## 20171214

### Physics quiz question: comparing changes in internal energies

Physics 205A Quiz 7, fall semester 2017
Cuesta College, San Luis Obispo, CA

An aluminum sample and an iron sample have the same mass. The aluminum sample initially has a higher temperature than the iron sample, and then they are brought in contact with each other to reach thermal equilibrium. Ignore heat exchanged with the environment. Specific heat of aluminum is 900 J/(kg·K); specific heat of iron is 452 J/(kg·K). After thermal equilibrium is reached, the __________ sample will have experienced the greatest change in internal energy.
(A) aluminum.
(B) iron.
(C) (There is a tie.)
(D) (Not enough information is given.)

Correct answer (highlight to unhide): (C)

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

Qext = ∆Ealuminum + ∆Eiron,

where for an isolated system, Qext = 0, so:

0 = ∆Ealuminum + ∆Eiron,

–∆Ealuminum = ∆Eiron.

This means that the aluminum sample experienced the same amount of thermal internal energy change (a decrease) as the change in the thermal internal energy of the iron sample (an increase).

Sections 70854, 70855
Exam code: quiz07Whu7
(A) : 7 students
(B) : 14 students
(C) : 19 students
(D) : 2 students

Success level: 45%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.31