PHYSICS · Single correct · AP · Physics 2: Algebra-based · Thermodynamics · Thermodynamics
Problem
Two metal cylinders, R and S, are brought into contact at t = 0 min. The temperature of R behaves as follows:
• Rapid decrease from 90 °C to 60 °C during 0 – 4 min.
• Gradual leveling off near 55 °C during 4 – 6 min.
• Sudden rise to 75 °C between 6 – 7 min owing to a brief external flame applied to R only.
• Slow decline from 75 °C to 40 °C during 7 – 12 min, after which both cylinders remain at 40 °C.
Which narrative most accurately describes the particle-level energy exchange throughout the entire experiment?
(a) Energy flows from S to R whenever R is cooling because R’s temperature is changing more rapidly; during the short heating interval, the flame—not S—supplies energy to R, but afterward energy again flows from S to R.
(b) Energy flows from R to S only during the first 4 min, after which their temperatures are equal and heat transfer stops; the flame heats R alone, and S receives no energy while R later cools to 40 °C by itself.
(c) Energy initially flows from R to S until their temperatures nearly match; the external flame then adds energy to R, increasing its particles’ kinetic energy; afterward, energy again flows from R to S until both reach 40 °C.
(d) Because both cylinders end at 40 °C, the total energy transferred must cancel out, so heat must have flowed equally in both directions throughout, with the flame adding just enough extra energy to keep the balance.