Heat or Work: Changes in Thermal Energy

[eraemia]

BRITNEY SPEARS: Heat or Work?

Homework Statement

1. Which of the following represent a change in an object's thermal energy due to the flow of heat, rather than work being done on the object? You may choose more than one response.

a. Liquid nitrogen poured on a slab of ice boils furiously. Liquid nitrogen boils at 77 K.
b. A meteorite entering the Earth's atmosphere becomes white hot.
c. A hot cup of tea rapidly cools on a wintry day.
d. A drill bit gets hot as it drills a hole through a hardened steel alloy.

2.
A 150-g granite stone is heated to an initial temperature of 100 degrees Celsius. It is then placed in an insulated tub holding 150 g of water at 0 degrees Celsius. The water will cool down the granite so that the final temperature of both is 50 degrees Celsius.

a. True
b. False



2. The attempt at a solution

1.
Well, I think that the answer is b, and c
B and c seem to involve temperature differences
while d is clearly work flowing across the object's boundary, I'm not so sure about a, which seems to be an intentional kind of work

2.
True
Tf-Ti = 150-100 = 50?

 

[Kurdt]

What is required to do work? When you know what it is the answers become clear.

For number two, what happens when two different substances are in thermal contact? That is, what properties determine how heat transfers?

 

[eraemia]

well i figured out 2
.15*790 = 118.5 J/K
.15*4186 = 627.9 J/K

[ (118.5)(373K) + (627.9)(273) ] / (118.5+627.9) = 16 C
so it's false

but about 1, i know the definitions of heat and work
but is my thinking correct, that both b and c are demonstrations of heat, whereas a and d that of work? if not, where has my thinking gone astray?

 

[Kurdt]

well i figured out 2
.15*790 = 118.5 J/K
.15*4186 = 627.9 J/K

[ (118.5)(373K) + (627.9)(273) ] / (118.5+627.9) = 16 C
so it's false

but about 1, i know the definitions of heat and work
but is my thinking correct, that both b and c are demonstrations of heat, whereas a and d that of work? if not, where has my thinking gone astray?

For 1 there need to be a force to do work. Think about the meteorite entering the atmosphere. It experiences friction. Now think about the liquid nitrogen. Is there any force acting there or are they just in thermal contact.

Question two looks ok.

 

[TVP45]

Hints

Use common-sense everyday approaches. Classical physics almost always makes sense. If you want to see if there is work producing heat, ask yourself "Are 2 or more things rubbing together?" If you want to know if there is heat transfer, ask yourself "Are there temperature differences big enough so the final temperature fits in between the original hot and cold temperatures?" These are very simple questions and not very numerically accurate, but they point the way to go.

In regards to number 2, remember there is a great difference between temperature and heat content (energy).