Question

So this is kind of a fun and thinking question...

Answer 1

For each of the scenarios below, determine how the total mass of the described object or system is changing.

A glass full of ice cubes melting
A cellphone battery discharging
A closed, thermally isolated box containing a controlled exothermic CHEMICAL reaction
A spring being slowly compressed
A hot stone cooling down
A closed, thermally isolated box containing a controlled exothermic NUCLEAR reacon

So, what is increasing in mass, decreasing in mass and unchanged.

@theEric

Answer 2

A glass full of ice cubes melting would be unchanged I'm thinking..

Answer 3

Well this is different! I agree with the first one! I mean, as long as you ignore evaporation and sublimation.

Answer 4

A cellphone battery discharging..that would be decreasing mass?

Answer 5

Yeah, it's my last question for now haha, thought I'd put up a fun one :p

Answer 6

I'd think the spring would be unchanged as well

Answer 7

Cellphone battery.. Well, the process of a battery is typically two chemical reaction requiring the loss or acceptance of electrons... But the chemicals stay inside the battery, right? And it needs to have about the same number of electrons let go from one terminal as accepted by the other for the reaction to continue...

As long as there is no net change in the number of electrons, I'd guess that the battery would have constant mass while discharging...

Answer 8

Cool.

And I agree. Spring changes shape, not mass.

Answer 9

A change in the temperature doesn't cause a change in the mass, so that hot stone should be constant as well.

Answer 10

That's a good observation, so the phone is unchanged

Answer 11

Obvious is A hot stone cooling down
here mass unchanged

Answer 12

A closed, thermally isolated box containing a controlled exothermic nuclear reaction, so since energy and mass are interchangeable, so this would kind of fit in every category I think, depending on rate of conversion right? But, for this case would be unchanged, as chemical reactions go they would be decreasing.

Answer 13

is*

Answer 14

The exothermic chemical reaction... Depending on the reaction, products could escape if the container is only thermally isolated, theoretically. But, at the same time, reactants could enter the system. Theoretically still, though, how would a particle escape without affecting the temperature. I'd say that it couldn't.... Maybe no change.

Nuclear; I don't know a lot about that. While radiation could escape, would it not have taken energy from within the system?

Thermodynamics is my weak point. Can you tell?

Answer 15

For the nuclear and chemical [exothermic] reactions, I really don't know! Does thermal insulation really limit the transfer of mass into or out of the system?

Answer 16

I know mass isn't conserved in chemical reactions right, but for nuclear I'm kind of lost, I'm not sure if it's unchanged or not...

Answer 17

Come on, @wolframalpha , keep it easy with special relativity :P
I just said that for irony.... I still think thermodynamics and statistical mechanics is just as tough or tougher.

Answer 18

Of course, I only got the basic special relativity stuff.

Answer 19

Yeah lol, I'm not sure! Wow, k so many of the things are wrong and we need to look at the energy conversions...

Answer 20

total mass depends on total energy

Answer 21

Oh.....

Answer 22

The first one; is that right? The energy is diffused, not gained nor lost?

Answer 23

In order to melt the ice, energy must be added to it, and a spring as the spring is being compressed potential energy increases, so the ice one is increasing mass? And spring is decreasing mass?

Answer 24

Battery discharging has lost energy... Spring has gained mechanical potential energy...

Answer 25

Stone has lost energy...

Answer 26

So it increases mass

Answer 27

Uh wth I tried the stone everywhere and it's all wrong haha.

Answer 28

losing energy increases mass?

Answer 29

The stone should be unchanged actually, since as energy is being added energy is also being lost internally?

Answer 30

Chemical reaction probably doesn't lose any energy? I still don't know!

Haha, three choices implies you should only need three guesses!
First guess is to check decrease, second to check unchanged. If one of them is right, you know it. If neither of them are right, it's the third option: increase.

If they were narrowed to 50% chance on two, it would take two guesses.

Why know things if you can answer multiple times and get feedback?

Answer 31

Naw, you lose marks every time.

Answer 32

The hot stone does have much energy added as it cools. And its energy is lost to its surroundings... Let me see if I can pull some thermo out...

Answer 33

Wait hold on it's the water and ice, I forgot about that

Answer 34

I'm sorry you've lost marks... That's prohibitive to my strategy....

Answer 35

It's alright, not much :P

Answer 36

I'm more into understanding then the marks anyways haha...

Answer 37

Yeah, water and ice seems to have energy exchange in the system more than to and from the system.

Answer 38

Bleh didn't do very well on it haha, but figured it out, here I'll take a pic for you :p http://puu.sh/bFYeG/9e07d4c797.png

Answer 39

Oh yay still got around a 100% in total :) so not a big deal.

Answer 40

Haha, well that's good! And thanks for the picture!!

Answer 41

So the glass is increasing in mass?

Answer 42

Np, it's good to know I suppose, if we ever encounter such a thing again haha...yeah the energy in the form of heat must be added to ice to melt it, so therefore the total mass of water increases as ice melts, I always thought it was unchanged, that's neat.

Answer 43

As a spring is compressed, its potential energy increases. Therefore, the mass of a spring increases as it is compressed.

Answer 44

A cellphone battery discharging, so that's decreasing.

A hot stone, as an object cools, its internal thermal energy decreases. Therefore, the mass of the stone decreases as it cools.

Nuclear reaction, in any exothermic reaction, potential energy (in this case, nuclear potential energy) is converted into thermal energy. However, since this reaction is taking place in a thermally isolated box, the thermal energy that is produced remains in the system, so the total energy is constant. Therefore, the total mass is constant.

Chemical, a closed, thermally isolated box containing a controlled exothermic chemical reaction, so it's unchanged as well.

Answer 45

Gaining energy? That one seems weird... The water that did not start as ice would lose energy I'd think (just because it would get colder and we're not assuming a pressure change). But the whole system... Maybe the surrounding air would pass energy to the ice and water...

Answer 46

Yeah, I'll ask my prof about it later, and get back to you :P

Answer 47

Okay. Are these all from the answers?

Answer 48

Yeah

Answer 49

Okay, cool! Feel free to post here what he says! I'll get a notification then.

Answer 50

Yup, np, btw thanks again for your help today :) you made it very fun and interesting!

Answer 51

You're welcome! It was a pleasure to work with you, so thank you! Haha, your questions helped, too. :) Take care!

Answer 52

Take care, and I'll get back to you sometime later, if not today or something I can always just message you with whatever reply I get back.

Answer 53

Sounds good! :D