Ohm's Law and Electrical Resistance
Simple Explanation
Ohm's law states that for many conductors, the current flowing through them is directly proportional to the potential difference (voltage) applied across them, as long as temperature stays constant. Resistance measures how strongly a component opposes the flow of current.
Why Do We Need It?
Ohm's law is one of the most fundamental and widely used relationships in all of electricity, essential for analysing and designing virtually every circuit.
Formula
Ohm's Law
V = IR
For many conductors (at constant temperature), the potential difference across them is directly proportional to the current flowing through them, with resistance as the constant of proportionality.
- V
- β Potential difference (voltage) across the conductor, in volts
- I
- β Current flowing through the conductor, in amperes
- R
- β Resistance of the conductor, in ohms (Ξ©)
When to use it: Use to find any one of voltage, current, or resistance when the other two are known.
Worked Example
Finding current using Ohm's law
A 12 V battery is connected across a resistor of 4 Ξ©. Find the current flowing.
Why Does This Work?
In an ohmic conductor, increasing voltage pushes charge carriers through the material with proportionally more force, driving proportionally more current β as long as the material's resistance itself doesn't change (e.g. due to heating), this produces a directly proportional, linear relationship between V and I.
Real-Life Example
Choosing a resistor for an LED
A small resistor is often placed in series with an LED to limit current and prevent it from burning out.
Using Ohm's law, an engineer can calculate exactly which resistance value limits the current to a safe level for a given supply voltage, protecting the LED from excessive current.
Practice
A current of 0.5 A flows through a resistor with a potential difference of 6 V across it. Find the resistance.
MediumCommon mistake
Assuming Ohm's law applies to every electrical component β some components (like diodes and filament bulbs at varying temperature) are non-ohmic, meaning their V-I relationship is not a simple straight line.
Quick Review
- V = IR (Ohm's law) for ohmic conductors at constant temperature.
- Resistance measures opposition to current flow, in ohms (Ξ©).
- Not all components are ohmic (e.g. diodes, filament bulbs at varying temperature).