1. Why is Cp always greater than Cv?
Let 1 mole of an ideal gas be heated so that its temperature arises by a certain amount. If the volume is allowed to be constant, no work will be done. Therefore heat is needed only to raise the temperature and nothing else. So less amount of heat (Q) will be required for the purpose. Then according to the relation,
the value of C, i.e. Cv will be less.
However, if pressure is allowed to be constant, the volume has to be increased. So the system would have to do work also and it has to increase the temperature also by the same amount as before. So, more amount of heat (Q) will be required. Then according to the same relation,
the value of C, i.e. Cp will be more.
2. Why is the latent heat of vaporization of water greater than the latent heat of fusion of water?
The latent heat of fusion and
vaporization both involve the heat required to change the state of a
substance without a change in temperature. In the case of the latent
heat of fusion it is the heat required to change a substance from a
solid (ice) to a liquid (water) or vice versa while the latent heat of
vaporization from a liquid (water) to a gas (steam) or vice versa.
3. It is well known that water expands when it freezes, which is why frozen pipes burst. This means that water can do work as it freezes. Where does the energy come from for this to happen? This is especially puzzling considering that energy is being removed from the water in order to freeze it.
The answer is in the question. When
water freezes, it releases significant amounts of energy, called latent
heat. If the water is in a pond, this energy flows into the surrounding
air as heat. But if the water is trapped inside a copper pipe, some of
this energy is used to burst the pipe so the water can expand. This
means there is less energy available to flow into the surroundings in
the form of heat.
4. Why does heat and cold make things expand and contract? Also why do some metals expand more than others?
Recall that all materials are made up
of atoms. At any temperature above absolute zero (-273 0C)
the atoms will be moving. In a solid they will be vibrating in fixed
positions, in a liquid thy will be jostling past each other and in a gas
they will be whizzing past each other at very high speeds. When a
material is heated, the kinetic energy of that material increases and
it's atoms and molecules move about more. This means that each atom will
take up more space due to it's movement so the material will expand.
When it is cold the kinetic energy decreases, so the atoms take up less
space and the material contracts.
6. If temperature is 'The average kinetic energy of particles' (i.e. if you measure the temperature of a cup of water it is the average of all the water molecules in the cup), then how does one determine the temperature of a vacuum?
One doesn't determine the temperature
of a vacuum. Just as 'nothingness' has no color, taste, smell, etc. it
also has no temperature. That is because, as you point out in your
question, there are no particles whose kinetic energy can be measured or
7. Since the sun is one giant ball of gas, what force holds its consistent shape and size? Why doesn't it expand and burn up quickly?
The Sun maintains its size and shape
against the outward pressure of fusion energy by the force of gravity.
In other words, its own weight keeps the Sun from growing larger.
8. Why is it that when one is taking a hot shower, the shower curtain tends to be pulled inward, inside the shower (as opposed to being pushed outward)?
Hot air is less dense than usual air and therefore rises up. This takes effect in a shower where the hot water heats up the air on the inside of the curtain. This hot air rises upwards creating a very partial vacuum in the inside of the shower. This results in decrease in the pressure on the inside of the curtain as compared to the pressure on the outside of the curtain. This results in the curtain being 'pushed' inwards.
11. If you left the refrigerator door open, what would happen to the room temperature and why?
The room would get warmer! Think of a
refrigerator as a device that transfers heat from inside a box to its
surroundings. The room around a refrigerator is warmed as it receives
the heat removed from inside the box.
13. Will a black container of hot water lose heat faster or slower than a white container of hot water?
A black body is assumed to be a
perfect radiator of heat.
14. Books say that heat transfer in an adiabatic process is equal to 0. How can this be if there is a change in temperature?
You are probably making the common
mistake among students by not realizing that heat and temperature are
two completely different things.
dQ = 0
or, dU + P dV = 0
or, dU = - P dV
Heat can not flow out of the system in an adiabatic process, but work can be performed on it from outside and it can also be allowed to do work.
When the system is allowed to do work, it requires energy, which it obtains at the cost of the internal energy it already possesses. So the total share of the internal energy decreases, thereby decreasing the temperature overall (Remember, no heat has been extracted out).
15. How can you boil a liquid without heating it? Why and how is this possible?
The boiling point of a liquid depends
on both temperature and pressure. As pressure increases, so does the
boiling temperature. Pressure cookers are used in cooking to raise the
temperature at which liquids within will boil. Conversely, the lower
atmospheric pressure on a mountain top makes it harder to get boiling
water hot enough for good tea or coffee.
16. Considering only temperature and disrespecting the rest, would I feel cold or hot with my hand in the vacuum?
Your hand 'feels' hot or cold when
heat is transferred to or from it, respectively. There are only 3 ways
that heat can be transferred--by conduction, convection, or radiation.
Only the third (radiation) can take place in a vacuum. That is how the
Sun's energy is able to reach the Earth.
17. Is it possible to make a perfect vacuum?
Practically, it is impossible to make
a perfect vacuum. A perfect vacuum is defined as a region in space
without any particles.
18. A student was messing around with the Bunsen burner when he noticed the following - He had put a wire gauze on top of the burner before turning the gas on. When he used the lighter on the bottom portion i.e. below the wire gauze, a flame was only visible at that portion but not above the wire gauze. When he used the lighter on the portion above the wire gauze, the flame only appeared on the top portion but not the bottom portion! Why?
What a great observation! Most people have never even thought of
this, though they observe it often.
19. When one looks at a bedside light which is pointing upwards, one sees particles of dust near the light being sucked downwards towards the bulb. Convection should cause the opposite to happen, so what is going on?
I have often observed the phenomenon you mention. The reason is simple. The dust particles do travel upwards, but only up a thin "chimney" of rising air, heated strongly and rapidly by the bulb, which would be at several hundred degrees Celsius. The dust moves so quickly up this chimney that it is actually quite difficult to see. The reason it moves so quickly is the relative narrowness of the chimney compared to the powerful heating supplied by the bulb.
The dust you see moving downwards is coming from the cooler air away from the bulb. The shape of the convection current is rather like that of a bagel: the air is rising through the hole in the middle and so it has to move downwards around the outside in order to supply more air to replace that which has been heated and is rising. Because the outside edge of this bagel is larger than the inside edge, the air does not have to move as quickly to keep up, and so it seems to drift serenely downwards towards the bulb. The dust which appears to float straight down onto the surface of the bulb is actually behind the bulb, but because the dust particles are so small, it is hard to judge distance. The dust between you and the bulb and to the sides of it is not visible, as it reflects light towards the source, and not towards you.
20. I have always believed that in hot, sunny weather it is best to wear white or light-colored clothing because this reflects the heat. But a friend insists that the opposite is true and that black clothing is better because it helps to radiate heat from the body. Who is right? Or is a combination of the two--black on the inside and white outside--the best option?
You are right and your friend is wrong. Although it is true that dark objects radiate heat more effectively than light-coloured ones, the amount of heat radiated from a body is proportional to its absolute temperature to the power of four. This means that a human body does not lose much heat to the environment by radiative transfer. Most of the heat loss is by conduction, convection and (through sweating) evaporative cooling.
However, all of the energy from the Sun is transferred radiatively, so wearing black or dark clothes will help you to absorb it much more efficiently (and so make you very hot), while making only a minute difference to your ability to lose heat to the environment.
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