Princeton Ch 7 - Thermodynamics

Question 1

Define work in terms of energy transfter.

Answer 1

Work is a transfer of mechanical energy into or out of a system, from or to the environment.

Question 2

System.

Answer 2

The objects or objects under examination.

Question 3

Environment.

Answer 3

The environment is just the other objects and external forces outside of the system. The environment may or may not interact with the system.

Question 4

Closed system.

Answer 4

The environment cannot contribute matter to it.

Question 5

Isolated system.

Answer 5

The environment cannot contribute matter or energy to it.

Question 6

Open system.

Answer 6

An open system is free to interact with the environment.

Question 7

Temperature.

Answer 7

Temperature(T) is the macroscopic measure of thermal energy per molecule. Absolute temperature is measured in Kelvins.

Question 8

Heat (Q).

Answer 8

The transfer of thermal energy between a system and its environment. Measured in joules.

Question 9

Differences between temperature and heat.

Answer 9

Units (K versus Joules). Temperature is an intensive property and thermal energy is an extensive one. Temperature (like density) does not depend on the amount of material present but thermal energy does. A block of stone cut in half with temp of 300K is still 300K. But if it had 20,000 J of thermal E, it is now cut in half.

Question 10

Zeroth law of thermodynamics.

Answer 10

If one object is in thermal eq with a second, and the second object is in thermal eq with a third object, then the first and third objects are in thermal eq with each other. The two bodies are in such a way that heat is free to pass between them but the same amount of heat passes each way.

Question 11

If we measured the temperature of two objects, a rod and wooden stick to be the same, then if we put the two in contact, how much heat would be transferred between the two?

Answer 11

No NET heat would be transferred between them.

Question 12

Heat transfer is the movement of thermal energy from one point to another. There are three mechs by which this is acheived. Name them.

Answer 12

Conduction, convection, radiation.

Question 13

You touch a hot stove and burn your hand. Que pasa? What kind of heat transfer is this?

Answer 13

Conduction is the transfer of heat by microscopic collisions of particles and movement of electrons within a body. The highly agitated atoms of the hot skillet bump into the atoms of your hand, making them move more rapidly, thus heating up your hand.

Question 14

Convection.

Answer 14

The movement caused within a fluid (liquid or gas) by the tendency of hotter and therefore less dense material to rise, and colder, denser material to sink under the influence of gravity, which consequently results in the transfer of heat. A convection current is formed, and the fluid is heated completely.

Question 15

In convection, what happens to fluids near the heat source.

Answer 15

They gain energy, become less dense, and rise.

Question 16

Radiation.

Answer 16

Absorption of energy carried by electromagnetic waves (like radiant energy from the sun’s fusion reactions). Remember sunlight on your face warms your skin.

Question 17

True or false. Most materials expand as their temperature increases.

Answer 17

True. Moving particles = less dense.

Question 18

First law of thermodynamics.

Answer 18

The total energy of the universe is constant. Energy may be transformed from one form to another, but it cannot be created or destroyed.

Question 19

The equation for the first law of thermodynamics and why we should pay attention to signs.

Answer 19

ΔE = Q - W. Q is positive when heat is moving into the sys and neg when it’s coming out. W is positive when the work is being done by the sys on the environment. W is negative when work is being done by the environment on the system. Sometimes you’ll see ΔE = Q + W. Just note when its positive or negative.

Question 20

When we talk about an ideal gas in a sealed container with a piston on top, typically, KE = PE = __, and the only form of energy present is ___.

Answer 20

When we talk about an ideal gas in a sealed container with a piston on top, typically, KE = PE = 0, and the only form of energy present is THERMAL E. We use E(internal) = E(thermal) to indicate the total E of the system. In those systems, we have ΔE(system) = Q(net into the sys) - W (net by the system).

Question 21

In a piston, ΔE(internal) is (greater/less/proportional) to the object’s absolute temperature.

Answer 21

Proportional.

Question 22

Assume we have a metal container with a pin that prevents the piston from moving. We heat the container. What happens to ΔE(internal)? Q?

Answer 22

Since the piston doesn’t move, no work is done, but energy is transferred as heat. ΔE = Q, and Q is positive. W = 0.

Question 23

Assume we have an metal container with a pin that prevents the piston from moving. We heat the container and then let it cool at room temp. Describe ΔE(internal) for this process of cooling down.

Answer 23

The hot cylinder and gas will cool down as they lose heat to the room, until they’re at room temp again. ΔE = Q, and Q is negative. W=0. Energy is lost form the system in the form of heat.

Question 24

Assume we have an metal container with a movable piston. We heat the container and then add insulation so no heat is lost. Describe what happens in terms of work.

Answer 24

If we heat the container, the piston will move up. When the weight on our piston moves up, it gains PE (mgh, h gets bigger) from work being done by the gas (the system). If piston is insulated, such that no heat (Q) can go in or out, ΔE(internal) = -W; W >0. Energy is lost from our sys in the form of doing work.

Question 25

Assume we have a metal container with a movable piston. We heat the container then insulate it. What happens to the temperature?

Answer 25

Because the gas is losing energy as it expands and raises the piston, and we know ΔE(internal) is proportional to temperature, the GAS COOLS AS IT EXPANDS.

Question 26

Assume we have a metal container with a movable piston. What happens when we add a weight in terms of ΔE(internal)?

Answer 26

Because ΔV is negative (gas has been compressed), this will lead to negative work being done by the gas (which is same as positive work being done on the gas) Since ΔE(internal) has to increase because energy is being gained by our system. So GAS WARMS as it is being compressed. ΔE(internal) = +W, W<0.

Question 27

Isobaric process.

Answer 27

An isobaric process is one that occurs at constant pressure. In a piston situation, this is accomplished by having the weight of the piston constant, so the force divided by the area of the piston remains constant. On a P-V graft, W = PΔV, which is the area under the curve.

Question 28

Isochoric process.

Answer 28

An isochoric process maintains a constant volume. Heating the gas with a locked piston would result in increasing pressure but no change in V. W = 0, because there is no displacement of the piston.

Question 29

Isothermal process.

Answer 29

When heat is allowed to pass freely between a system and its environment, an isothermal process can occur, where the temp of the system remains constant. For a moving piston, following Boyle’s law, for a gas to expand at a constant T, the pressure must decrease. Since E is proportional to T, for an isothermal process, ΔE = 0; Q = W

Question 30

Adiabatic process.

Answer 30

The process occurs when NO heat is transferred between the system and the environment, and all energy is transferred as work. Ex. insulated (no heat lost) piston container. ΔE(internal) = -W.

Question 31

Describe the thermodynamic process.

Answer 31

A series of reversible processes through which an ideal gas passes that return it to its initial state function. ΔE = 0 for the overall cycle. Q = W. During a cycle, the system either converts heat to work (for clockwise) or work to heat (for the counterclockwise cycle).

Question 32

If a gas is held under pressure and the source of the pressure is released, like suddenly removing the piston from the canister, the pressure will spontaneously DECREASE to match that of the surroundings. This phenomenon is ____.

Answer 32

Free expansion. It is nearly instantaneous, so there is no time for heat transfer. If some sort of membrane holding in the gas in and the MB ruptures, there is nothing to push against, and no work is done either. ΔE = Q-W = 0 - 0 = 0.

Question 33

Second law of thermodynamics.

Answer 33

Entropy (a measure of the disorder of a system). The entropy of an isolated system either stays the same or increases during any thermodynamic process.

Question 34

Whether or not a system is closed, entropy will always increase.

Answer 34

If the system is closed, its entropy CAN decrease, but not without a corresponding greater increase in the entropy of the surrounding environment.

Question 35

True or false. It is impossible for a system to convert all of the input heat (disordered E) into work (ordered E) during a thermodynamic cycle: it must always output some heat as well.

Answer 35

True.

Question 36

A block is sliding across a table and then comes to a stop. If you measure the temperature during the sliding, you notice the temp dropping. Why?

Answer 36

Thermal energy was being transformed into kinetic energy during the sliding. Once it comes to a stop, KE is converted to thermal; but entropy does increase. You never see the block accelerate backwards.