Question

The first law of thermodynamics In thermal physics, we are often interested in the internal energy (E) of a system. The wo 41 internal energy of an ideal gas is proportional to the temperature and the number of moles of the gas. The internal energy can change when energy is exchanged with the system's environment (ie., objects that are outside the system of interest). The case above is one in which the internal energy of a gas changes due to work done on the gas (the system) by the piston (an agent external to the system). When such a system is thermally insulated, the change in internal energy of the system is equal to the net work done on it: AE=Wym (for a thermally insulated system) B. Does the internal energy of a gas in an insulated cylinder increase, decrease, or remain the same when the piston is pushed inward? Explain. Does the temperature of the gas change? Explain. C. Two students are discussing process 1 (see part A): Student 1: The volume of the gas decreases, but the pressure increases. Therefore, by the ideal gas law, the temperature must remain the same. Student 2: "I disagree. The vollume is less, and therefore the particles collide more often with one another, so the temperature must go up. Neither student is correct. Find the flaws in the reasoning of cach student. Explain. Check your reasoning with a tutorial instructor before you continue. III. Energy transferred thermally A. Imagine that the cylinder from section II is no longer insulated, and the piston is locked in place. The gas is initially at room temperature. The cylinder is then placed into boiling water and reaches thermal equilibrium with the water. We will refer to this process as process 2. During process 2, do the following quantities increase, decrease, or remain the same? 1. Explain. the temperature of the gas the internal energy of the gas the volume of the gas the pressure of the gas mtion Group, Department of Physics

Answer 1

B) Piston pushed inside, work done on the gas. Work done is equal to the increase in internal energy. internal energy increases.

temperature also increases. Increase in internal energy increases temp.

C)

From ideal gas law

PV = nRT

T will remain const. only when the fraction of increase in P compensates the fractional decrease in V.

student -1 is not correct.

particles in gas always collide. temperature is not an indicator of the number of collisions. It is KE of the particles, not the frequency of collisions. The collisions are perfectly elastic.

IIIA:

temperature increases from room temp. to boiling water temp.

internal energy increases, because temp. increases.

piston locked - volume remain same

pressure increases , because T increases