Question
A system consists of two containers containing different liquids. The system is insulated, and no thermal energy is lost to or gained from the environment. Liquid A has a density of \(800kg/m^3\) and an initial temperature of 60°C , while liquid B has a density of \(1000kg/m^3\) and an initial temperature of 20°C . Which of the following statements accurately predicts what will happen once the containers are put into thermal contact? Select two answers.
A After a long time, the liquids will have the same total internal energy.
B After a long time, the liquids will have the same temperature.
C Thermal energy will be transferred from liquid B to liquid A because liquid A has a lower density.
D Thermal energy will be transferred from liquid A to liquid B because liquid A has more internal energy per molecule.
▶️Answer/Explanation
Ans:B , D
The liquids are in thermal contact so will eventually reach thermal equilibrium, that is, be at the same temperature.
The internal energy per molecule is related to temperature, and thermal energy is transferred from the liquid with the higher temperature to the liquid with the lower temperature.
Question
The figure shows the movement of a fluid through an automobile engine and a radiator. The system is designed to prevent overheating of the engine. Which of the following claims correctly describe the energy transfer processes involved? Select two answers.
A Thermal energy is transferred out of the hot fluid in the radiator, and the resulting cooler fluid is then used to cool the engine.
B Thermal energy from the air is added to the hot fluid to reduce the temperature of the hot fluid.
C The hot engine adds thermal energy to the cool fluid, which reduces the temperature of the engine and raises the temperature of the cool fluid.
D The cool fluid transfers thermal energy to the engine, providing it with the energy it needs to move the car.
▶️Answer/Explanation
Ans:A , C
The hot fluid loses energy to the cooler environment when it reaches the radiator through the process of radiation. The resulting lower-energy fluid is then directed to the engine to cool the engine.
When the cool fluid comes in contact with the engine, it will take thermal energy away from the engine. This will cool the engine and warm the cool fluid in the process, making the fluid hotter.
Question
When water boils, bubbles of water vapor form in the liquid and travel to the surface of the water. At the surface the bubbles burst, and the water vapor molecules collide with the surrounding air, which is at room temperature. Which of the following claims best describes the flow of energy between the water vapor molecules and the air molecules and the resulting change in temperature of the air and water vapor?
A Energy flows into the air. The temperatures of the water vapor and air both increase.
B Energy flows into the air. The temperature of the air increases, and the temperature of the water vapor decreases.
C Energy flows into the water vapor. The temperatures of the water vapor and air both increase.
D Energy flows into the water vapor. The temperature of the water vapor increases, and the temperature of the air decreases.
▶️Answer/Explanation
Ans:B
The water vapor is at a higher temperature than the air. As the molecules collide, energy is transferred from the higher-energy water vapor to the lower-energy air. So the temperature of the water vapor decreases, and the temperature of the air increases.
Question
The figure shows the pressure and volume of three moles of gas being taken through two different processes. Which of the following is correct concerning the two processes shown in the figure?
(A) \(\Delta U_{1}=\Delta U_{2}\) and \(W_{1}=W_{2}\)
(B) \(\Delta U_{1}=\Delta U_{2}\) and \(W_{1}>W_{2}\)
(C)\(\Delta U_{1}>\Delta U_{2}\) and \(W_{1}=W_{2}\)
(D) \(\Delta U_{1}>\Delta U_{2}\) and \(W_{1}>W_{2}\)
▶️Answer/Explanation
Ans:B
Both paths start and end at the same point. Therefore, the initial and final temperatures are the same, as are the initial and final thermal energies. Process 1 has a higher average pressure for the same volume change. Another way to think about it is to compare the area under the curves. Graph 1 has more area underneath and, therefore, a larger magnitude of work.
Question
The figure shows four samples of gas being taken through four different processes. Process 1 is adiabatic. In which process is heat being transferred to the gas sample from the environment?
(A) 1
(B) 2
(C) 3
(D) 4
▶️Answer/Explanation
Ans:C
In process 3, the work done is zero: W = 0. The temperature and the internal kinetic energy of the gas are increasing. Therefore, thermal energy must be entering the gas: \(\Delta U=Q\).