Specific Latent Heat and Thermal Physics

[Peter G.]

Hi

The mass of liquid nitrogen in an open beaker is found to have decreased by 46.3 g in 10 minutes. If the s.l.h of vaporisation of nitrogen at its boiling point is 1.99 x 10⁵, at what rate were the surroundings heating the beaker? Why is the heat capacity of the beaker irrelevant?

Ok, my attempt:

Q / T = m / T x L
Q / 600 = 0.0463 / 600 x 1.99 x 10⁵
P = 9213.7 J/s

The reason why the specific heat capacity of the beaker is irrelevant is because during the process, the temperature of the liquid nitrogen does not change and the beaker would already be at the same temperature as its contents due to thermal equilibrium.

2. An open dish of liquid is very slightly cooler than its surroundings. Why? Your answer should include an account of why it's temperature is steady, and the factors which determine the steady temperature.

I believe the reason why the temperature is slightly slower is because of evaporation: The molecules with the most energy are able to escape, leaving only the cooler, less "energetic" molecules behind. I don't understand however, the second part of the question. Could anyone give me any tips?

Thanks,
Peter G.

 

[Redbelly98]

#1 looks good. (Though if I were grading it, there would be a minor deduction for ignoring units in quantities )

#2. If the temperature is steady, what does that say about the net heat flow into/out of the liquid?

 

[Peter G.]

Hi,

Yeah, stupid me for not putting in the units

But, in number 2: If the temperature is steady there's not net heat flow into and out of the liquid even though it is cooler than the surroundings?

And if possible, can you help me with the factors? I can't find any information whatsoever in my textbook for this question.

Thanks once again,
Peter G.

 

[Redbelly98]

My apologies for not responding sooner.

Hi,
... But, in number 2: If the temperature is steady there's not net heat flow into and out of the liquid even though it is cooler than the surroundings?

Yes, that's right. And there is heat flowing both into and out of the liquid. So, also think about how or why heat flows into, as well as out of, the liquid.

And if possible, can you help me with the factors? I can't find any information whatsoever in my textbook for this question.

If you think about how heat flows into the liquid, next think about what factors affect the rate of that heat flow. Same for heat flowing out of the liquid.

 

[Peter G.]

Hi,

I handed in the H.W and for the factors I put: For the factors affecting the rate at which heat flows into the liquid: The temperature difference gradient. For the rate at which the heat flows out of the liquid I stated the factors that affect evaporation: Pressure, temperature, humidity, surface area presence/absence of breeze

Thanks for your help,
Peter G.