chapter 7
why do aluminum pains heat food much more evenly than stainless steel pans when you cook on a stove?
Aluminum has a much higher thermal conductivity than stainless steel.
How would you estimate the temperature of a glowing coal in a fireplace?
By the color of the coal.
Why are good thermal conductors generally good electric conductors also?
Electron motions is responsible for determining both properties
Trace the transfer of energy that comes from your food.
In food that human eat there is stored chemical energy that is only released after a chemical reaction occurs in your digestive system. After the chemical reaction has occurs in your digestive system it will converted into mechanical energy for muscle movement, thermal energy for heat, and it could also be stored as a chemical energy again in a different form to be store as fatty tissue in humans.
Boiling temperature can be increased by
Increasing pressure
If you add a little hot tea to ice water at 0 °C, the mixture will end up at 0 °C as long as some ice remains. Where does the tea's extra thermal energy go?
It melts some of the ice
If you put a warm bottle of wine in a container of ice water, the wine will cool but the ice water won't become warmer. Where is the wine's thermal energy going?
It melts some of the ice
Some air fresheners are solid materials that have strong odors. If you leave these air fresheners out, they slowly disappear. What happens to the solid material?
It sublimes into the surrounding air
your body is presently converting chemical potential energy from food in to thermal energy at a rate of about 100 J/s, or 100 W. If heat were flowing out of you at a rate of about 200 W, what would happen to your body temperature?
It would decrease.
Why does a pot of water heat up and begin boiling more quickly if you cover it?
No heat is being carried away by evaporation water
A blacksmith is forging a small piece of steel. It emerges yellow-hot ( 1000 C) from the furnace and emits more thermal radiation than your entire body. How can such a small object emit so much thermal radiation?
Radiated power is proportional to absolute temperature to the fourth power.
Even on a very humid day the hot air from your blow dryer can extract moisture from your hair. Why is heated air able to dry your hair when the air around you can't?
Relative humidity drops with rising temperature.
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 100o C can burn you much more than water at 1000 C. Please explain.
Steam can burn you more than water at a higher temp because steam contains more energy due to the fact it needs a lot of it to change liquid water to vaporize.
Putting a clear plastic sheet over a swimming pool helps keep the water warm during dry weather. Explain.
Sunlight can penetrate through to the water, and no heat is being carried away by evaporating water.
It's often a good idea to wrap food in aluminum foil before baking it near the red-hot heating element of an electric oven. Why does this wrapped food cook more evenly?
The aluminum foil conducts the heat evenly to all parts of the food.
The strongest evidence for the Big Bang theory of the origin of the universe is the thermal radiation emitted by that explosion. This radiation has cooled over the years to only 3 K and is now mostly microwaves. Why should 3K thermal radiation be microwaves?
The blackbody spectrum for a 3 K object has its peak wavelength in the microwave region of the electromagnetic spectrum.
Why is it that you can put a metal pot filled with water on a red-hot electric stove burner without fear of a damaging the pot?
The heat is conducted through the pot into the water.
A doctor can study a patient's circulation by imaging the infrared light emitted by the patient's skin. Tissue with poor blood flow is relatively cool. What changes in the infrared emissions would indicate such a cool spot?
The infrared emissions from the cool spot would be less bright and shifted toward longer wavelengths.
Doctors use an infrared camera to look for inflammation, which appears as a hot patch on otherwise cooler skin. What differences would the camera observe at this hot patch?
The light from the hot patch would be brighter and have a shorter wavelength than light from the surrounding skin.
Why do mats and vegetables cook much more quickly when there are metal skewers sticking through them.
The metal skewers conduct heat to the interior of the food.
When cooking a potato in a conventional oven it is useful to stick a nail in it, which make it cook faster. this is because
The nail is a good conductor of heat.
When you put a loaf of bread in a plastic bag, there is still air around the bread. Why doesn't the bread dry out as quickly in the bag as it does when there's no bag around it?
The relative humidity of the air in the bag reaches 100%.
Explain how convection contributes to the shape of a candle flame.
The rising warm air cause the flame to engate upward.
You have a roll of papered foil that is shiny on one side and black on the other. You wrap a cold potato salad in that foil. Which side should be facing outward to keep the potato cold longest?
The shiny side
You have a roll of papered foil that is shiny on one side and black on the other. You wrap a hot potato in that foil. Which side should be facing outward to keep the potato hot longest?
The shiny side
on a bitter cold day, the snow is light and powdery. This snow doesn't begin melting immediately when you go into a warm room. Why?
The snow's temperature needs to rise to 0 C before it can start melting
Why are concrete sidewalks divided into individual squares rather than being left as continuous concrete strips?
The space between the squares gives the concrete room to expand on a hot day.
Astronomers can tell the surface temperature of a distant star without visiting it. How is this done?
They use the color temperature of the star's light.
A glass of ice water contains both ice and water. After a few minutes of settling, how do the temperatures of the ice and the water compare?
They're at the same temperature
In thermal equilibrium at atmospheric pressure, ice can exist only
at or below 0 C (32 F)
The neighborhood children are playing one has lost a pocketful of change. As result they all decide to do a close inspection of the sidewalk. They notice that there are gaps in between the sidewalk filled with a felt-like material. These gaps will prevent
high stress due to thermal expansion.
A pedestrian bridge crosses a street. This bridge is entirely supported by columns from below. A gap at each end of the bridges separates the bridge's surface from the sidewalks leading to the bridge. The width of each gap changes with time. This width is smallest
on hot days
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 downwards toward the fighter and the aluminum tens reflects most of that radiation.
At 100% relative humidity
steam and liquid water can coexist
When relative humidity is less than 100%
the leaving rate of water molecules exceeds the landing rate.
Ice is
unusual because it is less dense than water of the same temperature