LearnTronics
Resistance
What is resistance?
Resistance tells us how strongly a material or component opposes electric current. It is measured in ohms, symbol Ω.
A useful first picture
R and Ω
R is the usual variable for resistance.
Ω is the Greek capital letter omega, and it is the unit symbol for the ohm. So R = 6 Ω reads: “the resistance is six ohms.”
Think of people trying to move through a hallway. A long, narrow hallway makes movement more difficult than a short, wide hallway. A conductor behaves somewhat similarly: longer generally means more resistance, while thicker generally means less.
Another way to think of this
Picture water moving through a hose. A long skinny hose restricts flow more than a short large hose. That analogy is useful for circuit intuition, though electrons in a conductor are not literally water molecules in a pipe.
What is really happening?
In a metal, moving electrons interact with the material's atomic structure. Those interactions oppose the orderly drift associated with current and convert electrical energy into thermal energy.
For a uniform conductor, a useful relationship is:
where ρ is resistivity, L is length, and A is cross-sectional area.
Where did the name come from?
The ohm is named for German physicist Georg Simon Ohm (1789–1854). In the 1820s he experimentally investigated the relationship between voltage, current, and resistance and published his major work on the subject in 1827.
We will use that relationship on the next page as Ohm's law.
Worked example
Question: Two copper wires have the same thickness. One is twice as long as the other. Which has more resistance?
Answer: The longer wire. With material and area unchanged, resistance increases with length.
Try these yourself
A little deeper
Resistance can also change with temperature. For many metals, resistance increases as temperature rises. Not every device obeys a constant resistance over all conditions: lamps, thermistors, semiconductor junctions, and many other devices can be non-ohmic.