Understanding Ohm's Law
Ohm's Law is one of the most fundamental principles in electrical engineering, describing the relationship between voltage, current, and resistance in an electrical circuit. This guide explains this relationship and how to apply it to solve practical electrical problems.
The Fundamental Principle
Discovered by German physicist Georg Ohm in 1827, Ohm's Law states that the current flowing through a conductor between two points is directly proportional to the voltage across the two points, and inversely proportional to the resistance between them.
Ohm's Law Formula
Where V is voltage in volts (V), I is current in amperes (A), and R is resistance in ohms (Ω)
Voltage (V)
The "pressure" that pushes electrons through a circuit, measured in volts (V)
Current (I)
The flow of electric charge, measured in amperes (A)
Resistance (R)
Opposition to current flow, measured in ohms (Ω)
The Water Analogy
Ohm's Law is often explained using a water analogy:
- •
Voltage
Like water pressure in a pipe - higher pressure pushes water faster
- •
Current
Like water flow rate - the amount of water passing through the pipe
- •
Resistance
Like the pipe's diameter - a narrower pipe restricts water flow
Practical Applications
Ohm's Law is used extensively in electrical engineering:
- •Designing circuits with specific voltage or current requirements
- •Calculating power consumption (P = V × I = I² × R = V²/R)
- •Sizing components like resistors, fuses, and wires
- •Troubleshooting electrical problems and malfunctions
Practical Examples and Applications
Let's explore how Ohm's Law is applied in real-world situations with practical examples.
Example 1: LED Circuit Design
Problem:
You want to connect a 2V LED to a 9V battery. LEDs need a specific current (typically 20mA) to operate safely. What resistor value should you use?
Solution:
The voltage across the resistor will be:
VR = Vbattery - VLED = 9V - 2V = 7V
Using Ohm's Law to find the resistance:
R = VR ÷ I = 7V ÷ 0.02A = 350Ω
You would choose a standard 330Ω or 360Ω resistor for this application.
Example 2: Power Calculation
Problem:
A 1500W electric heater runs on a 120V power supply. How much current does it draw, and what is its resistance?
Solution:
Using the power formula:
P = V × I
I = P ÷ V = 1500W ÷ 120V = 12.5A
Now using Ohm's Law to find resistance:
R = V ÷ I = 120V ÷ 12.5A = 9.6Ω
Limitations of Ohm's Law
While incredibly useful, Ohm's Law doesn't apply universally to all electrical components:
Non-Ohmic Devices
Certain components don't follow Ohm's Law:
• Diodes and LEDs
• Transistors
• Thermistors
• Vacuum tubes
These have non-linear V-I relationships that change with conditions like temperature or voltage level.
Complex Circuits
For circuits with multiple paths or components:
• Use Kirchhoff's Laws for complete analysis
• Calculate equivalent resistances for series and parallel combinations
• Consider time-varying signals in AC circuits
Ohm's Law is still valid for each individual resistive element, even in complex circuits.
Series Circuits
In series circuits:
• Same current flows through each component
• Voltage is divided across components
• Total resistance is the sum of individual resistances
Parallel Circuits
In parallel circuits:
• Same voltage across each component
• Current is divided among components
• Total resistance is less than the smallest individual resistance
Power Relationships
Power in electrical circuits:
• Measured in watts (W)
• Represents energy conversion rate
• Key for component ratings and heat generation