A sealed vertical cylinder contains an ideal gas confined by a movable piston. During a 30-s interval the piston is slowly pushed inward by an external force. No electrical devices are operating inside the cylinder, and the cylinder walls are warmer than the surrounding air, so energy is transferred from the gas to the environment during the interval. When the interval ends, the gas temperature is lower than it was at the start.
(a) Indicate the sign (positive, negative, or zero) of the following quantities for the 30-s interval and justify each choice.
(i) Energy transferred by heating between the gas and its surroundings
(ii) Mechanical work done on the gas
(b) Using conservation of energy, determine the sign of the change in internal energy of the gas for the interval and explain your reasoning.
(c) Describe, in terms of microscopic motion of the gas particles, how the processes you identified in part (a) lead to the temperature decrease observed at the end of the interval.