Physics Test 4

Which of the following temperature changes are equivalent?

1C = 1K

Which is possible: converting (i) 100 J of work entirely to 100 J of heat, (ii) 100J of heat entirely into 100J of work?

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.

Which of the following statements are true for an isobaric process?

An isobaric process is carried out at constant pressure. During an isobaric process, the internal energy of the system changes.

In an isobaric compression of an ideal 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.

An ideal gas undergoes an adiabatic expansion, a process in which no heat flows into or out of the gas. As a result...

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.

Two objects are made of the same material, but they have different masses and temperatures. If the objects are brought into thermal contact, which one will have greater temperature change?

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.

A thermally isolated system consists of a hot piece of aluminum and a cold piece of copper. The aluminum and the copper are in thermal contact. The specific heat of aluminum is more than double that of copper. Which object experiences the greater amount of heat transfer during the time it takes the system to reach thermal equilibrium?

Both experience the same magnitude of heat transfer

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 a higher specific heat?

Both liquids had the same increase in internal energy, because the same heat was added. But the cooler liquid had a lower temperature change. Since Q = mcDT, if Q and m are both the same and DT is smaller, then c (specific heat) must be bigger.

Which of the following statements are true for an adiabatic process?

During an adiabatic process, the internal energy of the system changes. During an adiabatic process, there is no heat transfer into or out of the system.

Which of the following statements are true for an isochoric process?

During an isochoric process, the internal energy of the system changes. During an isochoric process, the work done by the gas on its surroundings is zero. An isochoric process is carried out at constant volume.

The linear expansion of a material depends on which of the following?

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).

A typical thermos bottle has a thin vacuum space between the shiny inner flask and the shiny protective outer flask. The vacuum space is excellent at preventing...

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.

An ideal gas undergoes an isothermal expansion from state A to state B. In this process...

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).

In a mixture of the gases oxygen and helium, which statement is valid?

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.

Which temperature scale does NOT have negative values?

Kelvin

Which of the following statements are true?

Liquid water expands with increasing temperature above 4C. Solid ice is less dense than liquid water

Which of the following happens when a material undergoes a phase change?

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.

Radiation is emitted...

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.

Two drinking glasses are stuck, one inside the other. How would you get them unstuck?

Running hot water only over the outer glass will allow the outer one to expand, while the inner glass remains relatively unchanged. This should loosen the outer glass and free it.

Two different objects receive the same amount of heat. Which of the following choices is NOT a reason why the objects may have different temperature changes?

Since Q = m c DT and the objects received the same amount of heat, the only other factors are the masses and the specific heats. While the initial temperature is certainly relevant for finding the final temperature, it does not have any effect on the temperature change DT.

You want to take apart a couple of aluminum parts held together by steel screws, but the screws are stuck. What should you do?

Since aluminum has a larger b value, that means aluminum expands more than steel. Thus, by heating the part, the aluminum holes will expand faster than the steel screws and the screws will come loose.

When an ideal gas is warmed from 20C to 40C, the gas's temperature T that appears in the ideal gas law increases by a factor of...

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 specific heat of concrete is greater than that of soil. A baseball field (with real soil) and the surrounding parking lot are warmed up during a sunny day. Which would you expect to cool off faster in the evening when the sun goes down?

The baseball field, with the lower specific heat, will change temperature more readily, so it will cool off faster. The high specific heat of concrete allows it to "retain heat" better and so it will not cool off so quickly—it has a higher "thermal inertia."

Which of the following statements are true?

The first law of thermodynamics extends the principle of conservation of energy to include heat as well as mechanical energy. The work done by a thermodynamic system is equal to the area under the curve on a pV diagram. The amount of heat added or removed from a thermodynamic system depends on the path taken on a pV diagram.

An ideal gas is in a sealed rigid container. The average kinetic energy of the gas molecules depends most on

The temperature of the gas is a measure of the average kinetic energy of the gas molecules.

Will potatoes cook faster if the water is boiling faster?

The water boils at 100°C and remains at that temperature until all of the water has been changed into steam. Only then will the steam increase in temperature. Since the water stays at the same temperature, regardless of how fast it is boiling, the potatoes will not cook any faster

Is it possible to transfer heat from a cold reservoir to a hot reservoir?

Yes, if work is done, this transfer process can take place.

isolated system

closed system where no energy in any form is transferred (heat lost must equal heat gained)

By which primary heat transfer mechanism does one end of a steel rod become hot when the other end is placed in a flame?

conduction

isobaric

constant pressure, Q=DU+P(DV)

By which primary heat transfer mechanism does a stove heat a pot of soup?

convection

thermography

detailed measure of radiation from the body. Warmer areas may be a sign of tumors or infection, cooler areas may be a sign of poor circulation

Second Law of Thermodynamics

heat can spontaneously flow from a hot object to a cold object; it will not flow spontaneously from a cold object to a hot one (can be done but not spontaneously)

heat

heat is energy transferred from one object to another because of a difference in temperature

heat of vaporization

heat required to change 1.0kg of material from liquid to vapor

open system

mass may transfer as well

temperature

measures molecules average kinetic energy

adiabatic (isoentropic)

no heat flow in or out of the system, Q=0 so DU = -W

closed system

no mass enters or leaves, but energy may be exchanged

isovolumetric (isochoric)

occurs at constant volume, work done is 0, so Q=DU

conduction

occurs through molecular collisions

By which primary heat transfer mechanism does the Sun warm Earth?

radiation

One mole of an ideal gas in a sealed rigid container is initially at a temperature of 100C. The temperature is then increased to 200C. The pressure in the gas...

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%.)

On a very hot day, could you cool your kitchen by leaving the refrigerator door open?

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-temperature exhaust is the sum of the heat removed from the inside of the refrigerator and the work done by the refrigerator's compressor.

When you put an ice cube in a glass of warm tea, which of the following happens?

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."

As heat is added to water, is it possible for the temperature measured by a thermometer in the water to remain constant?

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.

The temperature of an ideal gas increases. Which of the following is true?

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.

An ideal gas undergoes an isobaric compression and then isovolumertric process that brings it back to its original temperature. Had the gas undergone one isothermal process instead...

A common misconception is that the work done in moving an object between two states is independent of the path followed. In the graph shown, the work done in going from point A to B to C by the isobaric and isovolumetric processes is equal to the area under the AB line. The work done by the isothermal process is the area under the curved line. Since the AC line includes all of the area under the AB line as well as the area between the AB and AC lines, more work is done on the gas in the isothermal process.

A heat engine operates between a high temperature of about 600C and a low temperature of about 300C. What is the maximum theoretical efficiency for this engine?

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, Eq. 15-5 gives the efficiency as only about 34%.

Which of the following are possible statements of the second law of thermodynamics?

All Carnot engines operating between the same two temperatures have the same efficiency, irrespective of the nature of the working substance.

A steel ring stands on edge with a rod of some material inside. As this system is heated, for which of the following rod materials will the rod eventually touch the top of the ring?

Aluminum is the only material that has a larger b value than the steel ring, so that means that the aluminum rod will expand more than steel ring. Thus, only in that case does the rod have a chance of reaching the top of the steel ring.

Two identical bottles at the same temperature contain the same gas. If bottle B has twice the volume and contains half of the number of moles of a gas 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.

Which of the following statements are true for an isothermal process?

During an isothermal process, the work done by the gas equals the heat added to the gas. An isothermal process is carried out at constant temperature.

Which statement is true regarding the entropy change of an ice cube that melts?

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.

You put 1 kg of ice at 0°C together with 1 kg of water at 50°C. What is the final temperature?

How much heat is needed to melt the ice? Q = m Lf = (1000g) (80 cal/g) = 80,000 cal How much heat can the water deliver by cooling from 50°C to 0°C? Q = cwater m DT = (1 cal/g °C) (1000g) (50°C) = 50,000 cal Thus, there is not enough heat available to melt all the ice!!

You put 1 kg of ice at 0°C together with 1 kg of steam at 100°C. What is the final temperature?

How much heat is needed to melt the ice? Q = m Lf = (1000g) (80 cal/g) = 80,000 cal How much heat is needed to raise the water temperature to 100°C? Q = cwater m DT = (1 cal/g °C)(1000g)(100°C) = 100,000 cal But if all of the steam turns into water, that would release 540,000 cal. Thus, some steam is left over, and the whole mixture stays at 100°C.

Metals such as brass expand when heated. The thin brass plate in the movie has a circular hole in its center. When the plate is heated, what will happen to the hole?

Imagine drawing a circle on the plate. This circle will expand outward along with the rest of the plate. Now replace the circle with the hole, and you can see that the hole will expand outward as well. Note that the material does NOT "expand inward" to fill the hole!!

A steel plate has a hole in it with a diameter of exactly 1cm when the plate is at a temperature of 20C. A steel ring has an inner diameter of exactly 1cm at 20C. Both the plate and ring are heated to 100C. Which statement is true?

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.

If the Sun's surface temperature falls to half the current surface temperature, by what factor will the radiant energy reaching the Earth change?

Radiation energy is proportional to T4. So if temperature is halved, radiation energy will decrease by a factor of 16.

A 1 kg block of silver (c = 234 J/kg °C) is heated to 100°C, then dunked in a tub of 1 kg of water (c = 4186 J/kg °C) at 0°C. What is the final equilibrium temperature?

Since cwater >> csilver it takes more heat to change the temperature of the water than it does to change the temperature of the silver. In other words, it is much "harder" to heat the water!! Thus, the final temperature has to be closer to the initial temperature of the water.

1 kg of water at 100°C is poured into a bucket that contains 4 kg of water at 0°C. Find the equilibrium temperature (neglect the influence of the bucket).

Since the cold water mass is greater, it will have a smaller temperature change! The masses of cold/hot have a ratio of 4:1, so the temperature change must have a ratio of 1:4 (cold/hot).

Two objects are made of the same material, but have different masses and temperatures. If the objects are brought into thermal contact, which one will have the greater temperature change?

Since the objects are made of the same material, the only difference between them is their mass. Clearly, the object with less mass will be much easier to change temperature since there is not much material there (compared to the more massive object)

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 subjected to the same change in temperature (but remain solid). How would the change in the rod's lengths compare?

Students may confuse thermal expansion with elasticity and surmise that the narrower rod would expand more for the same temperature change. However, in thermal expansion the change in length is independent of the rod's diameter or cross-sectional area.

Heat is...

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.

Given your experience of what feels colder when you walk on it, which of the surfaces would have the highest thermal conductivity?

The heat flow rate is k A (T1-T2)/l. All things being equal, bigger k leads to bigger heat loss. From the book: Steel=40, Concrete=0.84, Human tissue=0.2, Wool=0.04, in units of J/(s.m.C0).

A flat metal washer is heated. As the washer's temp increases, what happens to the hole in the center?

The hole in the center will expand.

Which of the following does the internal energy of an ideal gas depend upon?

The internal energy depends upon its temperature.

Which of the following possibilities could increase the efficiency of a heat engine or an internal combustion engine?

The maximum efficiency of an engine is given by Eq. 15-5, which can be written in the form e = (TH −TC )/TH. Increasing the temperature difference, as in (a), results in a higher efficiency. In (b) the temperature difference remains the same, while the hot temperature increases, which results in a lower efficiency. In (c) the efficiency increases as the temperature difference remains the same, but TH decreases. In (d) the temperature difference decreases, which lowers the efficiency.

Ten grams of water is added to ten grams of ice in an insulated container. Will all of the ice melt?

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 (0 010 kg)(4186 Q = mcΔT = . J/kg ⋅C°)(100C°) = 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.

It has been a hot summer, so when you arrive at a lake you decide to go for a swim even though its nighttime. 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?

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.

A steel tape measure is marked such that it gives accurate length measurements at room temperature. If the tape measure is used outside on a very hot day, how will its length measurements be affected?

The tape measure will expand, so its markings will spread out farther than the correct amount. When it is laid down next to an object of fixed length, you will read too few markings for that given length, so the measured length will be too small.

If both pressure and volume are doubled, what happens to the temperature of the ideal gas?

The temperature of the gas will increase by four times its original value.

Which of the following statements are true as a block of ice melts?

The temperature of the ice/water system remains constant.

About what percentage of the heat produced by burning gasoline is turned into useful work by a typical automobile?

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.

If the pressure acting on a given sample of an ideal gas at constant temperature is tripled, what happens to the volume of the gas?

The volume is reduced to one-third of its original value.

For objects at thermal equilibrium, which of the following is true?

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.

Which will cause more severe burns to your skin: 100°C water or 100°C steam?

While the water is indeed hot, it releases only 1 cal/g of heat as it cools. The steam, however, first has to undergo a phase change into water and that process releases 540 cal/g, which is a very large amount of heat. That immense release of heat is what makes steam burns so dangerous.

If you add some heat to a substance, is it possible for the temperature of the substance to remain unchanged?

Yes, it is indeed possible for the temperature to stay the same. This is precisely what occurs during a phase change—the added heat goes into changing the state of the substance (from solid to liquid or from liquid to gas) and does not go into changing the temperature! Once the phase change has been accomplished, then the temperature of the substance will rise with more added heat.

heat of fusion

heat required to change 1.0kg of material from solid to liquid

cyclic engines

internal energy over a cycle is zero, as the system returns to its initial state

heat removed from the system

negative

work done on the system

negative

convection

occurs when heat flows by the mass movement of molecules from one place to another. It may be natural or forced

isothermal

temp does not change, system remains in equilibrium throughout , Q=W

1cal

the amount of heat necessary to raise the temperature of 1g of water by 1 degree C

First Law of Thermodynamics

the change in internal energy of a closed system as work done by the system on its surroundings

Which of the following are IRREVERSIBLE thermodynamic processes?

the conversion of work to heat by friction the free expansion of a gas the spontaneous flow of heat from a hot body to a cooler body

metabolic rate

the rate at which internal energy is transformed in the body

heat of vaporization of water rises slightly as...

the temperature decreases

internal energy

total energy of all molecules

total heat required for a phase change depends on...

total mass and latent heat

radiation

what the Sun does

heat engines

work can be produced using thermal energy only when heat can flow from a higher temp to a lower temp