Honors Physics Ch.13-15 Conceptual Questions
What is the temperature of ice right after it is formed by freezing water?
0 C
Water expands significantly when it freezes: a volume increase of about 9% occurs. As a result of this expansion and because of the formation and growth of crystals as water freezes, anywhere from 10% to 30% of biological cells are burst when animal or plant material is frozen. Discuss the implications of this cell damage for the prospect of preserving human bodies by freezing so that they can be thawed at some future date when it is hoped that all diseases are curable.
A lot of those burst cells are needed. Those cells that cannot stretch to accommodate the frozen water are ruined. So which cells would be left up damaged when you thawed out. One work around is "flash freezing" where everything is frozen almost instantaneously so that the ice crystals don't have time to form in their snowflake patterns, but this has to be very quick and is hard to do on a larger scale.
Why does a beaker of 40.0oC water placed in a vacuum chamber start to boil as the chamber is evacuated (air is pumped out of the chamber)? At what pressure does the boiling begin? Would food cook any faster in such a beaker?
Boiling is when the vapor pressure equals the atmospheric pressure. As you decrease the outside pressure, the vapor pressure reaches this equilibrium point easier and the molecules of the liquid can escape more easily.
Heat transfer can cause temperature and phase changes. What else can cause these changes?
Changes in pressure, volume
What is the distinction between gas and vapor?
Gas is by itself, vapor is gas molecules that have escaped from a solid or liquid and hovering over the surface.
How is heat transfer related to temperature?
Heat energy is transferred from high temperature to low temperature and happens faster the greater the difference in temperature.
Under what circumstances would you expect a gas to behave significantly differently than predicted by the ideal gas law?
If the gas molecule is large, attracted to other gas molecules (polar like water), and at a cold temperature.
What effect does condensation on a glass of ice water have on the rate at which the ice melts? Will the condensation speed up the melting process or slow it down?
In order for water vapor to condense, it needs to lose heat energy, which would then go to the glass, then to the ice and increase the rate that the ice melts.
What does it mean to say that two systems are in thermal equilibrium?
It means that they are at the same temperature and that the rate of heat transfer in and out of the system is the same.
How does the latent heat of fusion of water help slow the decrease of air temperatures, perhaps preventing temperatures from falling significantly below 0oC , in the vicinity of large bodies of water?
It takes removing huge amounts of energy to freeze water. The energy removed from the water goes into the air, warming it. During a phase change, the ice/water temperature remains constant. In order for the ice to get below 0C, all of the water must freeze solid.
The brakes in a car increase in temperature by ΔT when bringing the car to rest from a speed v . How much greater would ΔT be if the car initially had twice the speed? You may assume the car to stop sufficiently fast so that no heat transfers out of the brakes.
Kinetic energy is 1/2 mv^2. Doubling the initial velocity increases the initial KE by 4 times. This energy has to be taken away by the work done by the brakes which converts the energy to heat. You would end up with 4 times heat and since q=mc delta T, you end up with a 4 times increase in temperature.
What is the vapor pressure of solid carbon dioxide (dry ice) at - 78.5oC ?
Looking at the diagram on page 469, the pressure is 1 atm. The normal freezing and boiling points of any material assumes that you are in normal air which is 1 atm.
What three factors affect the heat transfer that is necessary to change an object's temperature?
Mass, specific heat of the material and how much you want to change the temp
Gas is by itself, vapor is gas molecules that have escaped from a solid or liquid and hovering over the surface.
No, when we are talking about waters vapor pressure, we are talking about the pressure the water molecules exert which only depends on their mass and velocity and number. They are being held into the liquid by the same pressure as they would at sea level on Earth. The vapor densities would change with altitude, because as you go up, the atmospheric pressure decreases, which allows the liquid molecules to escape and become a gas easier.
Oxygen cannot be liquefied at room temperature by placing it under a large enough pressure to force its molecules together. Explain why this is.
Oxygen is a small, nonpolar molecule, so is not attracted to itself to form a liquid. You would have to squish it together to finally make it liquefy.
How is momentum related to the pressure exerted by a gas? Explain on the atomic and molecular level, considering the behavior of atoms and molecules.
Pressure is caused by the force of the molecules hitting the side of the container. Heavier and faster molecules will hit the side of the container harder, causing more pressure.
A constant-volume gas thermometer contains a fixed amount of gas. What property of the gas is measured to indicate its temperature?
Pressure. If the volume stays the same, the amount of gas stays the same (it's sealed), as the temperature increases, the pressure also increases.
Give an example of a physical property that varies with temperature and describe how it is used to measure temperature.
Resistivity - modern thermostats use the ability to conduct electricity to regulate when the heater turns on and off Density- not sure how used.... Volume and length - thermometers use volume of the liquid inside the thermometer, old thermostats use the length or the degree of curling of a bimetallic strip to complete a circuit turning on the heater.
Why does rubbing alcohol evaporate much more rapidly than water at STP (standard temperature and pressure)?
Rubbing alcohol molecules are less attracted to each other than water molecules, so they become a gas easier. Also, there are basically no alcohol molecules in the air so they don't have to compete as much as water molecules would.
If you place 0oC ice into 0oC water in an insulated container, what will happen? Will some ice melt, will more water freeze, or will neither take place?
Some of the ice will melt, but that will make some of the other water molecules freeze. The total amount of frozen and melted water will be constant, though. The rate of freezing will equal the rate of melting.
One method of getting a tight fit, say of a metal peg in a hole in a metal block, is to manufacture the peg slightly larger than the hole. The peg is then inserted when at a different temperature than the block. Should the block be hotter or colder than the peg during insertion? Explain your answer.
The block should be warmer than the peg so that the hole is larger, then when it cools, it closes up on the peg.
Why does condensation form most rapidly on the coldest object in a room—for example, on a glass of ice water?
The coldest thing is able to remove heat from the water the fastest since there is a greater difference in temperature. When it removes heat energy from the water vapor molecules, they slow down and change to a liquid.
When heat transfers into a system, is the energy stored as heat? Explain briefly.
The energy is changed to kinetic energy of the molecules. You cannot hold heat. Heat is energy transferred between two objects of different temperatures.
If you add boiling water to a cup at room temperature, what would you expect the final equilibrium temperature of the unit to be? You will need to include the surroundings as part of the system. Consider the zeroth law of thermodynamics.
The final temperature should be somewhere between room temp and boiling (100 C). Exactly where depends on the proportion of hot water to room temp water.
When a cold alcohol thermometer is placed in a hot liquid, the column of alcohol goes down slightly before going up. Explain why.
The glass expands first (feeling the heat from the hot liquid first), which causes it to dip down since the volume of the empty space has increases, then the liquid alcohol feels the heat from the glass and expands more than the glass does, shooting up.
Liquids and solids expand with increasing temperature, because the kinetic energy of a body's atoms and molecules increases. Explain why some materials shrink with increasing temperature.
The only example I can think of is water as it increases from 0 to 4 degrees C. This happens because at 0 degrees C the ice crystals are locked into their nice open hexagonal snowflake shapes. As they get warmer and melt, that crystal shape falls apart and the water molecules are free to move around more, and the crystal collapses making its volume smaller.
Find out the human population of Earth. Is there a mole of people inhabiting Earth? If the average mass of a person is 60 kg, calculate the mass of a mole of people. How does the mass of a mole of people compare with the mass of Earth?
There are about 7.5 billion people on earth, that is 7.5 x 10^9 people. A mole is 6.02 x 10^23 people..... not even close. You would need over a trillion earths (10^12) to get a mole of people.
A pressure cooker contains water and steam in equilibrium at a pressure greater than atmospheric pressure. How does this greater pressure increase cooking speed?
When the container is sealed, the boiling point (that equilibrium temperature) increases. Being hotter allows it to cook food faster.
Thermal stresses caused by uneven cooling can easily break glass cookware. Explain why Pyrex®, a glass with a small coefficient of linear expansion, is less susceptible.
With the smaller expansion coefficient, Pyrex does not expand as much, which puts less stress on the bonds holding the glass together
Does it really help to run hot water over a tight metal lid on a glass jar before trying to open it? Explain your answer.
Yes, the coefficient of expansion for a metal is larger than that of glass (25x10^-6 compared to 9 x 10^-6). So the metal will expand more than the glass and loosen the cap.
Can carbon dioxide be liquefied at room temperature ( 20oC )? If so, how? If not, why not? (See Figure 13.36.)
looking at the diagram, the bottom axis is temperature so finding 20 degrees C, and going up to the graph, you can see that if the pressure is greater than 56 atm, CO2 can be a liquid (l).
Describe a situation in which heat transfer occurs. What are the resulting forms of energy?
when your sweat evaporates - heat energy from your body is changed to the kinetic energy of the water gas molecules • When you heat water on a stove - heat from the burner increases the kinetic energy and temperature (really the same thing) of the pan, which then increases the kinetic energy of the water molecules.
