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/sci/ - Science & Math

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6355950 No.6355950 [Reply] [Original]

How is heat capacity measured?

Say you heat some unknown substance by one degree, how do you tell how much energy you have imparted it?

>> No.6355954

>>6355950
no it's, say you add some amount of heat to an object.
What's the temperature?
(measure temperature)
and derive the heat capacity

>> No.6355964

>>6355954
Well thanks but the question is how do you determine how much heat you have transferred to that object ?

>> No.6355974

>>6355964
Depends on the type of heat transfer.

eg you could use a resistor and the heat released is equal to the voltage times the current.
But then you have to know how effective the heat transfer is...and so on so forth.

What probably happens is that there's a flame like in your picture. They know the enthalpy of formation of the compounds created so they can calculate the flame temperature and they have some correction factor for the effectiveness of heat transfer.

>> No.6355982

>>6355974
They know the enthalpy of formation of which compound? The one that they are trying to measure the heat capacity of? How do they measure that enthalpy of formation in the first place?

Then how do they know how efficient is the heat transfer?

>> No.6355996

>>6355982
ok let me try again.
I stick a resistor inside a closed jar of water.
I create a voltage across it with a battery.
I measure the voltage across the resistor with a modified galvanometer.
I measure the current going into the resistor by setting up the galvanometer another way.
I now have the power lost to heat over the resistor.
I measure the temperature increase of the water by using a fluid that expands with temperature. I now have a relative temperature in which to compare other things to

>> No.6355999

>>6355996
and heat of course.
I measure the mass and then can derive a heat capacity.

Sorry I'm tired and not really thinking clearly.

>> No.6356010
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6356010

>>6355950
>not knowing what a BTU is

>> No.6356007

>>6355950
You can disregard every other post. The magic word you're looking for is calorimeter.

>> No.6356015

>>6355996
Okay nice I hadn't thought of putting a resistor inside the liquid directly, in my mind they were doing it with a flame or something.

However that only works for some liquids, how would they do it for a strong acid (can't put a resistor inside) or a solid?

>> No.6356023

>>6356015
> in my mind they were doing it with a flame or something.
yeah I knew they did it too in antiquity, but I couldn't think of how it was until >>6356007

> However that only works for some liquids, how would they do it for a strong acid (can't put a resistor inside) or a solid?
you need to start with something.
There's many many applications of heat transfer that you can abuse to find new ways to measure things.

>> No.6356031

Will look more into it thanks for the answers, calorimeter was indeed the word I was looking for. Cheers

>> No.6356032
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6356032

In order to determine heat capacity you must be able to quantify the amount of heat that has been imparted to the material/system.

Heat capacity is the amount of energy needed to change a material's temperature by one degree.

Fundamentally, heat capacity is a consequence of the internal energy of atoms/molecules. The more internal degrees of freedom a molecule has to vibrate, rotate, et cetera, the more energy it can possess in modes other than translational (which determine's temperature), so the higher its heat capacity will be. The internal and translational energy of molecules in a system interact though, so a translational interaction between two molecules (they bump together) will result in an exchange of both internal and translational energy. As you heat the material up, you "fill up" the translational and internal energy of the molecules, but the more capacity the molecules have for internal energy, the more energy you need to change the temperature. Theoretically this could be calculated from the bottom up using group theory and quantum mechanics but I'm not sure if this has actually been done.

>> No.6356660

This method will work with low degree of accuracy:

Step one: Start with 99°C water in a closed insulated container. Measure how long it takes the water to reach ambient temperature. Preferably, measure how long it takes for the water to drop 1 degree, between each degree (Because the water will exchange energy with the surroundings as a function of the temperature differential)

Step two: Do the same again but add some of your material at ambient temperature to the container. Measure how long it takes for your water to drop in temperature. The temperature will fall faster (Unless your material is interesting) because energy is leaving the container AND entering your material. The energy differential is now stored in your object.

Do keep in mind that the energy exchange with the environment is a function of dT so you can't just naively use your data directly.

>> No.6356670

>>6355950
Sounds like calories OP

>> No.6356873

>>6355950
Answer: http://en.wikipedia.org/wiki/Calorimetry