What is Resistance?
Resistance is the property of a material that opposes the flow of electric current. Every electrical conductor has some resistance, while components called resistors are specifically designed to provide a known amount of resistance in electronic circuits.
What is Resistance?
Electrical resistance is the opposition that a material offers to the flow of electric current.
When electrons move through a conductor they collide with atoms inside the material. These collisions make it more difficult for electrons to move, creating resistance.
Higher resistance means less current flows for a given voltage.
Water Pipe Analogy
Imagine water flowing through pipes.
- A wide pipe allows water to flow easily.
- A narrow pipe restricts the flow of water.
- The restriction is similar to electrical resistance.
The greater the resistance, the harder it is for electric current to flow.
Unit of Resistance
Resistance is measured in ohms, represented by the Greek letter Ω (Omega).
One ohm is the resistance that allows one ampere of current to flow when one volt is applied.
Smaller and larger units include:
| Unit | Symbol | Value |
|---|---|---|
| Milliohm | mΩ | 0.001 Ω |
| Ohm | Ω | 1 Ω |
| Kilohm | kΩ | 1,000 Ω |
| Megohm | MΩ | 1,000,000 Ω |
What Causes Resistance?
Several factors affect the resistance of a conductor.
- Material used.
- Length of the conductor.
- Cross-sectional area.
- Temperature.
Long wires have greater resistance than short wires, while thicker wires have lower resistance than thin wires.
Materials and Resistance
| Material | Relative Resistance |
|---|---|
| Silver | Very Low |
| Copper | Low |
| Aluminium | Low |
| Nichrome | High |
| Carbon | Higher |
| Rubber | Extremely High (Insulator) |
Resistance in Electronic Circuits
Resistance is deliberately used to control electrical current.
Common applications include:
- Current limiting for LEDs.
- Voltage dividers.
- Transistor biasing.
- Amplifier feedback networks.
- Pull-up and pull-down resistors.
- Heating elements.
Resistance and Heat
Whenever current flows through resistance, electrical energy is converted into heat.
This is known as the Joule heating effect.
Examples include:
- Electric heaters.
- Soldering irons.
- Hair dryers.
- Power resistors.
Measuring Resistance
Resistance is measured using an ohmmeter or a digital multimeter set to the resistance (Ω) range.
The circuit should be switched off before measuring resistance.
Measuring resistance on a powered circuit can produce incorrect readings and may damage the meter.
Practical Examples
| Component | Typical Resistance |
|---|---|
| LED Current Limiting Resistor | 220 Ω |
| Speaker Crossover Resistor | 2–20 Ω |
| Potentiometer | 1 kΩ – 1 MΩ |
| Carbon Film Resistor | 1 Ω – 10 MΩ |
| Heating Element | Depends on power rating |
Key Points
- Resistance opposes the flow of electric current.
- Resistance is measured in ohms (Ω).
- Higher resistance means lower current.
- Resistance depends on material, length, area and temperature.
- Resistors are used to control current and voltage.
- Resistance converts electrical energy into heat.