Physics for Exam 3
Which one releases more thermal energy when completely freezing, 2 kg of alcohol or 2 kg of water?
water
Why does jello stay cooler than sandwiches when both are removed from a picnic cooler on a hot day?
water in jello has more "thermal inertia," a higher specific heat capacity, than sandwiches
If 70°F air feels warm and comfortable to us, why does swimming in 70°F water feel cool?
Because water is a better conductor than air.
Why is whitewash sometimes applied to the glass of florists' greenhouses in the summer?
Because white paint reflects away the thermal radiation reaching it from the sun.
Suppose you do 100 J of work in compressing a gas. If 80 J of heat escapes in the process, what is the change in the internal energy of the gas?
20 J
which statement is true? 1. if object has a temperature of 100 degrees F it is twice as hot as an object that has a temperature of 50 degrees F 2. molecules inside the table and a chair in the same room will have the same total kinetic energy 3. 2 kg iron and 50 gram water will have the same average translational KE if they are in the same room
3. 2 kg iron and 50 gram water will have the same average translational KE if they are in the same room
Olive oil has a specific heat capacity of 2 kJ/Celsius Kg. How much energy do I need to supply to 5 kg of olive oil in order to increase its temperature by 3 degrees Celsius?
30 kJ
When can thermal energy in a system move from lower to higher temperatures?
when work is done on the system or adding energy from another source
The picture below shows what happens to a bimetallic strip when you heat it up. Which metal has a higher thermal expansion rate? Which way would it bend as it cools down?
white metal; downward
To keep your coffee hot, do you want a white cup or black cup? A shiny cup or a dull cup?
white reflects radiating energy back into cup (inside). Black outside energy absorbs outside energy. Same with shiny and dull.
Is it true that if I toss a coin 100 times, its more unlikely to have all outcomes landing on tails than half of outcomes landing on tails.
yes
Would a piece of watermelon stay cool for a longer time than sandwiches do when both are removed from a picnic cooler on a hot day? (hint: watermelon contains a lot of water, water has a very high specific heat capacity)
yes
Why can't you determine if you're running a fever by touching your own forehead?
your body is the same temperature
I put 1 kg of 40 degree Celsius iron in a 1 kg of water that is at 20 degrees Celsius. The final temperature of this closed system is:
below 30 degrees celsius
If 30 J of heat is added to a gas that does no external work, by how much is the internal energy of that gas raised?
by 30 J
Someone claims that they have created perpetual motion machine that does useful work without any energy input into the system. Which law of thermodynamics does their claim contradict?
first law of thermodynamics
Which law of thermodynamics consists of the conservation of energy applied to thermal systems?
first law of thermodynamics: when heat enters or leaves a system, the system gains or loses an amount of energy equal to the amount of heat transferred
heat always flows
from hot to cold
What three processes occur in a heat engine?
gaining heat, converting heat to mechanical work, and expelling the remainder
how does heat differ from thermal energy?
heat is energy in transit
radiation
heat transfer w/o matter
convection
heat transfers by movement of fluids
conduction
heat transfers by touching particles
Which contains the greater amount of thermal energy: a iceberg or a cup of hot coffee?
iceberg because it is bigger
When disorder in a system increases, does entropy increase or decrease?
increase
Same amount of heat is supplied to both a kilogram of water, a kilogram of granite and a kilogram of iron. Which one undergoes the greater change in temperature?
iron
Which undergoes a greater change in temperature when heat is applied: 1-kg water or 1-kg iron?
iron
What happens to the temperature of air when it expands?
it becomes less dense and moves up until the density of air is equalized
Gas loses 100 Joules of heat to surroundings and does 40 Joules of work on the surroundings. By how much does its thermal energy change?
it decreased by 140 joules
What happens to the density of water as it is cooled from 3°C to 1°C? Hint: ice crystals will start to form at 4 degrees Celsius.
it decreases
Does a good insulator prevent heat from getting through it, or does it simply delay its passage?
it delays passage
How does the specific heat capacity of water compare with the SHC of other common materials?
it has a higher specific heat capacity because there are more ways for water molecules to absorb energy without increasing translational KE
Desert sand is very hot during the day and very cool during the night. What does that tell you about its specific heat capacity?
it has a low specific heat capacity
What happens to the volume of water as it is cooled from 3°C to 1°C?
it increases
A great amount of water vapor changes phase to become water in the clouds that form a thunderstorm. Does this process release thermal energy or absorb it?
it releases thermal energy
if one object has a higher specific heat
it will be closer to it if mixed with a lower specific heat, it doesn't want to change
Which of the following could be units of power?
joules/hours
Why is air a poor conductor?
lack of constant contact between atoms/molecules
Which expands more for an equal increase in temperature: solids or liquids?
liquids
Heat transfer by convection takes place in:
liquids and gases
On a cold day you find that your key does not fit into the door lock. Which one has a higher coefficient of thermal expansion(contraction)?
lock
Water has a specific heat capacity of 4.2 (kJoules/kilogram/degree) while alcohol has a specific heat capacity of 2.4 (kJoules/kilogram/degree). If you mix equal amounts of water at 20C and alcohol at 0C what will the equilibrium temperature of the mixture be?
more than 10C
By what means is heat transferred by convection?
motion of fluid as it rises and sinks
gas compressing- work
negative
when heat flows out
negative
In a mixture of Neon, Helium , and Argon gases at a given temperature, the fastest molecules on average are those of:
neon
When the temperature of ice-cold water is increased slightly, does it undergo net expansion or net contraction?
net contraction
The first container with water is at 20 degrees Celsius. The second container is at 40 degrees Celsius. Is it accurate to say that the temperature of first is two times smaller than the second containers temperature?
no
a refrigerator moves heat from cold to warm. does this violate the second law of thermodynamics?
no
Which has a greater SHC: an object that cools quickly or an object that cools more slowly?
one that cools more slowly
under what conditions would a heat engine be 100% efficient?
only when the temperature is 0 degrees K, which is impossible
Which process creates a higher entropy state: Students are allowed to sit wherever they want, or students are told to keep one space (chair) between them (checkers pattern)?
sit wherever they like
we cannot change the temperature of something
that is changing phases
the lower something is
the faster it goes
temperature of an object is
the measure of the average translational kinetic energy per molecule that make up an object
radiation propagates at
the speed of light
Why are metals good conductors?
their molecules are close together
Does a hot object contain thermal energy, or does it contain heat?
thermal energy
Why does a bimetallic strip bend with changes in temperature?
thermal expansion
Why are liquids/gasses poor conductors?
they are thermal insulators because molecules are farther apart, particularly gasses
I have a plastic cup with water inside. Both, the cup and the water have the same temperature. Molecules of which one will have a greater average translational kinetic energy?
they have the same translational KE
You have equal amounts of Liquid A (initially at 0C) and Liquid B (initially at 100C). After mixing they reach an equilibrium temperature of 50C. Which liquid has higher specific heat?
they have the same value of specific heat
Why does a thermos often have a shiny light surface?
to reduce heat transfer by radiation
In lottery, which combination of numbers is more likely to come up and therefore have higher entropy?
5 numbers not in a sequential order.
which of the following does not violate any thermodynamic laws: 1. engine that converts all of the input energy into useful work 2. engine that does useful work and has cold and hot reservoirs at the same temperature 3. Engine that does mechanical work without any additional input energy and never stops working 4. Engine that does not waste any of the input energy 5. Engine that converts 90% of thermal energy into kinetic energy
5. engine that converts 90% of thermal energy into kinetic energy
What's the maximum theoretical efficiency of an engine that operates between a hot reservoir at 600K and a cold reservoir at 300K.
50%
If 100 joules of heat is added to a system that does 40 joules of work, by how much is the thermal energy of a system raised?
60 joules
thermal conductivity
A measure of the ability of a material to conduct heat
Suppose you do 100 J of work on the gas as you compress it. If 80 J of heat enters the gas from surroundings, what is the change in the internal energy of the gas?
Internal energy of the gas increases by 180 Joules.
A small piece of wood weights 0.0002 kilograms. If we were to make it disappear and convert all of its rest energy to a more useful type of energy how many Joules of energy would we have?
1.8*10^12 Joules
Suppose you do 90 J of work in compressing a gas. If 80 J of heat escapes in the process, what is the change in the internal energy of the gas?
10 J
If 100 joules of heat is added to a system that does no external work, by how much is the thermal energy of the system raised?
100 joules
A weightlifter lifts a 200kg barbell over 2 meters in 4 seconds. How much power does she exert in Watts?
1000 watts
A heat engine performs 5 000 J of work when energy exhausted to surroundings is 15 000 J. Find its efficiency. Hint: be careful to find total energy input first! (drawing a diagram might help).
25%
A heat engine performs 7000 J of work while discarding 21000 J into a cold reservoir. The temperatures of the hot and cold reservoirs are 400 K and 200 K respectively. Its efficiency is ___________ percent, and maximum possible efficiency that it could have is ________ percent. Hint for the first part: first figure out energy input into this engine. (energy input=work done by engine + energy discarded by the engine. )
25; 50
Which law of thermodynamics is related to a system reaching 0K?
3rd law: No system can reach absolute zero
What's the maximum efficiency of an Ideal engine that operates between a hot reservoir at 600K and a cold reservoir at 150K.
75%
A small piece of chalk weights 0.001 kilograms. If we were to make it disappear and convert all of its rest energy to a more useful type of energy how many Joules of energy would we have?
9*10^13 Joules
calculate the ideal efficiency for an engine in which fuel is heated to 3000 K and the surrounding air is 300 K
90%
Friends say cold "flows" from ice to your hand. What is your response?
Actually, it feels cold b/c energy moves from me to the ice as heat; heat moves from high to low
Which of the following can NOT happen because of the third law of thermodynamics?
An object was cooled down to temperature of -400 degrees Celsius
Why wouldn't you expect all the molecules in air to have the same speed?
Because heavier molecules will have a smaller speed if they have on average the same translational KE
During convection process, why does hot water rise?
Because it expands and therefore has less density
A heat engine performs 7 000 J of work when energy input is 14 000 J. The temperatures of the hot and cold reservoirs are 400 K and 100 K respectively. Find its efficiency, and maximum possible efficiency that it could have.
Efficiency = 50% . Max efficiency it could have is: 75%.
Which of the following is not a statement of first law of thermodynamics.
Efficiency of an engine can not be equal to 100%
Gas receives 100 Joules of heat. It does 60 Joules of work. By how much does its thermal energy change?
It increased by 40 Joules
What is the reason for ice being less dense than water?
Its crystalline structure has more space between water molecules than water
Which law of thermodynamics is related to the direction of heat transfer?
Second law: Heat never spontaneously flows from a cold substance to a hot substance
What is the reason why the interior of the car gets hot if you leave the car in the sun with closed windows.
The infrared radiation is trapped because glass reflects back the infrared thermal radiation emitted from the car's interior.
On a cold day, what type of doorknob would feel colder to touch?
a metal doorknob
Which will cool your finger faster: touching a nail stuck in ice or touching the surface of the ice?
a nail is a better conductor than ice
Which heat transfer mechanism is the reason why we place heater closer to the floor rather than ceiling? Which heat transfer mechanism do we try to avoid by covering the handle of the metal pan with plastic?
a) Convection. b) Conduction.
water is a better conductor than
air
how much energy can be removed from a system at 0 degrees kelvin?
all kinetic energy is lost so nothing can be removed
molecules of which substance experience smallest net restoring force acting on them? argon gas, honey, or wood
argon gas
In vacuum, heat transfer from radiation propagates:
at speed of light
You have equal amounts of Liquid A (initially at 0C) and Liquid B (initially at 100C). After mixing they reach an equilibrium temperature of 65C. Which liquid has higher specific heat?
b
Which of the following statements is FALSE about thermal radiation: a. Objects surface texture affects heat lost by radiation. b. Objects surface color affects heat lost by radiation. c. Table can not lose heat due to thermal radiation. d. Thermal energy from the sun can reach us only due to thermal radiation. e. Better thermal radiation emitters are also better thermal radiation absorbers.
c. Table can not lose heat due to thermal radiation.
Which statement is TRUE about the first law of thermodynamics? a. It states that efficiency can not be 100%. b. It states that heat spontaneously flows from hot to cold. c. It states that energy is conserved. d. It states that it is impossible for the object's temperature to reach absolute zero. e. It states that it is possible for the object's temperature to reach absolute zero.
c. it states that energy is conserved
Pick the statement below that is FALSE: a. Power is the rate at which work is done b. Power is the rate at which energy is transferred c. Power is energy transferred per unit area d. Unit of power is Watts
c. power is energy transferred per unit area
smaller specific heat
changes temperature more when mixed
As I walk barefoot, near seafront on a hot day, the pebbles feel hotter than the soil. Which heat transfer mechanism is responsible for that?
conduction
I notice that my thermos wall is made of glass rather than metal. This design probably reduces heat transfer through the wall by:
conduction
When cold, some animals fluff up their fur. Doing so reduces heat flow by:
conduction
I notice that my thermos wall has double layers of glass with vacuum between them. Having vacuum instead of air probably reduces heat transfer through the walls by:
convection
you put iron bar (irons atom's mass is 56 AU) next to copper bar (copper atom's mass is 64 AU). the iron and copper molecules have the same average velocity. which one has higher temperature? which way will heat flow?
copper; from copper to iron T= KEavg= mv^2avg/2
Which of the flowing is an example of heat transfer by conduction. a. Heat reaches us from the sun. b. Air above the fire is hotter than air besides the fire. c. Heat that leaves the house through the holes in the roof. d. The top of the spoon also gets hot if its other end is in hot tea.
d. The top of the spoon also gets hot if its other end is in hot tea.
Which of the following is FALSE about heat transfer by conduction. a. During this type of heat transfer heat flows from hot to cold. b. This type of heat transfer requires a medium. c. Good thermal insulators are bad thermal conductors. d. This heat transfer propagates at speed of light. e. Conducting objects need to be in contact.
d. this heat transfer propagates at speed of light.
Which of the following is a TRUE statement. a. Heat transfer by conduction propagates at speed of light. b. Thermal radiation propagates slower than conduction. c. Convection takes place in fluids: gases and solids. d. Humans constantly radiate and absorb but on average they radiate more heat than they absorb. e. If an object is a good absorber of radiation, it is a bad emitter of radiation and also a good reflector
dHumans constantly radiate and absorb but on average they radiate more heat than they absorb.
I removed hot milk from the stove. If I want to cool it down faster, I should pour it into:
dark matte surface cup
You have a shuffled deck of cards. You go through the deck, and first separate the suit, and then arrange cards in ascending order (from 2 to A) in each suit. What happened to the entropy of the deck?
decreased
Which of the following is a false statement? a. Unlike most of other substances water's volume can decrease with the increase of temperature for a certain temperature range. b. Interior of the car get hot by the same reason as the greenhouses temperature rises in summer and the same reason why Venus is hotter than we would expect it to be due to proximity to sun. c. If we are installing long telephone wires in summer we should make them a bit longer. d. When part of the glass cup is subjected to higher temperature the cup breaks due to thermal expansion. e. The temperature of air surrounding earth would be higher if there were no greenhouse gases in the atmosphere.
e. The temperature of air surrounding earth would be higher if there were no greenhouse gases in the atmosphere.
Which of the following is NOT the statement of second law of thermodynamics:
energy is conserved
term for amount of disorder in a system
entropy
All objects emit energy to their surroundings, so why don't all objects continuously decrease in temperature?
everything is also absorbing energy
heating an object
expands
thermal energy goes up
positive
when gas is expanding- work
positive
when heat flows in
positive
Heat transfer by which method do the astronauts reduce by wearing white shiny suits?
radiation
How does one feel "heat" laterally from a fire place?
radiation
What type of heat transfer does infrared thermometer use.
radiation
a glass of water is freezing, is it releasing heat or is it absorbing heat
releasing heat into surroundings
I often find headphones in my pocket tangled up but I have never found tangled headphones untangling themselves after removing them from my pocket. This is because of:
second law of thermodynamics
You put 1 kg of cold water at 20C next to 1kg of hot water at 80C. You come back in an hour and see that the first container is now at 10C and the second container is at 90C. Which law got violated?
second law of thermodynamics
cooling an object
shrinks
Which of the following actions would NOT decrease entropy of the system.
shuffling a deck of cards