Ohm’s Law

When the temperature is constant then the potential difference between two terminals is directly proportional to the current flowing through the circuit.

$$ V \propto I\\V = IR\text{ [R is the constant of proportionality ]}\\R=\frac{V}{I}

$$

R is Resistance along with the constant of proportionality.

Ohm’s Law does not always apply.$^1$

Ohmic Resistors

These are conductors that follow Ohm’s Law. Metals are usually ohmic conductors.$^1$

Non-Ohmic Conductors

These are conductors that do not follow Ohm’s Law. Semi-conductors are a good example.$^1$

What is Resistance?

It is the property of a material that opposes the flow of charge or electric current.$^1$

Its SI unit is ohms($\Omega$)

Factors that Affect Resistance

The following are the factors that affect resistance:

• Nature of the material
• Temperature:
  • For conductors, resistance increases with increasing temperature.
  • For insulators, resistance increases with increasing temperature.
  • For alloys, resistance is least dependent on temperature.
• Resistance is directly proportional to the length.
• Resistance is inversely proportional to the area of the cross section. $R \propto \frac{l}A\\R =\frac{l\rho}{A}$

Specific Resistance or Resistivity

Specific resistance($\rho$) or resistivity is a property of a material that only depends on the nature of the material but not on size and shape.