Thermal Energy Transfer and Equilibrium
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AP Physics 2 › Thermal Energy Transfer and Equilibrium
A student places a $25^\circ\text{C}$ glass beaker in direct contact with a $90^\circ\text{C}$ hot plate. The beaker is initially cooler than the hot plate, and they remain in contact until the beaker’s temperature stops changing. Which statement best describes the thermal energy flow while they approach equilibrium?
Thermal energy flows equally in both directions, so the beaker’s temperature stays at $25^\circ\text{C}$.
Thermal energy flows from the hot plate to the beaker until their temperatures become equal.
Thermal energy flows from the beaker to the hot plate until both are at the same temperature.
No thermal energy flows because the beaker’s temperature increases due to its material, not energy transfer.
Explanation
This question tests understanding of thermal energy transfer and equilibrium. The hot plate at 90°C is in contact with the glass beaker at 25°C, establishing a temperature gradient. Thermal energy flows from the higher temperature hot plate to the lower temperature beaker until they reach thermal equilibrium. Choice B incorrectly suggests no energy flows and attributes temperature change to the material itself rather than energy transfer, which violates the principle of energy conservation. The key concept is that thermal energy always flows from regions of higher temperature to regions of lower temperature through conduction.
A student places a $40^\circ\text{C}$ steel sphere into a large bath of oil maintained at $20^\circ\text{C}$. The sphere is fully submerged, and the bath is large enough that the oil temperature remains essentially constant. After a long time, the sphere’s temperature becomes constant. At equilibrium, which statement best describes the thermal energy transfer between the sphere and oil?
Thermal energy continues flowing from the sphere to the oil because the sphere started hotter.
No thermal energy flows because equilibrium means both objects contain zero thermal energy.
Thermal energy continues flowing from the oil to the sphere because the oil is denser.
No net thermal energy flows between the sphere and the oil because they are at the same temperature.
Explanation
This question tests understanding of thermal energy transfer and equilibrium. Initially, the 40°C sphere transfers thermal energy to the 20°C oil bath until the sphere cools to 20°C. At equilibrium, both the sphere and oil are at the same temperature (20°C), so there is no net thermal energy flow between them. Choice A incorrectly suggests energy continues flowing based on initial conditions, but thermal energy flow requires a temperature difference. The fundamental concept is that at thermal equilibrium, no net energy transfer occurs because there is no temperature gradient.
Two gases, A and B, are in thermal contact. The atoms of gas A have a higher average kinetic energy than the atoms of gas B. When an atom from A collides with an atom from B, which statement accurately describes the transfer of kinetic energy?
The transfer of kinetic energy is probabilistic, but on average, more energy is transferred from A to B than from B to A.
In every collision, the atom with greater instantaneous speed will transfer energy to the atom with lesser instantaneous speed.
In every collision, kinetic energy will be transferred from the atom in A to the atom in B.
The probability of energy transfer from A to B is exactly equal to the probability of transfer from B to A.
Explanation
Energy transfer in collisions is a statistical process. While a single collision could result in energy transfer from B to A (if a particularly fast atom from B hits a slow atom from A), the overall trend is governed by the average energies. Since gas A is hotter, it is far more probable that collisions will result in a net transfer of energy from A to B.
A $0.10,\text{kg}$ metal rod at $100^\circ\text{C}$ is pressed against a large wax block at $25^\circ\text{C}$. The rod and wax are in direct contact, and the wax is thermally insulated from the surroundings. They remain in contact until the rod is no longer cooling. Which statement best describes the thermal energy flow during contact?
No thermal energy flows because the rod cools due to radiation only, not contact transfer.
Thermal energy flows from the rod to the wax until they reach the same temperature.
Thermal energy flows from the wax to the rod until both reach the same temperature.
Thermal energy flows equally in both directions at all times, so the rod cannot cool.
Explanation
This question tests understanding of thermal energy transfer and equilibrium. The metal rod at 100°C is pressed against wax at 25°C, creating a significant temperature difference. Thermal energy flows from the hotter rod to the cooler wax until they reach the same temperature. Choice D incorrectly claims energy flows equally in both directions at all times, which would prevent any temperature change and violates the principle of net energy flow from hot to cold. The fundamental rule is that thermal energy flows from higher to lower temperature until thermal equilibrium is established.
Object X is at $$25°C$$ and object Y is at $$75°C$$. They are brought into thermal contact inside a thermally isolated container. Which statement correctly describes the process as they approach thermal equilibrium?
The net energy transfer is from X to Y, and the final temperature will be between $$25°C$$ and $$75°C$$.
The net energy transfer is from X to Y, and the final temperature will be greater than $$75°C$$.
The net energy transfer is from Y to X, and the final temperature will be exactly $$50°C$$.
The net energy transfer is from Y to X, and the final temperature will be between $$25°C$$ and $$75°C$$.
Explanation
Thermal energy spontaneously flows from a region of higher temperature to a region of lower temperature. Therefore, energy is transferred from object Y to object X. The final equilibrium temperature must lie between the initial temperatures of the two objects.
The spontaneous transfer of energy between two objects in thermal contact always occurs in a specific direction. Which of the following principles governs this directionality?
Energy is spontaneously transferred from the object with greater mass to the object with lesser mass.
Energy is spontaneously transferred from the object at a higher temperature to the object at a lower temperature.
Energy is spontaneously transferred from the object possessing greater total internal energy to the one with less.
Energy is spontaneously transferred from the object with a higher specific heat to the one with a lower specific heat.
Explanation
This is a consequence of the Second Law of Thermodynamics. The direction of spontaneous thermal energy transfer is determined by the temperature difference between the two objects, always flowing from hot to cold, regardless of their masses, internal energies, or specific heats.
An ice cube at 0°C is placed in a glass of warm soda. Considering the ice cube as the system of interest, which statement correctly describes the thermal process it undergoes?
The ice cube is cooling because it causes the temperature of the surrounding soda to decrease significantly.
The ice cube is heating because energy is transferred into it from the soda by thermal processes.
The ice cube is neither heating nor cooling, because its temperature remains constant at 0°C while it melts.
The ice cube is cooling because energy is transferred out of it to the soda by thermal processes.
Explanation
Heating is defined as the transfer of energy into a system by thermal processes. The ice cube absorbs energy from the warmer soda, which causes the ice to melt. Even though its temperature is constant during the phase change, its internal energy is increasing due to this energy transfer.
Two objects, A and B, are placed in an insulated container and are in thermal contact. Object A has a higher initial temperature than object B. Which of the following statements is true when the system reaches thermal equilibrium?
The temperature of object B is greater than the temperature of object A.
Object A must have more internal energy than object B.
The total energy transferred from object B to object A is greater than the total energy transferred from A to B.
The net transfer of energy between objects A and B is zero.
Explanation
By definition, thermal equilibrium is the state where there is no net flow of thermal energy between two systems in thermal contact. At this point, the rate of energy transfer from A to B equals the rate from B to A, resulting in a zero net transfer.
Which of the following statements correctly pairs a thermal energy transfer process with the requirement of a physical medium for that process to occur?
Convection requires a medium, but both conduction and radiation do not require a medium.
Conduction requires a medium, and convection requires a medium, but radiation does not require a medium.
Radiation requires a medium, and convection requires a medium, but conduction does not require a medium.
Conduction requires a medium, and radiation requires a medium, but convection does not require a medium.
Explanation
Conduction involves the transfer of energy through particle collisions, and convection involves the bulk movement of a fluid; both require a material medium. Radiation is the transfer of energy via electromagnetic waves, which can travel through a vacuum and do not require a medium.
A $1.0,\text{kg}$ iron pan at $150^\circ\text{C}$ is placed in contact with $0.50,\text{kg}$ cooking oil at $25^\circ\text{C}$ in an insulated setup so only pan and oil exchange energy. They are left until equilibrium. Which statement best describes the thermal energy flow?
Thermal energy flows from the pan to the oil only after the oil becomes hotter than the pan.
Thermal energy flows from the oil to the pan until equilibrium because liquids transfer heat faster.
No thermal energy flows at any time because equilibrium means both have zero thermal energy.
Thermal energy flows from the pan to the oil until both reach the same temperature.
Explanation
This question tests understanding of thermal energy transfer and equilibrium. The iron pan at 150°C is significantly hotter than the cooking oil at 25°C. When objects at different temperatures are in thermal contact, energy flows from the higher temperature object to the lower temperature object. Therefore, thermal energy flows from the hot pan to the cooler oil until both reach the same equilibrium temperature. Choice C incorrectly claims that equilibrium means zero thermal energy—equilibrium actually means equal temperatures, not zero energy. Always remember: thermal energy flows from regions of higher temperature to regions of lower temperature.