Exam 4 Physics MisConceptual Questions
When an ideal gas is warmed from 20 degrees C to 40 degrees C, the gas's temperature T that appears in the ideal gas law increased by a factor (s) of 2 (b) of 1.07 (c) that depends on the temperature scale you use
answer: (b) of 1.07 The absolute (Kelvin) scale must be used in the ideal gas law. If the Celsius scale is used, it appears that the temperature has doubled. However, 0°C is an arbitrarily chosen point on the temperature scale and cannot be used to determine temperature ratios. When the temperatures are converted to kelvins (293 K and 313 K) it can be seen that their ratio is about 1.07.
The linear expansion of a material depends on which of the following? (a) the length of the material (b) the change in temperature of the material. (c) the type of material (d) all of the above (e) Both
answer: (d) all of the above Equation 13-1a for thermal expansion shows that the change in length depends upon the initial length, the change in temperature, and the coefficient of thermal expansion (which depends upon the type of material).
The temperature of an ideal gas increases. Which of the following is true? (a) the pressure must decrease. (b) the pressure must increase. (c) The pressure must increase while the volume decreases. (d) the volume must increase while the pressure decreases (E) the pressure, the volume, or both may increase.
answer: (e) A common misconception is that if the temperature increases, both the pressure and volume will increase. However, by the ideal gas law, when the temperature increases, the product of the pressure and volume must increase. This increase can occur by increasing the pressure, increasing the volume, or both.
Two beakers contain a mixture of ice and water at equilibrium. Beaker A has less ice than Beaker B. Which beaker is the coldest, or are they equal in temperature? (a) Beaker A (b) Beaker B (c) Equal
Answer: c) Equal Students frequently interpret having more ice with being colder. However, whenever ice and water are mixed together and are in thermal equilibrium they will be at the melting/freezing point of the water. Therefore, the two containers will be at the same temperature.
Rod A has twice the diameter of rod B, but both are made of iron and have the same initial length. Both rods are now subject to the came change in temperature (but remain solid). How would the change in the rods' length compare? (a) Rod A > rod B (b) Rod B > rod A (c) Rod A = rod B (d) need to know whether the rods were cooled or heated.
answer: (c) Rod A = rod B in thermal expansion the change in length is independent of the rod's diameter or cross-sectional area.
An ideal gas is in a sealed rigid container. The average kinetic energy of the gas molecules depends most on (a) the size of the container (b) the number of molecules in the container (c) the temperature of the gas (d) the mass of the molecules
answer: (c) the temperature of the gas
Which of the following is not true about an ideal gas? (a) the average kinetic energy of the gas molecules increases as the temperature increases. (b) the volume of an ideal gas increases with temperature if the volume is held constant c) The pressure of an ideal gas increases with temperature if the volume is held constant (d) All gas molecules have the same speed at a particular temperature (e) The molecules are assumed to be far apart compared to their size
answer: (d) All gas molecules have the same speed at a particular temperature The temperature of an ideal gas is a measure of the average kinetic energy of the gas, so increasing the temperature will increase the average kinetic energy—(a) is true. Even though the temperature is proportional to the average kinetic energy of the molecules, each molecule canhave a random kinetic energy (according to the Maxwell distribution of speeds). The speeds of individual molecules will vary about this average, so (d ) is false. Per the ideal gas law, the product of the pressure and volume is proportional to the temperature. Therefore, if pressure or volume is held constant as the temperature increases, the other parameter (volume or pressure) must increase, so (b) and (c) are true. For a gas to be ideal, it is assumed that the space occupied by the gas is mostly empty. For this to be true, the molecules are assumed to be far apart compared with their size. Thus, (e) is also true.
An ideal gas undergoes an isothermal process. Which of the following statements are true? (i) No heat is added to or removed from the gas. (ii) The internal energy of the gas does not change. (iii) The average kinetic energy of the molecules does not change
answer: (i) F (ii) T (iii) T Students may misunderstand the difference between isothermal (temperature remains constant so ∆U = 0) and adiabatic (Q = 0). In an isothermal process heat can be absorbed, as long as an equal amount of work is done, so statement (i) is not true. For an ideal gas the temperature is proportional to the internal energy of the gas, statements (ii) and (iii) are equivalent, and both are true.
An ideal gas undergoes an adiabatic expansion, a process in which no heat flows into or out of the gas. As a result, a) the temperature of the gas remains constant and the pressure decreases b) both the temperature and pressure of the gas decrease c) the temperature of the gas decreases and the pressure increases d) both the temperature and plume of the gas increase (e) both the temperature and pressure of the gas increase
answer: b) both the temperature and pressure of the gas decrease As the gas expands, its volume increases and it does work on the surroundings. Since no heat is absorbed while the gas does this work, the first law of thermodynamics says that the internal energy and temperature of the gas must decrease. For the volume to increase as the temperature decreases, the ideal gas law requires that the pressure also decrease.
On a very hot day, could you cool your kitchen by leaving the refrigerator door open? a) Yes, but it would be very expensive. b) Yes, but only if the humidity is below 50% . c) No, the refrigerator would exhaust the same amount of heat into the room as it takes out of the room. d) No, the heat exhausted by the refrigerator into the room is more than the heat the refrigerator takes out of the room.
answer: d) No, the heat exhausted by the refrigerator into the room is more than the heat the refrigerator takes out of the room. A common misconception in this situation is not realizing that a heat cycle running in reverse, like a refrigerator, must have a high-temperature exhaust. Furthermore, that high-temp exhaust is the sum of the heat removed from the inside of the refrigerator, plus the work done by the refrigerator's compressor.
Two objects are made of the same material, but they have different massed and temperatures. If the objects are brought into thermal contact, which one will have the greater temperature change? (a) The one with the higher initial temperature (b) The one with the lower initial temperature (c) the one with the greater mass (d) the one with the lesser mass (e) the one with the higher specific heat (f) not enough information
answer:(d) the one with the lesser mass As the two objects are in thermal contact, the heat given off by the hot object will equal the heat absorbed by the cold object. The objects have the same specific heat, so the heat transfer is proportional to the product of the mass of each object and its change in temperature. The object with the smaller mass will then have the larger temperature change
About what percentage of the heat produced by burning gasoline is turned into useful work by a typical automobile? a) 20% b) 50% c) 80% d) 90% e) Nearly 100%
Answer: a) 20% The text states that "real engines that are well designed reach 60 to 80% of the Carnot efficiency." The cooling system of the engine keeps the high temperature at about 120°C (400 K) and the exhaust is about room temperature (300 K). The maximum efficiency would then be around 25%. Eighty percent of this maximum would be closest to 20% efficient. Any of the other choices for this question are not reasonable.
An ideal gas undergoes an isothermal expansion from state A to state B. In this process a) Q>0,ΔU=0,W>0. b)Q>0,ΔU=0,W<0. c)Q=0,ΔU=0,W>0. d)Q=0,ΔU>0,W>0. e)Q=0,ΔU<0,W<0
Answer: a) Q>0,ΔU=0,W>0 In an isothermal process the internal energy remains constant (ΔU = 0). In an expansion the gas does work on the surroundings (W = 0). Since the internal energy is constant and the work is positive, the first law of thermodynamics requires the heat absorbed also be positive (Q = 0).
Ten grams of water is added to ten grams of ice in an insulated container. Will all of the ice melt? (a) yes (b) no (c) more information is needed
Answer: c) more information is needed The problem does not specify the initial temperatures of the ice and water. If the ice and water are both initially at 0°C, then none of the ice will melt, since heat will not transfer between them. Alternatively, if the ice is at 0°C and the water is at 100°C, then the water can provide Q = mc∆T = (0.010 kg)(4186 J/kg ⋅ C°)(100 C°) = 4186 J of heat as it cools to 0°C. The ice only needs Q = mL = (0.010 kg)(333 kJ/kg) = 3330 J to melt completely, so in this case all the ice would melt. Since the water could provide more heat than needed, you need to know the initial temperatures of the ice and water to determine just how much of the ice would melt.
When using the ideal gas law, which of the following rules must be obeyed? (a) Always use temperature in kelvins and absolute pressure. (b) Always use volume in m^3 and temperature in kelvins. (c) Always use gauge pressure and temperature in degrees Celsius. (d) Always use gauge pressure and temperature in kelvins. (e) Always use volume in m^3 and gauge pressure
Answer:(a) Always use temperature in kelvins and absolute pressure. Students frequently do not understand that gauge pressure and temperature in Celsius are a comparison of pressure and temperature to an arbitrary zero point. For the ideal gas law to hold, the temperature, pressure, and volume must be measured relative to the true zero points: absolute zero pressure, absolute zero temperature, and zero volume. The volume units are not critical, since changing them only affects the value of the ideal gas constant. Gauge pressure and Celsius temperature have additive terms that change the functional form of the ideal gas law.
Two identical bottles at the same temperature contain the same gas. If bottle B has twice the volume and contains half the numbers of moles of gas as bottle A, how does the pressure in B compare with the pressure in A?
By the ideal gas law, when the temperature is held constant, the pressure is proportional to the number of moles and inversely proportional to the volume. If the second bottle has twice the volume with only half the number of moles, it would only experience one-fourth the pressure.
In a mixture of gases oxygen and helium, which statement is valid? (a) The helium atoms will be moving faster than the oxygen molecules, on average. (b) both will be moving at the same speed (c) the oxygen molecules will, on average be moving more rapidly than the helium atoms (d) the kinetic energy of helium atoms will exceed that of oxygen molecules (e) None of the above
answer: (a) The helium atoms will be moving faster than the oxygen molecules, on average. In the mixture, the oxygen molecules and helium atoms will be at the same temperature, which means that their average molecular kinetic energies will be the same. Since a helium atom has less mass than an oxygen molecule, the helium atoms will be moving faster than the oxygen molecules on average.
For objects at thermal equilibrium, which of the following is true? (a) Each is at the same temperature. (b) Each has the same internal energy (c) Each has the same heat (d) All of the above (e) None of the above
answer: (a) each is at the same temperature When two objects are in thermal equilibrium, heat does not transfer between them. This occurs when the two objects are at the same temperature. Internal energy is an extrinsic property that depends upon the amount of the substance present. Therefore, two gases in thermal equilibrium with each other would have different internal energies if one consisted of one mole of gas and the other consisted of two moles of gas. Heat is a transfer of energy between objects that are not in thermal equilibrium. Heat is not a property of an object.
Two equal-mass liquids, initially at the same temperature, are heated for the same time over the same stove. You measure the temperatures and find that one liquid has a higher temperature than the other. Which liquid has the higher specific heat? (a) the cooler one (b) the hotter one (c) both are the same
answer: (a) the cooler one The two objects absorb the same amount of heat from the stove. From Eq. 14-2, given that the masses are the same, the object with the higher specific heat will experience the smaller temperature increase and will therefore be cooler.
One mole of an ideal gas in a sealed rigid container is initially at a temperature of 100 degrees C. The temperature is then increased to 200 degrees C. The pressure of the gas (a) remains constant (b) increases by about 25% (c) doubles (d) triples
answer: (b) increases by about 25% A common error is to treat the temperature as doubling. If the temperature doubled, the pressure would also double. However, the temperature is given in degrees Celsius, not kelvins. When the temperature is converted to kelvins, it is easy to see that the temperature only increases by about 25%, from 373 K to 437 K. The pressure then also increases by about 25%. (The actual amount is 26.8%.)
A steel plate has a hole in it with a diameter of exactly 1.0 cm when the plate is at a temperature of 20 degrees C. A steel ring has an inner diameter of exactly 1.0 cm at 20 degrees C. Both the plate and the ring are heated to 100 degrees C. Which statement is true? (a) the hole in the plate gets smaller, and the opening in the ring gets larger. (b) The opening in the ring gets larger, but we need the relative size of the plate and the hole to know what happens to the hole. (c) the hole in the plate and the opening in the ring get larger. (d) The hole in the plate and the opening in the ring get smaller. (e) The hole in the plate and the opening in the ring gets smaller.
answer: (c) the hole in the plate and the opening in the ring get larger. Many students have the misconception that as the plate expands, the hole will get smaller. To understand what actually happens, imagine that the hole is filled with a steel disk. As the plate and the steel disk are heated, both will expand. After the plate and disk are heated, the disk is removed from the plate. Since the disk expanded, the hole that is left must also have expanded. As the steel ring is heated its circumference will expand, causing its interior to also expand.
The rms speed of the molecules of an ideal gas (a) is the same as the most probable speed of the molecules. (b) is always equal to the squared root of 2 times the maximum molecular speed. (c) will increase as the temperature of a gas increases. (d) All of the above
answer: (c) will increase as the temperature of a gas increases. The rms speed of a gas is an average molecular speed found by taking the square root of the average of the square of the molecule speeds. Consider a gas in which 2/3 of the molecules are at rest and 1/3 move at a constant speed. The most probable speed is 0, but the rms speed is greater than zero. Therefore, the most probable speed is not necessarily the rms speed, so (a) is false. The rms speed is an average speed; therefore, it will always be equal to or smaller than the maximum speed, so (b) is false. The temperature of a gas is determined by the average molecular kinetic energy. As the temperature increases, the molecular kinetic energy increases; therefore, the rms speed must also increase, so (c) is true.
Radiation is emitted (a) only by glowing objects such as the Sun (b) only by objects whose temperature is greater than the temperature of the surroundings (c) only by objects with more caloric than their surroundings (d) by any object not at 0 K (e) only by objects that have a large specific heat
answer: (d) by any object not at 0 K Radiation is emitted by all objects not at absolute zero. Very hot objects, such as the Sun, emit radiation in the visible spectrum, so they appear to be glowing. If the temperature of an object is less than that of its surroundings, it has a net gain in energy as it absorbs more radiation than it emits. However, it is still emitting radiation. The amount of radiation emitted is independent of the object's specific heat.
A typical thermos bottle as a thin vacuum space between the shiny inner flask (which holds a liquid) and the shine protective outer flask, often stainless steel. The vacuum space is excellent at preventing (a) conduction (b) convection (c) radiation (d) conduction and convection (e) conduction, convection, and radiation
answer: (d) conduction and convection Heat is able to transfer through a vacuum by radiation, but heat requires a medium to transfer by conduction and by convection. Therefore, the vacuum in a thermos prevents heat loss by conduction and convection.
Two ideal gases, A and B, are at the same temperature. If the molecular mass of the molecules in gas A is twice that of the molecules in gas B, the molecules' root-mean-square speed is (a) the same in both gases (b) twice as great in A (c) 1.4 times greater in A (d) twice as great in B (e) 1.4 times greater in B
answer: (e) 1.4 times greater in B Some students might erroneously relate the temperature of the gas to the velocity of the gas molecules and surmise that the rms speeds would be equal. However, when the two gases are at the same temperature, the molecules will have the same average kinetic energy. The kinetic energy is proportional to the mass of the molecule and the square of the rms speed. Since mass B is half the mass of A, the speed of molecules of mass B must be 2 ≈ 1.4 × greater than the speed of molecules of mass A.
Which of the following possibilities could increase the efficiency of a heat engine or an internal combustion engine? a) Increase the temperature of the hot part of the system and reduce the temperature of the exhaust. b) Increase the temperatures of both the hot part and the exhaust part of the system by the same amount. c) Decrease the temperatures of both the hot part and the exhaust part of the system by the same amount. d) Decrease the temperature of the hot part and increase the temperature of the exhaust part by the same amount. e) None of the above; only redesigning the engine or using better gas could improve the engines efficiency.
answer: a) Increase the temperature of the hot part of the system and reduce the temperature of the exhaust and c) Decrease the temperatures of both the hot part and the exhaust part of the system by the same amount. The maximum efficiency of an engine is given by equation eideal = 1−TLTH, which can be written in the form: e = (TH−TC)/TH.
Which statement is true regarding the entropy change of an ice cube that melts? a) Since melting occurs at the melting point temperature, there is no temperature change so there is no entropy change b) Entropy increases c) Entropy decreases
answer: b) Entropy increase Heat must be added to the ice cube to melt it. The change in entropy is the ratio of the heat added to the temperature of the ice cube. Since heat is absorbed in the process, the entropy increases.
Which is possible: converting (i) 100J of work entirely into 100J of near, (ii) 100 J of heat entirely into 100 J of work? a) only (i) is possible b) only (ii) is possible c) Both (i) and (ii) are possible d) neither are possible
answer: c) both are possible According to the second law of thermodynamics, it is impossible for heat to be entirely converted into work in a cycle or a heat engine. However, the question does not specify that we must consider a complete cycle. In an isothermal process Q = W . In an isothermal compression work is entirely converted into heat, and in an isothermal expansion heat is entirely converted into work.
An ideal gas undergoes an isobaric compression and then an isovolumetric process that brings it back to its initial temperature. Had the gas undergone one isothermal process instead, a) the work done on the gas would be the same b) the work done on the gas would be less c) the work done on the gas would be greater d) Need to know the temperature of the isothermal process
answer: c) the work done on the gas would be greater
In an isobaric compression of an ideal gas, a)no heat flows into the gas b) the internal energy of the gas remains constant c) no work is done on the gas d) work is done on the gas e)work is done by the gas
answer: d) work is done on the gas An isobaric process is one in which the pressure is kept constant. In a compression the volume of the gas decreases. By Eq. 15-3, the work done by the gas is negative, so an external force had to do work on the gas. In isobaric processes heat is allowed to flow into or out of the system and the internal energy changes.
A heat engine operates between a high temperature of about 600∘C and a low temperature of about 300∘C. What is the maximum theoretical efficiency for this engine? a) =100%. b) ≈66%. c) ≈50%. d) ≈34%. e) Cannot be determined from the given information.
answer: d) ≈34% A frequent misconception made in calculating the efficiency of an engine is to leave the temperatures in degrees Celsius, which would imply an efficiency of 50%. However, when the temperatures are properly converted to kelvins, equation eideal = 1−TLTH gives the efficiency as only about 34%.
As heat is added to water, is it possible for the temperature measured by a thermometer in the water to remain constant? (a) Yes, the water could be changing phase. (b) No, adding heat will always change the temperature. (c) Maybe; it depends on the rate at which the hear is added. (d) Maybe; it depends on the initial water temperature.
answer:(a) Yes, the water could be changing phase. A common misconception is that as heat is added to water the temperature will always rise. However, Fig. 14-5 shows that heat is added to water at its melting and boiling points without the temperature changing. At these temperatures the water is undergoing a phase change.
When you put an ice cube in a glass of warm tea, which of the following happens? (a) cold flows from the ice cube into the tea (b) cold flows from the ice cube into the tea and hear flows from the tea into the ice cube (c) Heat flows from the tea into the ice cube. (d) Neither heat nor cold flows . Only temperature flows between the ice and the tea..
answer:(c) Heat flows from the tea into the ice cube. A common misconception is that "cold" flows from the ice into the tea. When the ice is placed in the tea, the ice has less kinetic energy per molecule than the tea, so in molecular collisions between the tea and ice, energy transfers from the tea into the ice. This energy transfer cools the tea as it melts the ice and then heats up the ice. The transfer of energy from the warmer tea to the colder ice is called "heat."
Which of the following happens when a material undergoes a phase change? (a) the temperature changes (b) the chemical composition changes. (c) heat flows into or out of the material (d) the molecules break apart into atoms
answer:(c) heat flows into or out of the material Phase changes occur at specific temperatures (melting point, boiling point, or sublimation point) while heat is being added to or removed from the material. During the phase change the temperature remains constant. The thermal energy at this temperature is equal to the intermolecular binding energy. These energies are much lower than the energy necessary to break the molecules apart into their atoms or to change the chemical composition of the molecules.
Heat is (a) a fluid called caloric (b) a measure of the average kinetic energy of atoms (c) the amount of energy transferred between objects as a result of a difference in temperature (d) an invisible, odorless, weightless substance (e) the total kinetic energy of an ideal gas
answer:(c) the amount of energy transferred between objects as a result of a difference in temperature Students often think that heat is a substance or a property of a material. When two materials are at different temperatures (that is, they have different average kinetic energies per molecule), energy can transfer from the hot object to the cold object. This transfer of energy is called heat.
It has been a hot summer, so when you arrive at a lake, you decide to go for a swim even though it it nighttime. The water is cold. The next day, you go swimming again during the hottest part of the day, and even though the air is warmer the water is still almost as cold. Why? (a) Water is fairly dense compared with many other liquids. (b) Water remains in a liquid state for a wide range of temperatures. (c) Water has a high bulk modulus. (d) Water has a high specific heat.
answer:(d) Water has a high specific heat. The specific heat of an object is a measure of how much heat is required to change its temperature. Water has a high specific heat (much higher than air), so its temperature remains fairly constant even though the surrounding air may experience large temperature fluctuations.
