Re: Energy consumed by a thermometer?
- From: tadchem <tadchem@xxxxxxxxxxx>
- Date: Mon, 26 Nov 2007 15:13:55 -0800 (PST)
On Nov 25, 12:15 am, timeOday <timeOday-UNS...@xxxxxxxxxxxx> wrote:
I was thinking tonight about thermometers (the kind that work on the
expansion/retraction of mercury or metal, not a digital one with a battery).
Moving the needle, or moving the mercury up and down, must take some
energy, dissipated as heat. But aren't these powered by heat in the
first place? So where's the entropy? Doesn't seem like the thermometer
should be able to change its configuration "for free," energy-wise.
Google: "Specific heat" AND "mercury"
The top hit is this:
http://www.iun.edu/~cpanhd/C101webnotes/matter-and-energy/specificheat.html
It explains the definition of specific heat in terms accessible to an
8th grader. The table gives the specific heat of mercury as 0.140
Joules per gram-degree C. This means that a gram of mercury (roughly
the amount in one ordinary mercury thermometer) will ABSORB (not
dissipate) 0.140 Joules as its temperature is raised 1 degree C.
[Technically, that number will vary a little as the temperature
changes, but not very much.] Raise the temperature 7 degrees C and
the mercury will absorb 7 * 0.140 = 0.98 Joules.
The heat provides the energy to make the mercury droplet *expand*. It
does not move the mercury "up and down". The thermometer works in all
orientations. Rectal thermometers will still work even if you are
standing up.
Tom Davidson
Richmond, VA
.
- References:
- Energy consumed by a thermometer?
- From: timeOday
- Energy consumed by a thermometer?
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