Ohm's law and the resistivity of a wireStarter example
A starter example, adapted by LABKickstart from the OpenStax College Physics for AP Courses Lab Manual and shared under CC BY 4.0. It is not a classroom submission from a teacher on Benchtop.
Build a simple battery circuit to show current is proportional to voltage, then measure how a wire's resistance depends on its length, its thickness and what it is made of.
Materials
- Battery holder and batteries, or a bench supply capped at about 5 V
- Leads with alligator clips
- Miniature bulb and holder
- Four to six resistors of different values
- Multimeter
- Wire samples differing in diameter, length and material
- Metre rule and a caliper or micrometer
- A table of resistivities for the wire materials
Procedure
- Build a series circuit of battery, resistor and bulb. Measure the voltage across the resistor and the current through it.
- Swap in each resistor in turn, recording voltage and current for every one.
- Plot voltage against current. The slope is the resistance, which is Ohm's law.
- Now measure the resistance of each wire sample directly with the multimeter.
- Measure each sample's length and diameter, and compute its cross-sectional area.
- Plot resistance against length, and against the inverse of area, then use the slopes to work out each material's resistivity and compare with the table.
Expected results
The voltage against current plot is a straight line through the origin, so resistance is constant and current is proportional to voltage. Resistance rises in proportion to length, falls in proportion to cross-sectional area, and differs between materials even at the same size. Your computed resistivities should land within a few tens of a percent of the table values, which is normal for classroom wire and a hand-held meter.
Safety
Keep the supply at or below about 5 V and never connect it straight across a wire sample with no resistor: thin wire heats fast and can burn. Disconnect the battery between measurements so nothing stays warm, and let a hot sample cool before handling it.
Source
Adapted from OpenStax, College Physics for AP Courses Lab Manual (CC BY 4.0). Access for free at openstax.org. AP is a trademark registered by the College Board, which was not involved in and does not endorse this adaptation.
Downloads
- OpenStax Lab 14: Ohm's LawPDF324 KBDownload
Shared under CC BY 4.0. Use it, remix it, credit the author.