What is the SI unit of electrical resistance?
Quick answer
The SI unit of electrical resistance is the ohm, written with the Greek letter Ω (omega). It’s named after German physicist Georg Simon Ohm. One ohm is the resistance that lets one ampere of current flow when one volt is applied.
What is resistance?
Resistance measures how much a material opposes the flow of electric current. Think of electricity like water in a pipe:
| Electricity | Water comparison | Unit |
|---|---|---|
| Voltage (V) | Water pressure | Volt (V) |
| Current (I) | How much water flows | Ampere (A) |
| Resistance (R) | How narrow the pipe is | Ohm (Ω) |
Ohm’s law
Georg Ohm published his law in 1827:
V = I × R (voltage = current × resistance)
So if a 12-volt battery is connected to a 6-ohm resistor, the current is 12 ÷ 6 = 2 amperes.
What affects resistance?
- Material: Copper and silver have low resistance (good conductors); rubber and glass have very high resistance (insulators).
- Length: Longer wires have more resistance.
- Thickness: Thicker wires have less resistance, which is why heavy appliances need thick cables.
- Temperature: Most metals resist more when hot.
Everyday examples
| Device | Why resistance matters |
|---|---|
| Toaster and kettle | High-resistance wires heat up on purpose |
| Old light bulbs | A thin tungsten filament glows white-hot |
| Electronics | Resistors control current to protect components |
| Power lines | Very high voltage reduces energy lost to resistance |
Superconductors
Some materials, when cooled to extremely low temperatures, have zero resistance. These superconductors are used in MRI scanners and particle accelerators.
Electric current is the flow of electrons; see which part of an atom has a negative charge. Filaments with high resistance lit the first bulbs: who invented the light bulb.
Still curious?
Jump to a random question from the archive.