Why mole and kelvin are basic units?

the_emi_guy

Interesting idea, but is it practical? Let's say I am in a biology lab and I need to measure the temperature of a blood sample. To have the thermometer read out in units of energy, it would have to be calibrated to the specific heat of this particular blood sample.

Khashishi

No specific knowledge of the specific heat of the sample is needed. The only difference is the unit of measurement. 1 K = 1.3806488*10^-23 J.
However, it isn't practical for everyday usage simply because of the huge difference in magnitude. Room temperature is on the order of 10^-20 joules.

Both Boltzmann's constant and Avogadro's number are used to connect the microscopic scale to human scale, and therefore have similar values. We could get rid of kB and just express temperatures in terms of J/mol, and the scales would be reasonable.

the_emi_guy

Khashishi,

Forgive me, I am still unclear on this. I thought that heat capacity was the conversion from energy to temperature.

As you mentioned earlier, the constant of proportionality between E and T is 1/2(K) per degree of freedom.

For simple substances (monatomic gasses, perhaps plasmas) we know how many degrees of freedom are involved so we can convert freely between E and T.

But how many degrees of freedom should we assign to a blood sample?

D H

No, we couldn't. Molar energy loses something in translation. Different substances at the same temperature do not necessarily have the same molar energy. Another problem is that the relation between energy and temperature is not linear.

The equipartition theorem is an idealization. It works great for ideal gases. It is not valid for real gases. It doesn't account for phase transitions, it doesn't account for the fact that specific heat is not constant for real substances, and it doesn't work for solids at low temperatures.

Khashishi

I thought I was clear. I was not suggesting to get rid of temperature altogether and just use internal energy. I was suggesting using energy units to measure temperature. It's still temperature, just with a different unit.

jd12345

I am a bit unclear about temperature. What is it actually?
I thought its the average energy of the molecules/atoms of a compound. But if it is that then we can write temperature in joules but we don't!

the_emi_guy

No.

For example, consider 1 liter of argon gas, and 1 liter of chlorine gas, both at the same temperature. The chlorine gas will contain about twice the thermal energy in Joules as the argon.

Khashishi -
Would those two bottles of gas be at the same temperature in your proposed units?