potential difference, V, across the ends of a given metallic wire in an electric circuit is directly proportional to the current I flowing through it, provided its temperature remains the same.
This is called Ohm’s law and can understood as,

V ∝ I

Or, V/I = constant = R (Resistance)

Or, V = IR

Resistance (R): It is the property of a conductor to resist the flow of charges through it.
R is a constant for the given metallic wire at a given temperature.
Its SI unit is ohm, represented by the Greek letter Ω.
Thus, the current through a resistor is inversely proportional to its resistance.
If the resistance is doubled the current gets halved.
According to Ohm’s law

R = V/I

One ohm: According to Ohm’s Law, If the potential difference across the two ends of a conductor is 1 V and the current through it is 1 A, then the resistance R, of the conductor is 1 Ω.

1 ohm = 1 volt/ 1 ampere

Or, I = V/R

Variable resistance: A component used to regulate current without changing the voltage source is called variable resistance.
- In an electric circuit, a device called rheostat is often used to change the resistance in the circuit.
Resistor: The electrons are not completely free to move within a conductor.
- They are restrained by the attraction of the atoms among which they move.
- Thus, motion of electrons through a conductor is retarded by its resistance. A conductor having some appreciable resistance is called a resistor.

Factors Influencing Resistance in Electrical Conductors

On applying Ohm’s law, it is clear that the resistance of the conductor depends on

its length,
its area of cross-section, and
the nature of its material.

Resistance of a uniform metallic conductor is directly proportional to its length (l) and inversely proportional to the area of cross-section (A).

R ∝ l and

R ∝ 1/A

Combining both we get

R ∝ l/A

Or, R = ρ lA (ρ is electrical resistivity)

Electrical resistivity (ρ): ρ (rho) is a constant of proportionality and is called the electrical resistivity of the material of the conductor.
- The SI unit is Ω m.
It is a characteristic property of the material.
- The metals and some alloys have very low resistivity in the range of 10–8 Ω m to 10–6 Ω m.
- Conductor: They are good conductors of electricity.
- Resistivity: The resistivity of an alloy is generally higher than that of its constituent metals.
  - Alloys do not oxidize (burn) readily at high temperatures.
  - For this reason, they are commonly used in electrical heating devices, like electric iron, toasters etc.
  - Tungsten is used almost exclusively for filaments of electric bulbs, whereas copper and aluminum are generally used for electrical transmission lines.
  - Insulators like rubber and glass have resistivity of the order of 1012 to 1017 Ω m.
Both the resistance and resistivity of a material vary with temperature.

Electrical resistivity of some substances at 20°C

POINTS TO PONDER

Resistance to the electric current is one of the major reasons for the transmission losses in the national power grids. Also this resistance causes the heating of devices and reduced efficiency in computing. Have you heard of Superconductors? Can you look for the status of research on superconductors?