The resistance R of a resistor is to be determined. The current I in the resistor and the potential difference V across it are measured.

Question 10

The resistance R of a resistor is to be determined. The current I in the resistor and the potential

difference V across it are measured.

The results, with their uncertainties, are

I = (2.0 ± 0.2) A                       V = (15.0 ± 0.5) V.

The value of R is calculated to be 7.5 Ω.

What is the uncertainty in this value for R ?

A ± 0.3 Ω          B ± 0.5 Ω          C ± 0.7 Ω          D ± 1 Ω

Reference: Past Exam Paper – November 2014 Paper 13 Q5

Solution:

Answer: D.

Ohm’s law: V = IR

R = V / I = 15.0 / 2.0 = 7.5 Ω

ΔR / R = ΔV/V + ΔI/I

ΔR / 7.5 = 0.5/15 + 0.2/2

 
ΔR = (0.5/15 + 0.2/2) × 7.5 = ± 1