Heat and Thermodynamics

While polishing a 1 kg brass statue you do 760 J of work against sliding friction. Assuming all of the resulting heat flows into the statue how much does the temperature rise pecific eat of Brass is 380 j/kg degrees C

2 degrees C (Q=mct --> 760=1*380t --> 760/380=2)

Why are good thermal conductors generally good electric conductors also?

Electron motion is responsible for determining both properties and you need a carrier to transfer heat and electricity

Which of the following quantities is not conserved?

Entropy (change in disorder)

Why does the handle of a metal spoon submerged in boiling soup feel hot?

Heat from the soup is transferred through the spoon by conduction

Why does an ice cube feel cold?

Heat from your hand enters the cube by conduction because heat goes from hot to cold

A shiny white transparent surface prevents heat transfer because

It has emissivity of zero and reflects all light that

Which temperature scale has 373 as the boiling point of water?

Kelvin

What makes it hard for radiation to transfer heat to the center of the potato?

Light can't go directly from the outside of the potato to its center so it has to go through layers. The potato blocks thermal radiation at every step along the way. Thermal radiation does participate in the heating processes, but it moves only short distances before being absorbed and must then be reemitted over and over again.

You are talking with your fiancée and looking up into a beautiful night sky and she comments "You know it is sad that we couldn't tell if there were any black objects out there no matter how hot they were because a black ob ect would never emit light at any temperature." You would comment

Nothing. Even though you know it is a misconception and that black objects absorb and emit radiation perfectly you don't want to ruin the evening.

"The entropy of an isolated system never decreases." Which Law of Thermodynamics is this?

Second

Suppose you are teaching a science course and have prepared a demonstration to teach how a thermometer works. When you plunge the thermometer into hot water one of your students points out that the mercury in the thermometer actually went down a little bit before it shot up. It behaved in such a way because

The glass expanded before the mercury had a chance to, creating space

You fill two identical mugs with coffee but the coffee in one mug is at a higher temperature than that in the other mug. You place the two mugs simultaneously in a microwave oven and turn it on briefly. As a result you add 1 joule of thermal energy to each mug. Which mug experiences the larger increase in entropy (if any)?

The mug containing the colder coffee experiences the larger increase in entropy

What makes it hard for conduction to transfer heat to the center of the potato?

The potato is a poor conductor of heat because it has no mobile electrons --> nonconducting so it can only conduct heat via the atom by atom bucket brigade. Heat flows slowly to the center of the potato via this mechanism.

Why does inserting a metal skewer through the potato help it cook more quickly?

The skewer is an excellent conductor of heat (a metal with mobile electrons) and carries it quickly into the center of the potato from outside. The skewer has mobile electrons to carry heat through it and provides an alternate path for heat to enter the potato and flow to its center. The skewer dramatically shortens the time it takes for the potato's center to become hot.

Soil heats up much faster than water when the two are exposed to sunlight. Use that fact and your understanding of heat transfer to predict which way the wind will blow near the surface of the earth as the sun rises near the seashore

The surface wind will blow from the water toward the land

Why do you feel cold (even on a very hot day) when you step out of a swimming pool and are wet?

The water in contact with your body evaporates because it is slowly vaporizing. This means that molecules are breaking free from the water droplets' surfaces. It takes a considerable amount of energy to break the molecules free and this energy comes from what the water is in contact with - your body.

"Two objects in thermal equilibrium with a third ob ect are in thermal equilibrium with each other." Which Law of Thermodynamics is this

Zeroth

People fighting forest fires carry emergency tents that have shiny aluminum outer surfaces. If there is trouble

a fire fighter can lie under the tent to block the heat from burning trees overhead. The tent helps because, radiation carries heat downward toward the fire fighter and the aluminum tent reflects most of that radiation.

The first law of thermodynamics is a re - statement of the law of

conservation of energy

Wind is one of nature's ways of transferring energy through

convection

It is a hot summer day and your friend's apartment has no air conditioning. However a brand new refrigerator lies unopened in the center of the living room. Being a creative person, your friend closes all the doors and windows of the apartment, unpacks the refrigerator, plugs it in, and turns it on. Your friend then opens the refrigerator door and uses a small fan to circulate the air past the refrigerator and throughout the room. After the refrigerator has operated for a few minutes, the average temperature in the room is

higher than it was before.

A pedestrian bridge crosses a street. This bridge is entirely supported by columns from below. A gap at each end of the bridge separates the bridge's surface from the sidewalks leading to the bridge. The width of each gap changes with time. The width is the smallest on ____ days

hot

Consider water that freezes into ice when put in your freezerso the water goes from being disordered to being relatively ordered. Please explain if the freezing of the ice violated the second law of thermodynamics

no because entropy of the universe will still increase and greater entropy increases because heat is displaced outward from the freezer

What heat transfer mechanisms would be inhibited by a clear thermos not evacuated

no transfer

Melting is a _______ ______ that requires energy ________ for the melting system

phase transition; input

What heat transfer mechanisms would be inhibited by a silvered thermos only

silvering prevents radiative heat transfer

You get a job as a dish washer to help with your college bills. The first thing the management tells you is to be very careful of the steam - it is exceedingly dangerous. In fact

steam at 100 degrees C can burn you much more than water at 1000 degrees C. steam energy > water energy because it takes energy to vaporize liquid water. steam retains energy and transfers it when in contact with other objects

What heat transfer mechanisms would be inhibited by a clear evacuated thermos

the evacuation prevents mainly convection and a small amount of conduction through air

entropy

the measure of a system's thermal energy per unit temperature that is unavailable for doing useful work or the amount of entropy is also a measure of the molecular disorder/randomness of a system.

What heat transfer mechanisms would be inhibited by a silvered evacuated thermos

the silvering prevents radiative heat transfer and the evacuation inhibits all convection and a small amount of conduction through the air

A popular classroom demonstration involves placing a paper cup with water in it on a burner and boiling the water in the cup. Although part of the cup may burn, the part containing the water does not because

the water absorbs most of the heat from the flame

What makes it hard for convection to transfer heat to the center of the potato?

the water in the potato is trapped in cells and cannot undergo convection so heat flows slowly to the center