. The increase in kinetic energy will be the same factor, because the kinetic energy of the molecules is directly proportional to temperature.2) When a mercury thermometer is warmed, the mercury expands. What happens when the mercury expands? Do more molecules of mercury appear or does the distance between molecules increase?
Answer: Clearly there is no new mercury added or lost to the sealed thermometer. The mercury expands because the atoms are further apart from each other. This means they are not as tightly packed and that the density has gotten smaller.
3) Why is it sometimes easier to remove the lid from a tightly closed jar after warming it under hot running water?
Answer: Typically when stuff food is sealed in jars, it is sealed while hot. When the jar cools, and the air above the stuff cools, the pressure of the air decreases. By heating the jar, the air above the stuff in the jar is heated and will raise the pressure slightly, making it closer to atmospheric pressure and easier to open.
4) A closed bottle placed in a fire will suddenly explode. Why?
Answer: As the gas in the bottle heats up in the closed bottle, the pressure will rise. When the pressure is more than the bottle can hold, it will explode.
5) On the average, on a hot day do you think the air molecules are closer together, farther apart, or the same distance apart as on a cold day? Assume that atmospheric pressure is the same on both days.
Answer: The air molecules will be further apart on the hot day. When a gas is heated, it will try to expand, and make the density lower, so that the molecules are further apart.
6) A cuircular ring is heated from 20 °C to 80 °C. Will the hole in the ring become larger or smaller?
Answer: The hole will become larger. Remember the demonstration with the aluminum ball and ring I did in class. Another way to think about it is to imagine cutting the ring and bending it out into a straight piece of metal. When the straight metal bar is heated, it will expand in length. Now bend it back into a circular ring. The circumference has gotten larger, so the radius of the hole must have as well.
12) If the temperature of a gas is raised from 20 °C to 100 °C, by what factor does the kinetic energy of the molecules increase?
Answer: Converting the temperatures to Kelvin, 20 °C = 293K and 100 °C = 373K. The temperature has increased by a factor of . The increase in kinetic energy will be the same factor, because the kinetic energy of the molecules is directly proportional to temperature.
1) Room temperature is often defined as 68 °F. What temperature is this on the Celsius scale? On the absolute scale?
Answer: Using the formula 
we find 
. On the absolute scale, we just add 273K to the °C, so that this is 
.
2) On a hot day it is 90 °F outside. What is this in degrees Celsius?
Answer: 
9) If a gas is compressed to one-fifth its original volume but the temperature is kept constant, what happens to the pressure?
Answer: From Boyle's Law we know that 
. If the volume is one-fifth as big as it was originally, then the pressure must be five times as big. This is because five times one-fifth equals one, or 
, keeping the product PV constant.
11) If the pressure on a gas is tripled, what happens to the volume if the temperature is kept constant?
Answer: As in the previous problem, in order to keep the product PV constant, the volume must be one-third its original volume.
12) The temperature of a gas in a glass container is increased from 300K to 600K. How does the pressure change?
Answer: Because the temperatures are already in K, we can directly calculate the factor by which the temperature has been increased. We can easily see that the temperature has been doubled. In that case, from Charles' Law, the pressure will also double.
3) Why would you expect the water temperature at the bottom of a waterfall to be slightly higher than at the top?
Answer: The water in the waterfall loses gravitational potential energy and gains kinetic energy as it falls. When it reaches the bottom, it collides (inelastically) with the water below the falls. In an inelastic collision, the mechanical energy is transformed into heat energy, which should heat the water slightly.
5) Does an electric fan cool the air? Why or why not? If not, why use it?
Answer: An electric fan uses electricity, and the fan motor has resistance, so it actually generated heat energy, heating up the air around it. The reason a fan can cool us off is that it brings drier air to our bodies which can then carry moisture which has evaporated away from our bodies. The moisture carries away heat energy from our bodies when it evaporates, cooling us off.
7) The specific heat of water is quite large. Explain why this fact makes water particularly good for heating systems (that is, hot-water radiators).
Answer: Because water has a large specific heat, water can carry large amounts of heat. This heat can be used to heat a lot of air and make rooms warm.
8) Why does water in a canteen stay cooler if the cloth jacket surrounding the canteen is kept moist?
Answer: The water evaporating from the canteen requires thermal energy to turn into a gas and it gets part of this energy from the canteen, keeping it cool.
10) Why do you feel cool when you step out of a swimming pool into a warm breeze?
Answer: The water evaporating from our bodies requiers thermal energy to become a gas, so it gets part of that energy from us, cooling us off.
12) When your body overheats, it perspires. What is the value of the process?
Answer: In order to evaporate, the perspiration requires a large amount of thermal energy . This thermal energy is taken from our bodies and cools us off. As long as you have enough fluid in your body, this process can continue and will maintain your body temperature.
15) Explain why skin burns caused by steam are often so severe.
Answer: In order to become steam, each gram of water had to absorb 540 calories of heat energy. When the steam contacts your skin (which is a relatively cool 37°C ) it condenses and releases this heat energy to your skin. The more steam that touches your skin, the more serious the burn will be.
16) Will potatoes cook faster if the water is boiling faster?
Answer: No. The temperature of the boiling water can never get above 100°C, and the temperature determines the speed with which food cooks. The extra energy from the high flame goes into creating steam, not into cooking the potatoe.
21) At the same temperature, why does a tile floor seem so much cooler to your bare feet than a carpet?
Answer: The tile is a much better heat conductor than carpet. When your skin contacts the tile, heat will be readily conducted away from you; you feel cool. When your feet contact carpet, which is a poor thermal conductor, very little heat flows away from you; you feel warm.
25) Why do Bedouins wear several layers of clothes in the desert even when the temperature reaches 50°C (122°F)?
Answer: The layers of clothes trap air between them and air is a very good thermal insulator (poor thermal conductor). This keeps the bedouins cool even on the hottest day.
28) Why does the earth cool at night more quickly when the sky is clear than when it is cloudy?
Answer: The clouds act as an insulating layer and reflect heat radiation back to the earth as well as keep air from convecting freely upward.
30) Why does dirty snow melt faster than clean snow?
Answer: The dirty snow, which is darker, is more efficient at absorbing the suns radiation than clean snow. This energy can then be used to heat the snow and melt it.
1) How much heat (joules) is required to raise the temperature of 2.0kg of water from 0°C to 20°C? (for example when a two-liter bottle of water warms to room temperature from near freezing)
Answer: The heat energy required to raise the water temperature is 
. The starting temperature is 
, and the final temperature is 
. The specific heat of water is 
, which means it takes 4180J of heat energy to raise the temperature of 1kg of water by 1°C. The heat energy needed to heat the water is then: 
3) How much work must a person do to offset eating a 400 Cal piece of cake?
Answer: The food unit of energy is the Cal which is equivalent to the kcal, so 
. That means a person must do a total work of 
. This is the same amount of work expended by an 84kg man as he climbs an approximately 2000m mountain!