thermometer experiment

[chiddler]

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Clicky.

I've been staring at this for way too long. I can't seem to figure out how exactly this is functioning. I'd really like some help, please.

How is this thermometer working? Why must the mercury level on the left be 0?

 

[theseeker4]

Clicky.

I've been staring at this for way too long. I can't seem to figure out how exactly this is functioning. I'd really like some help, please.

How is this thermometer working? Why must the mercury level on the left be 0?

The gas bulb is placed in the sample to measure, and is warmed or cooled to the sample's temp. The gas expands against the mercury with an increase in temperature (and contracts with a decrease, obviously). Since the gas's pressure will obey ideal gas laws (or closely enough), the measurement of the pressure, as determined by the mercury height, can be used to find the temperature.

I imagine the mercury must be at 0 for the calculation to work simply because some baseline is needed for consistent comparison.

 

[chiddler]

The gas bulb is placed in the sample to measure, and is warmed or cooled to the sample's temp. The gas expands against the mercury with an increase in temperature (and contracts with a decrease, obviously). Since the gas's pressure will obey ideal gas laws (or closely enough), the measurement of the pressure, as determined by the mercury height, can be used to find the temperature.

I imagine the mercury must be at 0 for the calculation to work simply because some baseline is needed for consistent comparison.

can't the increased pressure push mercury back into the reservoir?

 

[chiddler]

When it is mentioned that "the mercury level on the left is brought to 0 on the manometer" this means the side nearer to the gas bulb, right? So as the temperature of the bulb increases, that level will decrease as pressure goes up. How is temperature measured in such a case?

Or how about the way it's on the image? What's the temperature on that?

 

[theseeker4]

can't the increased pressure push mercury back into the reservoir?

Yes, and this is counteracted by raising the reservoir to bring the mercury level on the left back to 0.

When it is mentioned that "the mercury level on the left is brought to 0 on the manometer" this means the side nearer to the gas bulb, right? So as the temperature of the bulb increases, that level will decrease as pressure goes up. How is temperature measured in such a case?

Or how about the way it's on the image? What's the temperature on that?

The reservoir is raised or lowered, and in the same way that a barometer measures pressure, the height of the reservoir relative to the 0 level on the left is used to determine the pressure. The higher the pressure, the higher the reservoir needs to be raised to bring the level of the mercury to the 0.

The 0 is a reference point, standardized based on the size and shape of the thermometer, the amount of gas in the gas bulb, the amount of mercury in the system, etc. Moving the level of the mercury to the 0 is simply a pre-set reference to allow use of the formula provided to calculate the pressure, and hence, the temperature.

For the picture provided, the reservoir would have to be raised until the mercury level on the left was at the 0. There is too little information to determine the temperature without it being moved.

The pressure of the gas on the mercury is equal to the pressure of the mercury on the gas since the level is stationary. Moving the reservoir up increases the mercury pressure in the left-hand tube, which is why the mercury level moves up that tube. The weight of the mercury is able to do work on the gas and compress it. When the mercury level is at the 0, the formula provided can be used to determine the pressure of the gas, and thus the temperature of the sample, because the 0 point was pre-determined when building the thermometer.

 

[chiddler]

I'm having difficulty understanding this. I'm not seeing why a 0 is necessary. Why can't any point be arbitrarily picked?

So the amount the reservoir is raised tells us how much pressure is exerted on the mercury and therefore the temperature?

Or are we using the ideal gas law somehow?

Is this right: We are finding out how much volume increase. We can use any arbitrary point. The volume increase can be used in the ideal gas law to find the new temperature. Yes?

We would have to know mol of gas in the bulb, initial temperature, initial pressure, and initial volume that the gas displaces. Then measure change.

 

[theseeker4]

I'm having difficulty understanding this. I'm not seeing why a 0 is necessary. Why can't any point be arbitrarily picked?

So the amount the reservoir is raised tells us how much pressure is exerted on the mercury and therefore the temperature?

Or are we using the ideal gas law somehow?

A 0 is necessary so the pre-calculation of the constant C can be made. If the same point is not used every time, the same constant C can't be used every time.

The system has a certain volume in the gas bulb, a certain mass of mercury, a certain n of gas molecules in the bulb, etc. Based on all of that, the 0 point is determined (exactly how, I don't know, but it would involved the ideal gas law and calculations).

Yes, the amount the reservoir is raised does tell you how much pressure is exerted on the mercury, and therefore the temperature. That is correct. The ideal gas law comes in in determining where, exactly, the 0 point is, which is already done for you so you don't need to worry about it.

Think about it this way: the ideal gas law is PV=nRT. n is fixed. R is fixed. Setting 0 at a single point that can be used consistently makes V fixed, meaning whenever the mercury is at the 0, the volume of the gas is x. This x is the same no matter what the pressure or temperature, so now V is fixed (as long as the mercury is at the 0). Since everything other than P and T are now fixed, the formula provided can now be used.

Another way to look at it based on the above: The formula T=Cp determines the temperature based on the pressure. The constant C is based (in part) on the volume of the gas when the mercury is at 0. You could use, say, the third line below 0 instead of 0, but that would require a different C.

 

[theseeker4]

Is this right: We are finding out how much volume increase. We can use any arbitrary point. The volume increase can be used in the ideal gas law to find the new temperature. Yes?

We would have to know mol of gas in the bulb, initial temperature, initial pressure, and initial volume that the gas displaces. Then measure change.

You could do this, but you either have to keep the pressure constant (which would require a manometer to measure the pressure exerted on the manometer...) or you have three variables instead of two (volume, pressure and temperature.) It is simpler to keep volume a constant as described in my reply a moment ago than to create new variables.

You are essentially correct, though, that a system that used volume change instead of pressure change would also work.

 

[chiddler]

oh! so the right side doesn't really matter, does it? as long as the left side is at 0 and we can measure the change in height of the reservoir, we are golden in finding temperature.

What is the purpose of the right side tube?

 

[theseeker4]

oh! so the right side doesn't really matter, does it? as long as the left side is at 0 and we can measure the change in height of the reservoir, we are golden in finding temperature.

What is the purpose of the right side tube?

I am pretty sure the right-side tube has to be open to the atmosphere for the manometer to work. The height of that tube shouldn't be looked at, only the mercury in the left-had tube.

 

[chiddler]

I am pretty sure the right-side tube has to be open to the atmosphere for the manometer to work. The height of that tube shouldn't be looked at, only the mercury in the left-had tube.

thanks so much for the excellent explanation