1. Resistance is measured in ohms

The SI unit of electric resistance is the ohm, represented by the Greek letter omega (Ω). A resistance of 1 ohm means that a potential difference of 1 volt produces a current of 1 ampere through the component under the applicable conditions.

2. Resistance is connected to voltage and current

Ohm's law is V = IR, where voltage equals current multiplied by resistance. For a fixed voltage across an ohmic resistor, increasing resistance reduces current.

3. Copper has low electrical resistance

Copper is widely used in electrical wiring because it conducts electricity well. Silver is more electrically conductive at room temperature, but copper is often more practical and economical for wiring.

4. Thicker wires usually have lower resistance

For the same material and length, a wire with a larger cross-sectional area has lower resistance. This is one reason electrical cables must be sized appropriately for their current.

5. Longer wires usually have higher resistance

For the same material and cross-sectional area, doubling a wire's length doubles its resistance, assuming uniform temperature and material properties.

6. Resistance can produce heat

When current flows through a resistance, electrical energy can be converted into heat. This effect powers electric kettles, toasters, heaters, and traditional incandescent bulbs.

7. Resistance can waste energy in power lines

Wires carrying current generate heat because of their resistance. The power dissipated is P = I²R, so reducing current can substantially reduce transmission losses.

8. Resistance can be measured with a multimeter

A multimeter's resistance mode measures resistance, typically by applying a small test current or voltage. For reliable readings, the circuit should generally be de-energized, and the component may need to be isolated from other circuit paths.

9. Temperature changes resistance

For many metallic conductors, resistance increases as temperature rises. Some thermistors behave in the opposite way: their resistance decreases as temperature increases, making them useful in temperature sensors.

10. Some materials can have zero electrical resistance

Certain materials become superconductors below a specific critical temperature and under suitable conditions. In their superconducting state, they can carry direct current without ordinary electrical resistance.

11. Resistors control current in electronic circuits

Resistors help set current, divide voltage, establish operating points, and protect sensitive components such as LEDs. Without appropriate resistance or another current-limiting method, excessive current can damage components.

12. Resistor color bands reveal their value

Many through-hole resistors use colored bands to indicate resistance and tolerance. For example, a resistor with red, red, and brown bands has a nominal resistance of 220 Ω, before considering its tolerance band.

13. Parallel resistors reduce total resistance

When ordinary resistors are connected in parallel, the equivalent resistance is lower than the resistance of the smallest individual resistor. Adding another parallel path makes it easier for current to flow.

14. Insulators have extremely high resistance

Materials such as glass, rubber, and many plastics strongly resist electric current under normal conditions. They are used to insulate wires and protect people from contact with conductive parts.

15. Resistance matters for electrical safety

Resistance in wires and connections can cause overheating, particularly when connections are loose or undersized. Fuses and circuit breakers protect against specified overcurrent conditions, while proper wiring and insulation help reduce hazards.