Ohm’s Law
Ohm’s Law is one of the most fundamental laws of electrical engineering. It explains the relationship between voltage, current, and resistance in an electrical circuit. Almost every topic in Basic Electrical Engineering, such as Kirchhoff’s Laws, network theorems, and circuit analysis, is built on this simple but powerful law.
This tutorial is designed for BEE, Diploma, Engineering students, and JEE aspirants. The explanation is kept simple and conceptual, with emphasis on clear understanding and exam application.
What Is Ohm’s Law?
📌Statement: Ohm’s Law states that at constant temperature, the current flowing through a conductor is directly proportional to the voltage applied across it.
This means that if the voltage across a conductor increases, the current through it also increases, provided the physical conditions of the conductor remain unchanged.
Mathematical Form of Ohm’s Law
Ohm’s Law is mathematically expressed as:
\[ V = I R \]
where \( V \) is the voltage across the conductor in volts, \( I \) is the current flowing through the conductor in amperes, and \( R \) is the resistance of the conductor in ohms.
The above equation can also be written as:
\[ I = \frac{V}{R} \] \[ R = \frac{V}{I} \]
Physical Meaning of Ohm’s Law
The physical meaning of Ohm’s Law lies in the behavior of charge carriers inside a conductor. When a voltage is applied, an electric field is created, which causes electrons to move through the conductor.
Resistance opposes the motion of electrons. A higher voltage produces a stronger electric field and hence a larger current, while a higher resistance reduces the flow of current.
Resistance and Its Role
Resistance is the property of a material that opposes the flow of electric current. It depends on the length of the conductor, its cross-sectional area, the nature of the material, and temperature.
Metals such as copper and aluminum have low resistance, whereas materials like rubber and glass have very high resistance.
When a voltage is applied across a conductor:
- Free electrons start drifting under the applied electric field.
- Inside the material, electrons collide with atoms, ions, and lattice vibrations.
- These collisions slow down electron motion and convert electrical energy into heat energy.
This opposition to electron flow due to collisions is called electrical resistance.
Energy Point of View
Resistance indicates the ability of a material to:
- Dissipate electrical energy as heat
- Limit current to protect circuits and devices
This is why resistors get warm when current flows through them.
Factors Affecting Resistance
Resistance depends on:
- Length (L) of conductor → longer wire = more resistance
- Area (A) of cross-section → thicker wire = less resistance
- Material (resistivity ρ)
- Temperature (for metals, resistance increases with temperature)
Mathematically:
\[ R = \rho \frac{L}{A} \]
Conditions for Validity of Ohm’s Law
Ohm’s Law is valid only when certain conditions are satisfied. These include constant temperature, unchanged physical dimensions of the conductor, and operation within the ohmic region of the material.
If these conditions are not maintained, the relationship between voltage and current may no longer remain linear.
Limitations of Ohm’s Law
Ohm’s Law does not apply to all electrical devices. It is not valid for semiconductor devices such as diodes and transistors, non-linear elements like thermistors, gas discharge tubes, and electric arcs.
In such cases, the voltage–current relationship is non-linear.
Graphical Representation of Ohm’s Law
When voltage is plotted on the vertical axis and current on the horizontal axis for an ohmic conductor, the graph is a straight line passing through the origin.
The slope of the voltage-current graph represents resistance. A steeper slope indicates higher resistance, while a smaller slope indicates lower resistance.
Simple Numerical Example
A resistor of resistance 2 Ω is connected across a 20 V battery. Find the current flowing through the circuit.
Using Ohm’s Law:
\[ I = \frac{V}{R} = \frac{20}{2} = 10~\text{A} \]
Therefore, the current flowing through the circuit is 10 A.
Practical Applications of Ohm’s Law
Ohm’s Law is widely used in electrical engineering and daily life. It is applied in designing electrical circuits, selecting appropriate resistors, calculating current ratings of devices, understanding power consumption, and troubleshooting electrical faults.
Importance of Ohm’s Law in Circuit Analysis
Ohm’s Law serves as the starting point for understanding advanced topics such as Kirchhoff’s Current Law, Kirchhoff’s Voltage Law, nodal analysis, mesh analysis, and network theorems like Superposition and Thevenin.
A strong understanding of Ohm’s Law makes these advanced topics easier to learn and apply.
Common Mistakes Students Make
Common mistakes include using incorrect units, forgetting to convert milliampere to ampere, applying Ohm’s Law to non-ohmic devices, and confusing series and parallel resistance values.
Exam-Oriented Tips
Always write the formula before substitution, mention units clearly, verify whether the device obeys Ohm’s Law, and apply Ohm’s Law before using Kirchhoff’s Laws whenever possible.
Related Topics
Now that you have studied Ohm’s Law, you should study Kirchhoff’s Laws to understand circuit analysis better.
Ohm’s Law is one of the most important and basic laws in electrical engineering. It establishes a clear relationship between voltage, current, and resistance and forms the foundation of circuit analysis. Mastering Ohm’s Law is essential before moving on to advanced topics such as Kirchhoff’s Laws and network theorems.
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