Resistors, Resistance and Ohm's Law

The electrical resistance is the tendency to resist the flow of electric current. A wire having a desired resistance for use in an electric circuit is called a resistor.

Resistance can be both either desirable or undesirable in a conductor or circuit. In a conductor, to transmit electricity from one place to another place, the resistance is undesirable. Resistance in a conductor causes part of electrical energy to turn into heat, so some electrical energy is lost along the path.

On the other hand, it is the resistance which allows to use electricity for light and heat. For example, light that we receive from electric bulb and heat generated through electric heaters.

Ohm's Law

The current flowing through a wire is directly proportional to the potential difference applied across its ends.

V ∝ i

V = Ri

Here, R is a constant of proportionality and is called the resistance of the given metallic wire. This observation was first made by Georg Simon Ohm and is known as Ohm's Law.

Ohm's Law states that the current flowing through a conductor is directly proportional to the potential difference applied across the ends of the conductor provided temperature of the conductor remains the same.

The law can be applied only to conducting wires and that too when its temperature and other physical conditions remain unchanged. If the temperature of the conductor increases its resistance also increases.

Resistance of Wire

The R, resistance of wire, is a constant for a given wire. The resistance of a wire depends on:

  • Its length - longer the wire, more the resistance
  • Its thickness - thicker the wire, lesser the resistance
  • Its width - more the width, lesser the resistance

Therefore, the resistance of the wire is directly proportional to the length and inversely proportional to the cross-sectional area.

The nature of material: Copper wire has lesser resistance than iron wire of same length and thickness. The resistance of a wire can never be negative.

Resistance is a scalar quantity and its SI unit is ohm denoted by the symbol Ω (omega). 1 ohm is the resistance of a wire across which when 1 V potential difference is applied, 1 A current flows through the wire.

1 ohm = 1 volt / 1 ampere

High resistances are measured in kilo ohm (kΩ) and mega ohm (MΩ)

1 kΩ = 103 Ω

1 MΩ = 106 Ω