# Homework Help: Why does sprinkling of water reduces temperature of a closed room?

1. Jan 26, 2006

### gaganpreetsingh

Two vibrating tuning forks produce progressive waves given by
y1 = 4 sin(500pt) ........ p = pi = 3.14159 and t is time
y2 = 2 sin(506pt).
Then how many beats are heard per minute?
-- How do I deal with it? If I try to add them trigonometrically, I am unable to express it as a product of two trigonometrical functions, which is necessary.

In a stationary wave pattern that is formed as a result of reflection from an obstacle, the ratio of the amplitude at an antinode and a node is b = 1.5. What percentage of energy passes through the obstacle?
-- Any hint please because I have no idea how to begin with this question.

Some gas at 300K is enclosed in a container. Now it is placed in a fast moving train. Then what will happen to the temperature of the gas?

The initial pressure (P) and Volume(V) are same. Two processes one isothermal and other isobaric are carried out both ending at the same volume. In which case is more energy given to the gas as heat?
-- I know Q(isothermal) = nRT ln(Vf/Vi) Vf = final volume, Vi = initial volume
Q(isobaric) = n Cp (T2-T1) Cp= Molar specific heat at constant P.
How do I compare these expressions?

Why does sprinkling of water reduces temperature of a closed room?
-- I think this is so because water absorbs the heat to get to the same temperature as that of the room. Am I right?

What will happen if a substance has infinite specific heat?

Which is better to extinguish a fire - hot water, cold water, ice?
-- I am confused with this as the answer is given hot water. Ice must be the answer, according to me, because it can absorb maximum heat before evaporating.

And last but not the least,
The capacity of a capacitor is 1F. It's resistance in a DC circuit will be infinite or zero?
-- If in a circuit there is a capacitor we initially replace it with a wire and after a long time we replace it with a broken wire. So the answer must be both. Am I right?

2. Jan 26, 2006

### Staff: Mentor

Yikes, what, are you saving these up?! :surprised

Do you know what causes beats? You should be able to answer this one by inspection, as long as you are clear what causes beats.

The reflection coefficient equation should help you on this one. What is the definition of the reflection coefficient? How is the shape of a standing wave altered if the left-going wave is smaller in amplitude than the right-going wave?

Sorry, I'm of no help on this one. Seems like the temperture might go up a little bit from the acceleration to the speed of the train, but I don't know how to calculate it.

Seems to depend on the temperature of the water going into the room. Hot water would heat the room up, not cool it off.

Dunno. It would suck the heat out of everything else?

Interesting question. Water does two things to help put out a fire. It blocks oxygen from getting to the fire, and it absorbs heat. Maybe the hot water turns to steam more easily, and the energy absorbed in the phase change is higher than just from the fire heating up cold water. Sounds like a good thing for some experimental trials.... (or a google search would be safer, I guess...)

Think about what a simple capacitor is. Two parallel plates with non-conductive stuff between them, like air or plastic or vacuum. What's the DC resistance going to be....? In a practical capacitor, you will get leakage current through the dielectric, so the DC resistance is just large, not infinite.

3. Jan 26, 2006

### gaganpreetsingh

>Two vibrating tuning forks produce progressive waves given by
>y1 = 4 sin(500pt) ........ p = pi = 3.14159 and t is time
>y2 = 2 sin(506pt).
>Then how many beats are heard per minute?
Beats are caused by waves that have close frequency and they interfere. For same amplitude we just subtract their frequency. But the question here has different amplitudes. Is the rule still applicable?

>In a stationary wave pattern that is formed as a result of reflection from >an obstacle, the ratio of the amplitude at an antinode and a node is b = >1.5. What percentage of energy passes through the obstacle?
>-- Any hint please because I have no idea how to begin with this >question
I have no idea about it. Because this question is the first one I have seen of this type. Looks like I need to read some theory about it.

4. Jan 27, 2006

### Staff: Mentor

Yes. Think more about what is causing the beat -- it's the temporary alighment of crests and troughs in amplitude as one waveform rolls in phase throught the other. For sound, our ears hear a louder sound at the moment that the two sound waves have aligning crests in the pressure waves flowing past us, and the beat is the quietest when the crests of one wave line up with the troughs of the other. It helps to use Excel or Mathematica or Matlab to play with waveform plots and additions to get a better feel for what is going on.

Definitely read about the reflection coefficient, how it relates to the characteristic impedance in the different domains that the wave is travelling through, and what you can use it for (like for solving this problem).

5. Jan 27, 2006

### gaganpreetsingh

The initial pressure (P) and Volume(V) are same. Two processes one isothermal and other isobaric are carried out both ending at the same volume. In which case is more energy given to the gas as heat?

Any help with this question?

6. Jan 27, 2006

### Staff: Mentor

You might want to post that last question as a new thread, with a descriptive title to help folks know if they can be of help in answering it. Something like "Isothermal versus Isobaric Process Question..."