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PHYSICS TEST REVIEW
In an experiment, a student releases an object from rest in four different liquids and observes whether it floats. The results are listed in the table. Which of the following is true about the density of the object? - It is between the density of distilled water and the density of salt water. A student wants to determine whether the density of a solid cube of copper will decrease as its temperature is increased without melting the cube. Graphing which of the following will allow the student to study this question? - Volume as a function of temperature Students are investigating the change in the density of water as the temperature of the water increases. The students measure the mass and the volume of a quantity of water and then heat the water to various temperatures in the range using a thermometer to measure the temperature. They then attempt to determine the density of the water at the different temperatures. Assume any changes of equipment or measuring tools due to temperature changes are negligible. Which of the following methods would allow the students to obtain data from which they could determine the change in density of the water at different temperatures? - Record the temperature of the water and precise measurements of the volume of the water. In an experiment, a student puts each of three objects, A, B, and C, in four different liquids and observes whether the objects float. The results are listed in the table. Which columns contain information that is most useful for estimating the density of object C? - Gasoline and Oil A physics teacher gives a solid object to each student in her class. The objects are each made of the same material but have different shapes and sizes. The teacher asks the students to measure the mass and volume of their object and to work together to determine the density of the material from which the objects are made. Which of the following methods will give the best estimate of the material's density? - Each student shares the mass and volume of their object with the class. The class then creates a graph plotting mass as a function of volume, draws a best-fit line through the data, and uses the slope of the best-fit line as the density of the material. Water is poured into a U-shaped tube and allowed to come to rest, as shown in the figure. Which of the following can help explain why the water in the left and right sides of the tube reaches the same height? Select two answers. - -The force exerted by the atmosphere on the surface of the water is the same on both sides. -The weight of water in the left and right halves of the tube is the same. A wooden cube is floating at rest on the surface of a liquid. Which of the following is an action-reaction pair of forces that can explain why the force exerted by the liquid on the
bottom of the container is greater with the floating cube than without it? - The liquid exerts a net upward force on the cube, and the cube exerts a contact force on the liquid. A block of weight W is floating in water, and one-third of the block is above the surface of the water. Which of the following correctly describes the magnitude F of the force that the block exerts on the water and explains why F has that value? - F=W. The block is at rest, so the net force on it must be zero. The water must exert an upward force of magnitude FF on the block to make this so. The block then exerts a force of equal magnitude on the water. A cube submerged in a rectangular tank of liquid is at rest. Which of the following is a Newton's third law pair of forces in this situation? - The force that the liquid exerts on a side of the tank and the force the side of the tank exerts on the liquid The figure shows a block of wood floating in a beaker of water. Which of the following is true of the force exerted by the wood on the water? - The wood exerts a downward force on the water that is equal in magnitude to the weight of the wood. A block of mass M is floating in a container of liquid and is partially submerged. The block is then replaced by a block of mass 2M. Which of the following describes two forces that are a Newton's-third-law force pair whose magnitudes are the same for the two situations? - The gravitational force Earth exerts on the liquid and the gravitational force the liquid exerts on Earth A sphere made of gold is moving downward through liquid mercury at an increasing speed. The mercury exerts a net buoyant force in the upward direction on the gold sphere. Which of the following describes a Newton's-third-law pair for this force? - The gold sphere exerts a force on the mercury with magnitude equal to the buoyant force. A submarine of mass 3.6×106kg is submerged in water and at equilibrium. Inside of the submarine are ballast tanks filled with water. Some of the water in the ballast tanks is then ejected, decreasing the density of the submarine. The free-body diagram in the figure shows the magnitude and direction of the forces acting on the submarine after the water is ejected. The acceleration of the submarine is most nearly - 1.1m/s A sphere of mass 0.5kg is dropped into a column of oil. At the instant the sphere becomes completely submerged in the oil, the sphere is moving downward at 8m/s, the buoyancy force on the sphere is 4.0N, and the fluid frictional force is 4.0N. Which of the following describes the motion of the sphere at this instant? - The sphere's speed is decreasing while the sphere continues to move downward. A solid cube is submerged in a container of water of density ρw. The cube has mass M , volume Vc , and density ρw2. The water exerts an upward buoyant force on the cube of magnitude ρwVcg. Which of the diagrams is the best representation of the forces exerted by the water on the cube? - Larger force up ^ all other forces are equal in length.
will float in the water. If resistance is negligible, what is the shape of the path followed by the sensor? - A parabolic path toward the north and curving upward A sphere of metal is placed in a liquid. The sphere sinks with an acceleration less than g due to the vertical buoyant force FB exerted by the liquid and the gravitational force Fg on the sphere. Which of the following best represents these two forces? - Force of gravity is stronger A sphere is moving vertically in a liquid. The free-body diagram shows the magnitudes and directions of the forces exerted on the sphere at a particular instant. What is the magnitude of the sphere's acceleration and the direction of the sphere's motion? - Acceleration: 0.7 m/s^ Direction of motion: downward A block is submerged in a container of water and remains at rest with the only forces exerted on it being gravity and forces exerted by the water. The force exerted by the water on the bottom of the block is greater than the force exerted by the water on the top of the block. Which of the following is an explanation of the microscopic cause of the difference in the forces? - The molecules of water hitting the bottom of the block have a greater average speed than those hitting the top of the block, and thus they exert a greater force. Four cubes with equal volumes are at rest in a container of water, as shown in the figure. A student claims that cube D is experiencing a greater number of water molecule collisions per second with its bottom surface than the other cubes. Which of the following indicates whether or not the student is correct and provides evidence? - The student is correct, because the bottom surface of cube DD is deepest, which means the pressure is greatest. Two metal cylinders of equal density and volume are submerged in water and remain at rest with their top surfaces at the same height, as shown. Cylinder A has a smaller height and a larger cross-sectional area than cylinder B. Which of the following explains why the force exerted on the top of cylinder A is greater than the force exerted on the top of cylinder B? - The water molecules striking the top surface of cylinder A have the same average speed as the water molecules striking the top surface of cylinder B, but more water molecules strike the top surface of cylinder A than the top surface of cylinder B. The small dots in the figure represent the molecules of a solid object, and the dashed lines represent the molecular bonds between them. The large dots represent the molecules of a liquid in which the object floats. Which of the following best describes the force that explains the interaction of the object and the liquid? - An electrical force that repels the electrons of the object's molecules from the electrons of the liquid's molecules
An ideal fluid flows from left to right in the horizontal pipe shown in the figure. The fluid enters the left side of the pipe with speed v, where the diameter of the pipe is d. The pipe then narrows to a diameter of d/2. What is the speed of the fluid in the narrow section of the pipe with diameter d/2? - 4v An ideal fluid flows from left to right in the horizontal pipe shown in the figure. The fluid enters the left side of the pipe with speed v, where the diameter of the pipe is d. The pipe then narrows to a diameter of d/2. Does the pressure of the fluid change as the pipe becomes narrower, and why or why not? - Yes, because the kinetic energy of the fluid changes but the total gravitational potential energy of the fluid-Earth system is constant. A plastic bowl is floating in a sink full of water. Which of the following describes the microscopic cause of the buoyant force exerted on the bowl by the water? - Electrostatic repulsion between the electrons in the molecules in the bowl and the electrons in the molecules of the water A block is submerged in a container of fluid. Which of the following describes the microscopic cause of the force exerted on the bottom of the block by the fluid being greater than the force exerted on the top of the block by the fluid? - The electrostatic force between the electrons in the molecules of the block and the electrons in the molecules of the fluid is greater at the bottom than at the top. The figure shows a horizontal pipe with sections with different cross-sectional areas. Small tubes extend from the top of each section. The cross-sectional area of the pipe at location C is half that at A, and the areas at A and D are the same. Water flows in the pipe from left to right. Which of the following correctly ranks the height h of the water in the tubes above the labeled locations? - (hA=hD)>hB>hC Which of the following graphs best represents the absolute pressure Pabs at a point in a liquid near Earth's surface as a function of the depth h of the point below the surface of the liquid? - Linear graph with x=0 at y=# above axis In the figure, water flows from a section of pipe with a smaller radius to a section of pipe with a larger radius. How do the pressure and velocity in the wider pipe compare to those in the narrower pipe? - Pressure: greater Velocity: less The figure shows a container filled with water to a depth d. The container has a hole a distance y above its bottom, allowing water to exit with an initially horizontal velocity. Which of the following correctly predicts and explains how the speed of the water as it exits the hole would change if the distance y above the bottom of the container increased? - The speed would decrease because the water pressure at the depth of the hole would decrease. The figure shows a top view of three connected, horizontal pipes. Water moves from left to right in pipe A and then divides when it reaches the junction with pipes B and C.
temperature. Which of the following is a reason why the pressure in the second sphere is less than that in the first sphere? - The molecules in the second sphere have to go travel a longer distance between collisions with the walls of the sphere than the molecules in the first sphere do. A gas enclosed in a cylinder has a pressure of 2.0×105Pa. The ends of the cylinder have a diameter of 0.40m and the cylinder has a height of 0.30m. The magnitude of the force exerted by the gas on the wall at one end of the cylinder is most nearly - 2.5×104N F=PA=P(πr2)F=PA=P(πr2) Which of the following correctly explain why the pressure of a gas in a rigid container increases with increasing temperature? Select two answers. - -The average molecular kinetic energy increases with temperature, so the molecules exert a larger average force on the walls of the container when they collide with the walls of the container. -The average molecular speed increases with temperature, so the molecules collide with the walls of the container more frequently. Compressed air in a vertical cylinder with a piston of radius 0.30m is used to lift a crate. The minimum pressure of the air in the cylinder needed for the piston to lift the crate is 1.61×105N/m2, and the pressure in the room outside the piston is 1.01×105N/m2. If the mass of the piston is negligible, the weight of the crate is most nearly - 1.7×104N F=ΔP⋅A A student watches a soap bubble drift from inside a warm room, out an open window, and into the cold air outside. The student notices that the bubble gets smaller once it is outside. Assuming no air leaks out of the bubble, what conclusion could the student reasonably draw from this observation? - If the bubble were cooled even further, there is a temperature at which the volume of the bubble approaches zero. A quantity of an ideal gas, initially at zero degrees Celsius, is placed in a container of unknown fixed volume. The gas is heated, and the temperature and pressure are measured, resulting in the graph shown. Which of the following quantities can be determined from a best-fit line to the data? - Absolute zero A student has a sample of gas with a known temperature and volume in an insulated cylinder with a piston and wants to investigate the relationship between temperature and volume. The student quickly pushes on the piston to reduce the volume a small amount and measures the new temperature and volume of the gas. This procedure is repeated to get measurements for increasingly smaller volumes of the gas. The data show that as the volume of the gas decreases, its temperature increases, which appears to contradict the ideal gas law. Which of the following modifications to the experiment would demonstrate the relationship between temperature and volume in the ideal gas law? - Perform a new experiment with hot gas that uses an object on top of the piston to keep the pressure of the gas constant while the gas is allowed to cool.
Students perform an experiment to determine the number of moles in a sample of an ideal gas. The students place the sample in a container with a movable piston. A thermometer and a pressure sensor are attached to the container. The gas is initially at a temperature of 293K. The students cool the gas to 273K in an ice bath, recording the temperature and pressure at regular intervals during the process. The students are surprised to find that the pressure remains constant as the temperature decreases. How could the students refine their experiment to determine the number of moles of gas present in the sample? - Perform the same experiment with the piston held in place so that the volume is constant. Students want to investigate the inverse relationship between the pressure and volume of an ideal gas as predicted by the ideal gas law. Their plan is to use a gas-filled cylinder with a movable piston on one end and compress the piston. The students will then measure the volume and pressure of the gas when the piston is in various positions. Which of the following additions to this procedure will allow the students to observe the predicted relationship between pressure and volume? Select two answers.
- -Maintain a constant gas temperature by surrounding the cylinder in a constant temperature water bath. -Ensure the piston and cylinder walls don't allow gas in or out of the cylinder. A student conducts an experiment to measure the universal gas constant R using the apparatus shown. A cylinder with volume markings is filled with 0.45mol of helium gas at room temperature and pressure. The piston is connected to a lever system, as shown in Figure 1, with a pivot that is connected to a fixed support and moving hinges that allow the lever to push the rod and piston down. The student pushes on the lever and moves the piston down, as shown in Figure 2, to compress the gas as slowly and steadily as possible while the pressure, volume, and temperature of the gas are measured. The graph on the left shows pressure as a function of volume for this process. The graph on the right shows PVnT as a function of time, which should show that R is constant. The student suspects that there is a problem with the experimental apparatus that caused the variation in data between 1 and 2 atm pressure. Which of the followi - Hinges that stick and cause uneven motion of the lever An ideal gas is confined within a rigid container with a fixed lid. The container is placed on a heat source. Which of the following correctly compares the force exerted on the lid by the ideal gas and the force exerted on the ideal gas by the lid, and correctly relates the force to pressure? - The force of the ideal gas on the lid is equal to the force of the lid on the ideal gas, but the force is increasing because of the increased kinetic energy of the ideal gas, leading to an increase in pressure. To launch a rocket, fuel in its engines is ignited, and the combustion produces gases at high pressure. As the combustion gases escape downward from one end of the rocket, the rocket is accelerated upward. Which of the following claims correctly describes forces in this situation? - The combustion gases are pushed out of the rocket because the pressure inside the engine is greater than the pressure outside the rocket. The
The temperature of an ideal gas is held constant as its volume is slowly decreased. Which of the following claims correctly describes the cause of the increase in the force exerted by the gas on the walls of the container? - The gas molecules strike the walls more frequently. A gas is in a sealed cylinder fitted with a movable piston, as shown in the figure, so that none of the gas escapes. The cylinder and piston are made of an insulating material. The cylinder is fitted with pressure and temperature sensors, and the volume of the confined gas can be measured from markings on the side of the cylinder. The gas is initially in a state with pressure Pi, temperature Ti, and volume Vi. The piston is slowly compressed and data are recorded until the gas reaches a final state with pressure Pf, temperature Tf, and volume Vf. A graph of the pressure as a function of volume is shown, with dotted lines indicating isotherms. Which of the following describes the energy transfer process that the insulation prevents? - Collisions of gas molecules with the inner surface of the cylinder cause disturbances of molecules of the insulating material that travel through the cylinder, transferring energy to air molecules that hit the outer surface of the cylinder A gas is in a sealed cylinder fitted with a movable piston, as shown in the figure, so that none of the gas escapes. The cylinder and piston are made of an insulating material. The cylinder is fitted with pressure and temperature sensors, and the volume of the confined gas can be measured from markings on the side of the cylinder. The gas is initially in a state with pressure Pi, temperature Ti, and volume Vi. The piston is slowly compressed and data are recorded until the gas reaches a final state with pressure Pf, temperature Tf, and volume Vf. A graph of the pressure as a function of volume is shown, with dotted lines indicating isotherms. Which of the following claims identifies the change in internal energy of the gas between its initial and final states and describes the corresponding changes in energy due to work and heating? - Internal energy increases. Energy is added to the gas via work and none is lost by heating. A gas is in a sealed cylinder fitted with a movable piston, as shown in the figure, so that none of the gas escapes. The cylinder and piston are made of an insulating material. The cylinder is fitted with pressure and temperature sensors, and the volume of the confined gas can be measured from markings on the side of the cylinder. The gas is initially in a state with pressure Pi, temperature Ti, and volume Vi. The piston is slowly compressed and data are recorded until the gas reaches a final state with pressure Pf, temperature Tf, and volume Vf. A graph of the pressure as a function of volume is shown, with dotted lines indicating isotherms. Which of the following claims correctly indicates the changes in the collisions between gas molecules and the cylinder wall that contribute to the change in pressure of the gas as it goes from its initial to its final state? - The volume decreases, so the number of collisions per unit time increases. The average speed of the molecules increases, so the average impulse of the collisions increases.
A sample of an ideal gas is in a sealed cylinder with a movable piston and is initially in state A, as shown in the graph of pressure P as a function of volume V. The gas's temperature in state A is 300K. The gas can be taken from state A to state D via three separate paths, AD, ABD, or ABCD. For which of the paths, if any, is the change in the internal energy of the gas greatest? - None; the change in internal energy is the same for all three paths. A sample of an ideal gas is in a sealed cylinder with a movable piston and is initially in state A, as shown in the graph of pressure P as a function of volume V. The gas's temperature in state A is 300K. The gas can be taken from state A to state D via three separate paths, AD, ABD, or ABCD. What is the temperature of the gas in state B? - 600K A sample of an ideal gas is in a sealed cylinder with a movable piston and is initially in state A, as shown in the graph of pressure P as a function of volume V. The gas's temperature in state A is 300K. The gas can be taken from state A to state D via three separate paths, AD, ABD, or ABCD. In which process is energy transferred from the gas to the environment by heating only?
- CD The graph shows pressure as a function of volume for a sample of an ideal gas that is confined in a cylinder. A moveable piston sits on top of the gas, and the piston can move vertically with negligible friction. The weight of the piston is significantly less than the force exerted by the gas on the piston during the process shown. Which of the following is true of the acceleration of the piston during the process? - The acceleration stays the same since the net force on the piston stays the same. A container filled with gas is sealed on the top by a piston of negligible mass and area 0.005m2. The container and gas are allowed to reach thermal equilibrium with the surrounding air, and a block on top of the piston keeps the piston in place. If the pressure inside the container is 3×105Pa, what is the total downward force exerted on the piston? - 1500N A cylindrical container with a movable piston contains a fixed amount of ideal gas. Initially, it is in thermal equilibrium with an ice-water bath. The diagram shows the forces exerted on the piston. The cylinder is then removed from the ice water and placed into a hot-water bath. The system slowly comes to thermal equilibrium with its surroundings. The forces exerted on the piston once the system reaches equilibrium are shown in the figure. What is the relationship among the forces exerted on the piston? - Fgas=(Fatm+mg) Gas molecule A is moving to the right with speed v, as shown in the figure, when it collides with identical gas molecule B, which is at rest. After the collision, molecule A is moving directly toward the top of the page with the same speed v. Which of the following best represents the net force on molecule A during the collision? - Arrow pointing to the left at an angle
work done on the gas. Which of the following describes a correct analysis of information from the graph that will determine the work? - Multiply the total change in pressure by the total change in volume. Divide the result by two. Add to this the product of the final pressure and the total change in volume. A student is sitting on the ground a few meters away from a campfire on a cool, windless night. The student feels noticeably warm on the side facing toward the fire, and noticeably cool on the side facing away from the fire. Which of the following models describes the primary method by which the side of the student facing toward the fire is warmed? - The fire emits electromagnetic radiation. Some of the radiation is absorbed by the student and is converted to thermal energy. An ideal gas is confined in a container with a moveable piston. The graph of pressure as a function of volume shows four different processes in which the gas is compressed. Which of the following correctly ranks the magnitude of the work W done on the gas in the four processes? - W2>W1>(W3=W4) A gas is in a sealed cylinder fitted with a movable piston, as shown in the figure, so that none of the gas escapes. The cylinder and piston are made of an insulating material. The cylinder is fitted with pressure and temperature sensors, and the volume of the confined gas can be measured from markings on the side of the cylinder. The gas is initially in a state with pressure Pi, temperature Ti, and volume Vi. The piston is slowly compressed and data are recorded until the gas reaches a final state with pressure Pf, temperature Tf, and volume Vf. A graph of the pressure as a function of volume is shown, with dotted lines indicating isotherms. Which of the following describes the energy transfer process that the insulation prevents? - Collisions of gas molecules with the inner surface of the cylinder cause disturbances of molecules of the insulating material that travel through the cylinder, transferring energy to air molecules that hit the outer surface of the cylinder A gas is in a sealed cylinder fitted with a movable piston, as shown in the figure, so that none of the gas escapes. The cylinder and piston are made of an insulating material. The cylinder is fitted with pressure and temperature sensors, and the volume of the confined gas can be measured from markings on the side of the cylinder. The gas is initially in a state with pressure Pi, temperature Ti, and volume Vi. The piston is slowly compressed and data are recorded until the gas reaches a final state with pressure Pf, temperature Tf, and volume Vf. A graph of the pressure as a function of volume is shown, with dotted lines indicating isotherms. Which of the following claims identifies the change in internal energy of the gas between its initial and final states and describes the corresponding changes in energy due to work and heating? - Internal energy increases. Energy is added to the gas via work and none is lost by heating. A gas is in a sealed cylinder fitted with a movable piston, as shown in the figure, so that none of the gas escapes. The cylinder and piston are made of an insulating material. The cylinder is fitted with pressure and temperature sensors, and the volume of the
confined gas can be measured from markings on the side of the cylinder. The gas is initially in a state with pressure Pi, temperature Ti, and volume Vi. The piston is slowly compressed and data are recorded until the gas reaches a final state with pressure Pf, temperature Tf, and volume Vf. A graph of the pressure as a function of volume is shown, with dotted lines indicating isotherms. Which of the following claims correctly indicates the changes in the collisions between gas molecules and the cylinder wall that contribute to the change in pressure of the gas as it goes from its initial to its final state? - The volume decreases, so the number of collisions per unit time increases. The average speed of the molecules increases, so the average impulse of the collisions increases The figure shows a cylinder that has a movable piston and contains an ideal gas initially in state 1 at room temperature. The cylinder is sealed. Blocks of known mass can be added to or removed from the top of the piston. The gas is taken through the process represented by the graph of pressure P as a function of volume V. Which of the following actions could cause the gas to go through the process represented by the graph? - Placing the cylinder on a block of ice while keeping the mass on the piston constant The figure shows a cylinder that has a movable piston and contains an ideal gas initially in state 1 at room temperature. The cylinder is sealed. Blocks of known mass can be added to or removed from the top of the piston. The gas is taken through the process represented by the graph of pressure P as a function of volume V. Which of the following energy bar charts could represent the process as the gas is taken from state 1 to state 2, where U1 represents the initial internal energy of the N molecules of gas, Q represents the energy transferred to the gas by heating, W represents the work done on the gas, and U2 represents the final internal energy of the gas? - Above, below, above, above A sample of gas X initially has a higher temperature than a sample of gas Y. The molecules of gas X have more mass than the molecules of gas Y. The samples are mixed together in an insulated container. Which of the following claims best describes the collisions between gas X and gas Y molecules? - Molecules of both gases will sometimes gain and sometimes lose momentum, and sometimes gain and sometimes lose kinetic energy. An ideal gas is in a sealed cylinder with a moveable piston, as shown. In collisions between any two molecules of the gas, the linear momentum and the kinetic energy of the two-molecule system are the same before and after the collision. The gas is then heated until it reaches a certain temperature, and the heat source is removed. Which of the following claims correctly describes the linear momentum and kinetic energy in a collision between two gas molecules of the hotter gas? - Both the linear momentum and the kinetic energy have the same values before and after the collision. A rigid container holds an ideal gas. A gas molecule of mass m moving at speed v collides with a container wall. The molecule's velocity is perpendicular to the wall before
An oxygen atom (of mass m) has a speed of 2v0 collides with another oxygen atom that has a speed of v0 in the same direction. As a result, the two atoms exert forces on each other and form an oxygen molecule. Assume that the two-atom system is completely isolated and no energy is released. Which of the following correctly predicts the resulting changes in the momentum and kinetic energy of the two-atom system as the atoms form a molecule? - The momentum does not change. The kinetic energy decreases. The figure shows an experimental setup with two thermal reservoirs connected by a uniform solid cylindrical bar. The experiment is performed twice, once with bar X and then with bar Y. The graph shows the amount of energy Q transferred through each bar as a function of time t. If there is only one difference between the experimental situations, which of the following could account for the differences in the lines on the graph for each bar? Select two answers. - -The thermal conductivity of bar XX is greater than that of bar YY. -The length of bar XX is less than that of bar YY. A student has four square plates. The two large plates are the same size but made of materials with different thermal conductivity, and are initially at thermal equilibrium with their surroundings. The two identical small square plates are initially heated to a temperature greater than that of the surroundings. Each small plate is placed in contact with one of the large plates in the configuration shown. For each configuration, the student will measure the time it takes for the temperature to begin to rise at one or both of the labeled points. A shorter time means a higher thermal conductivity. Measurement at which of the points gives the best information for determining which material has higher thermal conductivity? - Measurement at either point AA or point BB, because the relative times for either point will give sufficient information. Students are given a solid rod made of an unknown material and are asked to determine the thermal conductivity k of the rod. The rod is initially at room temperature. They measure the length L and the cross-sectional area A of the rod. Which of the following procedures would provide the most reliable estimate of k? - Place one end of the rod in a water bath at 100°C100°C. At regular time intervals, measure the temperature at the other end of the rod and the amount of energy needed to keep the water bath at 100°C100°C. The table lists experimental data related to the conduction of energy by three slabs of metal. The slabs have identical dimensions but are made of different metals. Which of the following correctly ranks the metals' thermal conductivities k? - k1>k2>k When a room-temperature spoon is placed in a cold bowl of ice cream, some of the ice cream quickly melts where it comes into contact with the spoon. Which of the following is the best explanation for why the ice cream melts? - Energy is transferred to the ice cream molecules and increases their average speed, because the higher-energy
molecules in the spoon are vibrating and colliding with the lower-energy molecules of the ice cream. The figure shows a horizontal metal bar with the left end in thermal contact with a constant temperature reservoir that is much hotter than the initial temperature of the bar. The reservoir-bar system is thermally isolated from the surroundings. The atoms of the metal bar are represented by the black dots. Which of the following provides the best reasoning for how the system reaches thermal equilibrium? - The hotter atoms on the left are vibrating faster than the colder atoms on the right and collide with the colder atoms, transferring kinetic energy until the bar reaches thermal equilibrium with the reservoir. The figure shows a sample of an ideal gas enclosed within a cylinder that has been fitted with a movable piston. The piston and the sides of the cylinder are thermally insulated. The bottom of the cylinder is in contact with a thermal reservoir. The gas is compressed isothermally while thermal equilibrium is maintained with the reservoir. The piston, gas, and reservoir form a closed, isolated system. True statements about entropy for this reversible situation include which of the following? Select two answers. - -The entropy of the gas decreases because work is done on the gas while its temperature remains constant. -The entropy of the reservoir increases because thermal energy is transferred to it from the gas. Which of the following describes the microscopic difference between the change in entropy of a gas during reversible processes (which are theoretical) and irreversible processes (which actually occur)? - Reversible: The number of combinations of microscopic states that can combine to create the macroscopic state of the gas remains the same. Irreversible: The number of combinations of microscopic states that can combine to create the macroscopic state of the gas increases. 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? - Energy flows into the air. The temperature of the air increases, and the temperature of the water vapor decreases. Two identical samples of helium gas are in identical sealed flasks at room temperature. One flask is just sitting in the room, and the other is inside a large, insulated vacuum container. Both flasks are opened and the samples are released, so the helium in one flask spreads throughout the room and the helium in the other flask spreads in the sealed container, as shown in the figures. Which of the following is true of the change in entropy that occurs in each case when the flasks are opened? - The entropy increases in both cases because each system is now in a more disordered state.
