Ohm's Law & Resistivity
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flagWhat you'll discover
- arrow_forwardVerify Ohm’s law from a straight V-I graph
- arrow_forwardWire ammeter in series and voltmeter in parallel correctly
- arrow_forwardDetermine R from the slope and ρ = πd²R/4L from the wire’s dimensions
- arrow_forwardExplain why heating of the wire must be avoided during readings
Ohm’s law and what can break it
Ohm’s law states that at constant temperature the current through a metallic conductor is proportional to the potential difference across it: V = IR. On a V-I graph the data should fall on a straight line through the origin whose slope is the resistance R.
The law is not universal — filament bulbs curve (they heat up), diodes conduct one way only. Even your test wire disobeys if the current heats it, which is why alloys like constantan and manganin, whose resistance barely changes with temperature, are the standard samples, and why you close the key only briefly for each reading.
Circuit and procedure
The circuit: battery → plug key → rheostat → ammeter, all in SERIES with the test wire, and the voltmeter in PARALLEL across the wire alone. The ammeter has tiny resistance so it does not disturb the current; the voltmeter has very high resistance so it steals almost none.
Slide the rheostat to set five or six well-spread currents, recording V and I each time. Plot V against I, draw the best-fit line and read R from the slope — more reliable than any single V/I ratio. Then measure the wire’s length L with a metre scale and diameter d with a screw gauge (several places, two perpendicular directions), and compute ρ = RA/L = πd²R/4L.
Why your ρ may be off
The diameter enters squared, so a 2% error in d becomes 4% in ρ — measure d most carefully of all. Connections add contact resistance; clean the wire ends. The voltmeter, if cheap, draws current and makes the ammeter read slightly high. And if you leave the key closed, Joule heating raises R as you watch.
Typical values to expect: constantan ρ ≈ 49 × 10⁻⁸ Ω·m, nichrome ≈ 110 × 10⁻⁸ Ω·m, copper just 1.7 × 10⁻⁸ Ω·m (too small to measure well this way). Quoting your result with units and comparing against the accepted value is exactly what the examiner wants in the conclusion.