April 30 Discussion

Q1, P1: Ideal Gas LawsQ2, P2: HeatFinal Review Questions (posted on Oncourse), particularly problems 18-27

Q1: Ideal Gas Law

Which of the following is a consequence of theideal gas law, PV=NkT=nRT?

A) Boyle’s law, PV=constantB) Charles’ law, V ~ T for constant PC) Gay-Lussac’s law, P ~ T for constant VD) All of the above.

A scuba tank, when fully charged, has a pressure of 195 atm at 20°C. The volume of the tank is 11.3 L.

(a) What would the volume of the air be at 1.00 atm and at the same temperature? (b) Before entering the water, a person consumes 2.0 L of air in each

breath, and breathes 12 times a minute. At this rate, how long would the tank last?

P1: Air in a Scuba Tank

Q2: Transfer of Heat

Two 1kg blocks of different materials are put in thermalcontact with one another, but are otherwise isolated. Block 1 is initially at temperature 100oC, and block 2 is at 130oC. The specific heat of block 1 is twice that of block 2. After the two blocks equilibrate, the final temperature is:

A) 105oC B) 110oC C) 115oC D)120oC E) 125oC

The heat into the colder object equals the heat out ofthe warmer one. Use Q=mcΔT.

2

1

1

2

2

12211

c

c

T

TTmcTmc

1302

1100 FF TT

So TF=110oC.

You are helping a friend who is a veterinarian to do some minor surgery on a cow. She has asked you to sterilize a scalpel and a

hemostat by boiling them for 30 minutes. You boil them as ordered and then quickly transfer the instruments to a well insulated tray

containing 200 grams of sterilized water at room temperature (23 oC) which is just enough to cover the instruments. After a few minutes the

instruments and water will come to the same temperature, but will they be safe to hand to your friend without being burned? You are

both wearing surgical rubber gloves, but they are very thin. You know that both the 50 gram scalpel and the 70 gram hemostat are made

from steel which has a specific heat of 450 J / (kg oC). Take the specific heat of water to be 4200 J / (kg oC).

P2: Preparing for Surgery

We want to know what is the final temperature of the instruments.Since they were immersed in boiling water, their initial temperature

was 100oC. Heat out of the instruments = heat into water.

minstruments = (50+70)g = .12kg csteel = 450J/kg-oC

mwater = .2kgcwater = 4200/kg-oC

minstruments csteel (TF – 23)= mwater (100-TF)

Solve this to find TF = 27.7oC or 82oF

…perfectly safe to touch. Water has a verylarge specific heat, so is good at cooling.