Resistance of a wire: length, thickness and metal
Change the length and thickness of a wire and the metal it is made of, and read its resistance on an ohmmeter. Find why house wiring uses thick copper and heaters use thin nichrome.
Core concept
A wire's resistance is R = ρL/A: it doubles when the wire is twice as long, halves when the cross-section is twice as big, and depends on the metal's resistivity ρ
Topic: Electricity
resistance: How much something slows down the flow of electricity, measured in ohms (Ω). All key words is how much a wire opposes the flow of current: The flow of electricity through a wire, measured in amperes (A). All key words, measured in ohms (Ω). It depends on three things. A longer wire has more resistance, in proportion to its length L. A thicker wire, with a bigger cross-section A, has less resistance. And the metal matters: each has a resistivity ρ (rho). Together, R = ρL ÷ A. Copper and aluminium have a very low resistivity, so they are used for wires that carry current with little loss. Nichrome, an alloy of nickel and chromium, has a resistivity about 65 times that of copper, so it is used in heaters, irons and toasters where the wire must get hot.
What you will learn
- State how resistance depends on the length, cross-section and material of a wire.
- Calculate resistance with R = ρL/A.
- Explain the choice of copper for wiring and nichrome for heating elements.
Before you start
- Know that resistance is measured in ohms.
Materials
- Pieces of copper and nichrome wire of different lengths and thicknesses
Equipment
- A multimeter set to measure resistance
Procedure
- Record 1 m of copper wire of 1 mm² cross-section.
- Double the length and record.
- Double the cross-section and record.
- Change the metal to nichrome and record.
How this simulation is simplified
- The wire is at room temperature; ρ is in Ω·mm²/m, so L is in metres and A in mm².