Electricity plays a vital role in our daily lives, powering devices, industries, and communication systems. At the heart of electrical science lies Ohm’s Law, a fundamental principle that relates the current flowing in a conductor to the potential difference across it. The concept of “resistance” arises directly from this law.
Statement of Ohm’s Law
Ohm’s Law states that:
“At constant temperature and physical conditions, the current (I) flowing through a conductor is directly proportional to the potential difference (V) applied across its ends.”
Mathematically, V α I → V = IR
Here:
* V = potential difference across the conductor (in volts)
* I = current flowing through the conductor (in amperes)
* R = resistance of the conductor (in ohms, Ω)
Thus, resistance is the constant of proportionality between voltage and current.
Concept of Resistance
Resistance is the property of a material or conductor to oppose the flow of electric current. Whenever charges flow through a conductor, they collide with the atoms or ions of the material, losing energy in the form of heat. This obstruction to charge flow is called resistance.
The SI unit of resistance is ohm (Ω). One ohm is defined as the resistance of a conductor when a potential difference of 1 volt produces a current of 1 ampere through it.
1 Ω = 1 volt / 1 ampere
Factors Affecting Resistance
The resistance of a conductor depends on several factors:
1.Length of the conductor (L): Resistance increases with length. R α L
2.Cross-sectional area (A): Resistance decreases with an increase in area.
R α 1 / A
3.Nature of material: Materials with high free electron density (like copper and silver) offer less resistance, while insulators like rubber or glass have very high resistance.
4. Temperature: For most metals, resistance increases with temperature due to increased vibrations of atoms. For semiconductors, resistance decreases with rise in temperature.
Combining the first two factors, resistance can be expressed as:
R = ρ L / A
Where ρ (rho) is the resistivity of the material.
V-I Characteristics
The relation between current and potential difference in ohmic conductors (like metals at constant temperature) is a straight line passing through the origin, showing proportionality between
V and I.
* Ohmic conductors: Follow Ohm’s law (linear graph).
* Non-ohmic conductors: Do not obey Ohm’s law (non-linear graph), e.g., diodes, filament lamps, thermistors.
Significance of Ohm’s Law
1.Foundation of Circuit Theory: Ohm’s Law forms the basis for analysing simple and complex electric circuits.
2.Design of electrical appliances: Resistance values are chosen carefully to control current in devices like heaters, lamps, and resistors.
3,Measurement of unknown resistance: Using voltmeters and ammeters, Ohm’s Law helps calculate the resistance of a conductor.
4. Safety in circuits: It allows engineers to design circuits that prevent overheating and damage by controlling current flow.
Limitations of Ohm’s Law
Although highly useful, Ohm’s Law is not universal. It holds true only under specific conditions:
1.It is valid only at constant temperature and physical conditions.
2, It does not apply to non-linear devices like vacuum tubes, semiconductors, or electrolytes.
3. For materials whose resistance changes with applied voltage (like filament bulbs), Ohm’s Law is not strictly obeyed.
Practical Applications
* Resistors in circuits: Used to limit current and divide voltage.
* Electric heaters and toasters: High resistance wires like nichrome are used to convert electrical energy into heat.
* Light bulbs: Filament resistance provides both light and heat.
* Measuring instruments: Ohmmeters are based on the principle of Ohm’s Law.
Conclusion
Ohm’s Law and the concept of resistance are keystones of electrical science. The law provides a clear mathematical relation between current, voltage, and resistance, enabling us to design, analyses and use electrical circuits effectively.
Resistance, as a property of materials, explains why different substances conduct electricity differently and how we can utilise this property for practical applications like heating devices, resistors, and circuit protection.
