## 20191204

### Physics quiz question: comparing specific heat capacity values

Physics 205A Quiz 7, fall semester 2019
Cuesta College, San Luis Obispo, CA

A 0.25 kg glass sample (400 K initial temperature) and a 0.75 kg graphite sample (300 K initial temperature) are brought in contact with each other to reach thermal equilibrium with a final temperature of 330 K. Ignore heat exchanged with the environment. The specific heat capacity values of these two substances are not known. The __________ sample has the larger specific heat capacity value.
(A) glass.
(B) graphite.
(C) (There is a tie.)
(D) (Not enough information is given.)

Correct answer (highlight to unhide): (A)

The transfer/balance energy conservation equation for this system is given by:

Qext = ∆Eglass + ∆Egraphite,

and since there is no heat exchanged with the environment, Qext = 0, such that:

0 = mglass·cglass·ΔTglass + mgraphite·cgraphite·ΔTgraphite,

and:

mglass·cglass·ΔTglass = mgraphite·cgraphite·ΔTgraphite,

–(0.25 kg)·cglass·(–70 K) = (0.75 kg)·cgraphite·(+30 K),

(17.6 kg·K)·cglass = (22.5 kg·K)·cgraphite.

From inspection, for the equality to hold, cglass > cgraphite, and thus glass must have a greater specific heat capacity value than graphite.

Sections 70854, 70855
Exam code: quiz07VlnC
(A) : 25 students
(B) : 17 students
(C) : 3 students
(D) : 6 students

Success level: 49%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.82