# Thread: thermodynamics

1. ## thermodynamics

Hi all on MHF,

I was wondering if anyone on here was good at physics (thermodynamics)? that stuff is out of this world.. lol

2. Originally Posted by dadon
Hi all on MHF,

I was wondering if anyone on here was good at physics (thermodynamics)? that stuff is out of this world.. lol
Some us used to be (that would be real thermodynamics), but I suspect that
thermodynamics that qualifies as highschool maths or physics is pretty elementary.

RonL

3. ## re

thanks for the reply,

oh thats good. il try posting a question on the weekend that im stuck on, if anyone one could help that would be great.

thanks

4. ## Re:

Hi all,

Is there any book you recommend for thermo?

Here is the question for thermodynamics:

The volume of my bicycle tyre is 1.5L. It is meant to be pumped to 40 psi guage ("guage" means above atmosphere pressure, ie. 55 psi absolute). The dimensions of the bicycle pump are 20mm diameter with a stroke of 300mm. How many times will I have to pump if the pump is 80% efficient on average?

I hope someone can help! lol

Thank you

5. Originally Posted by dadon
Hi all,

Is there any book you recommend for thermo?
I do not know thermodyanmics but I can bet that any textbook just on this topic involves very complicated math. I would recommend to buy a general physics book and just read the topic they have on thermodynamics.

6. Originally Posted by dadon
Hi all,

Is there any book you recommend for thermo?

Here is the question for thermodynamics:

The volume of my bicycle tyre is 1.5L. It is meant to be pumped to 40 psi guage ("guage" means above atmosphere pressure, ie. 55 psi absolute). The dimensions of the bicycle pump are 20mm diameter with a stroke of 300mm. How many times will I have to pump if the pump is 80% efficient on average?

I hope someone can help! lol

Thank you
If by efficiency it means that 80% of the mass of gass in the cylinder is
transfered into the tyre on one stroke we can do something.

The perfect gass law tells us that at constant temprature:

pV=Km,

where p is the pressure, V the volume, m the mass, and K a constant
which in this case will be related to the gas constant per unit mass,
temprature and the units that everything else is measured in.

(and while the temprature of the air in the pump will change during a stroke
the initial temprature in the pump will be ambient, and the final temprature
in the tyre will also be ambient. So the required calculations can all be done
using assuming constant initial and final temprature)

Then the mass transfered on a single stroke of the pump is:

m_p=0.8 x 10 x (60)/K.

The initial mass in the tyre is:

m_0=10 x (1500)/K

and the final mass:

m_1=55 x (1500)/K

(1500 is 1.5 litres in cc's, and 60 is the volume of the pump
cylinder in cc's)

So the change in the mass of air in the tyre is:

deltam=45 x (1500)/K

which is the mass transfered by:

N= [45 x (1500)/K] / [0.8 x 10 x (60)/K] =140.625

strokes of the pump.

(Note we can use dreadfully mixed units here as all the conversion
factors are absorbed into K, which convieniently cancells out)

RonL

7. ## Re:

Thanks for that captainblack! and the way you explained is very useful.

I have the answers somewhere but can't find them right now, when I do I will get back to you.

Thanks again.

8. ## re:

Hi captainblack,

I found the answer sheet for that question I get the answer of 75 times. I was told using the $P_1V_1=P_2V_2$ sould lead to the answer.