Resistance Slows Current - Electricity and Electronic Circuits Step by Step

A resistor's job is to oppose current. Its opposition is called resistance and is measured in ohm; more ohm means less current for the same push.

electricity A resistor is the brake of a circuit: its resistance in ohm measures how strongly it opposes current. Here the one exact value shown is a 4 ohm resistor.

Example

A resistor's job is to oppose current. Its opposition is called resistance and is measured in ohm; more ohm means less current for the same push. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.

highlighted = computed this step

A resistor opposes current

A resistor is a circuit element whose job is to oppose the flow of current. How strongly it opposes is called its resistance, and the unit of resistance is the ohm. Here the resistor's resistance is 4 ohm.

R = 4\ \text{ohm} \quad (\text{ohm} = \text{unit of resistance})

More resistance means less current

Resistance and current pull opposite ways: with the same battery pushing, a larger resistance in ohm means a smaller current flows, and a smaller resistance means a larger current. The resistor is the brake; the battery is the push.

R \uparrow \;\Rightarrow\; I \downarrow \qquad R \downarrow \;\Rightarrow\; I \uparrow

From opposition to a precise rule

So far this is qualitative. The natural question is exact: for a given battery voltage and a given resistance in ohm, what current actually flows? That rule is Ohm's law, built in the next lesson around this same 4 ohm resistor.

\text{Ohm's law: how } V, R, I \text{ fit together}