How Things Work Chapter 3
An ideally efficient heat pump adds 1100 J of heat to a warm room. How much work does it perform if it removes 1400 J of energy from the outdoors? (Think very carefully about this problem before you answer. If you look logically, the answer is obvious.)
0, It can't be done
Go to Wikipedia and look up the thermal conductivity of air. Which value below is closest to the correct value?
0.024 [Wm^-1K^-1]
One atmosphere is roughly the same as (select the best choice):
100,000 Pascals, 14.7 psi, 1 bar, 101325 Pascals
The load bearing piston in a hydraulic system as an area 50 times larger than that of the the input piston. If the larger piston supports a load of 6000N, how large a force must be applied to the input piston?
120 Newtons
If you submerge a 5-kg log in water and it displaces 14 kg of water, what will be the BUOYANCY force on the log in the moment you let go of the log?
140 Newtons Up
If you double the absolute temperature of a star, its power output increases by a factor of
16
An ideally efficient freezer cools food to 210K. If room temperature is 300K, and the machine consumes 75 J of work, how much heat was removed from the food?
175 J
If you double the radiating surface area of a star, its power output increases by a factor of
2
A log weighing 150 Newtons is pressed down into the water. The water it displaces would have weighed 200 Newtons. What is the buoyancy force on the log?
200 Newtons
A log weighing 150 Newtons is pressed down into the water. The water it displaces would have weighed 200 Newtons. What is the TOTAL force on the log?
50 Newtons up
An ideally efficient airplane engine provides work as heat flows from 1500 K burned gases to 300 K air. If 100 J of heat leaving the burned gases is converted into work, how many Joules of work can you get from this engine?
80 J
If you submerge a 5-kg log in water and it displaces 14 kg of water, what will be the net force on the log in the moment you let go of the log?
90 Newtons Up
An ideally efficient solar energy system produces work as 100 Joules of heat flows from the 6100 K surface of the sun to the 300 K room air. How much of that solar heat is transformed into work?
95 J
Why does it take so much more energy to warm water by 2 degrees C than to warm air by 2 degrees C?
All choices are correct
What is the turboencabulator good for? (Choose best answer from below.)
All of the above (lol.)
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.
Although the two substances are at the same temperature, steam has much more energy than the same mass of water because it takes energy to vaporize liquid water. Steam retains this energy and transfers it when in contact with other objects.
Why are good thermal conductors generally good electric conductors also?
Electron motion is responsible for determining both properties.
Which of the following quantities is NOT conserved?
Entropy
Balloons are buoyant because they have MORE air molecules in the balloon area than the normal air which the balloon is displacing.
False
Heat naturally flows from cold to hot.
False
In an evaporator, the coolant condenses. This phase change allows for heat to flow out of the evaporator even though the refrigerant remains at the same temperature.
False
Pressure is area per unit force.
False
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.
The throttling valve in a refrigerator
Is an adjustable nozzle, can make things cold (both)
In making ice cream, the flavored cream is in a container in contact with melting ice. When the ice cream freezes it give energy up to the ice / water mixture. What happens to the temperature of the ice / water mixture?
It will not change because the energy is going into its melting the ice.
What is a heat pump primarily designed to do?
Move heat from a low temperature region to a high temperature region.
Which temperature scale has 273 as the boiling point of water?
No scale
Consider water that freezes into ice when put in your freezer, so the water goes from being disordered to being relatively ordered. Although locally the ice froze and its entropy went down, there is a greater entropy increase due to the heat being dissipated outward from the freezer coils. So when you add it all up, the entropy of the universe decreased.
No the second law is not violated; the entropy of the universe still increased. Although locally the ice froze and its entropy went down, there is a greater entropy increase due to the heat being dissipated outward from the freezer coils. So when you add it all up, the entropy of the universe increased.
Who invented the internal combustion engine?
Otto
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.
What does the "R" in "R-value" stand for?
Resistance
Moments before it is ignited by the spark plug, the mixture of fuel and air inside an automobile cylinder is compressed to very high density. During the compression process, the mixture's
Temperature rises dramatically and so does its pressure.
What causes knocking in your car's engine?
The fuel ignites before it should.
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 hotter coffee experiences the smaller increase in entropy.
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.
A perfect "black" object has an emissivity of 1.0. That means it absorbs and then re-emits whatever radiation falls on it.
True
A refrigerator compressor takes liquid and turns it into gas.
True
Bernoulli's equation says that the total energy per unit volume along a streamline stays constant.
True
Evaporation is a forth method of heat transfer--it carries off energy when a molecule "leaps the bonds" of a liquid's surface and careens away as a gas.
True
Heat pumps can be used to both cool things off (as with a refrigerator), and warm things up (as with warming a house in the winter).
True
If I send heat from a tank of hot water to an immediately adjacent tank of cold water, the amount of energy leaving the hot tank is the same as that arriving in the cold tank.
True
If two panes of glass are separated by more than about five eights of an inch, and have only air in between the panes, they can begin to have convection. The air begins to swirl in between the panes, making the windows very poor insulators.
True
In most situations, gauge pressure will be smaller than absolute pressure.
True
It is possible for a process to decrease entropy in one area, as long as it more than makes up for it by increasing even more entropy elsewhere.
True
It is possible to get hypothermia (lowering your body temperature to a dangerous level) by spending a long time without any protective clothing in water that is kept as warm as 85o F. This is because water has a very high specific heat, so it is able to store and release a lot of energy. Since the surrounding water is at a lower temperature than your body, heat transfers from you to the water. Since the water's temperature is kept steady, heat will continue to transfer until you start to run out of energy.
True
It may be a waste of money for someone to invest in a tank full of premium gasoline because premium gasoline is designed to prevent knocking, meaning that it will ignite at a higher temperature than lower grade blends of fuel. This means, however, that to take advantage of premium fuel your car must have a high compression engine which will have a high enough ignition temperature and most ordinary car engines are not in this category.
True
Low e-glass reflects 95% of ultraviolet light, and 70% of infrared light, but it allows most visible light to pass through the window. The reflection of ultraviolet light helps keep your carpet and furniture from fading. The reflection of infrared light helps keep heat from the outside away during the hot summer months, and helps keep heat in your house during the cold winter months.
True
Some biologists imagine what it would be like to have warm - blooded fish. If fish really were warm - blooded they would have to increase their food intake by a considerable amount. Whales and marine mammals get around this by having thick layers of fat. Water has a very high specific heat and drains a lot of heat energy from something in it whose temperature is higher than the surrounding water temperature. Fat, however has a very low thermal conductivity and is insulating, greatly reducing the amount of energy the animal loses to its surroundings. Therefore marine mammals are able to maintain their body temperatures with reasonable diets because of the large amount of insulating fat they have on their bodies.
True
Ultraviolet light is what causes carpets and paint to fade.
True
Vitamin D is naturally produced by your body when your skin is exposed to the electromagnetic radiation that is sunlight. If you live in northern climates, especially if you have naturally darker skin, or you don't have much exposure to the sun, it's wise to explore supplementing your diet with vitamin D to avoid cancer, depression, and many other negative outcomes.
True
You cannot see infrared radiation, just as you cannot see microwave and radio wave radiation. But even so, infrared radiation can hurt your eyes (and even give your skin a "sunburn").
True
You feel cold (even on a very hot day) when you step out of a swimming pool and are wet because even on a hot day, the water in contact with your body evaporates—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.
True
Your car engine is generally more efficient when its ignition temperature is higher because engine is basically a thermodynamic heat engine, complete with an operating temperature, exhaust temperature and useable work output. Since the available work output is proportional to the difference of the hot (ignition) temperature and the cool (exhaust) temperature, a greater ignition temperature means more available work output and hence more efficiency.
True
How can you reduce knocking in your car's engine?
Use higher-octane gasoline
In the classic movie The Sand Pebbles, "dead steam" was used as to refer to
Water
Wind is one of nature's ways of transferring energy through
convection
Pressure times area is equal to
force
The "strokes", or "cycles" in a four - stroke engine are
induction, compression, power, exhaust
The _______ the temperature difference between hot and cold, the ________ the fraction of heat you can divert and transform into __________.
larger; work
Which of the following are forms of electromagnetic radiation?
microwave, infrared, ultraviolet, radio
You are a mechanic in a garage and are working on a carburetor that is made of metal, has a glass inspection window and plastic gears for the choke. If all three types of material are at the same temperature and each type of material is in contact with the other two, then (hint: yes, this is really easy)
no heat will flow between any parts.
Melting is a _______ ______ that requires energy ________ for the melting system.
phase transition; input
Freezing is a _______ ______ that requires energy ________ for the melting system.
phase transition; release
A thermoelectric cooler is a type of heat pump that uses electric power to move heat against its natural direction of flow. In other words, it takes energy from a system and transfers it to another system which is at a higher temperature. In such a heat pump
the cooler requires power input because the flow of heat from a cool region to a warm region alone would decrease the entropy (disorder) of the system.
Housing and natural animal insulation works because
the insulating materials trap air and prevent convection.
When cooking a potato in a conventional oven it is useful to stick a nail in it, which makes it cook faster. This is because
the nail is a good conductor of heat
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.
