A cool question for a hot hot weather - ice cream anyone?

Question

On a hot hot summer day I like to go to local supper market to buy a tub of my favourite ice cream that bring home in shopping bag. It is hot hot outside the bag, and it is frozen cold ice cream inside the bag.

Now, according to the first law of thermodynamics, heat always flows from a reservoir (an object at high temperature) to a sink (a object at low temperature) when both are placed in contact with each other. The mode of heat transfer this way is called conduction of heat energy.

The question is - if heat flows from hot outside the bag – the big giant reservoir at about 35 degrees – to the cold ice cream inside the bag - a sink for heat below zero degree. Then why the bag, in the path of the heat flow, should get cold?

Answer 1

The bag doesn't have much mass. The air doesn't have much mass. The ice cream has more mass near the contact point.

The ice cream can remove heat from the bag faster than the air can add heat to the bag.

Note: Why not just make your own ice cream. One of the best demonstrations of heat transfer is ice, whose melting point has been lowered below freezing by adding salt, surrounding the ice cream mix in the cannister. The hand cranked method is a lot of fun (uh, kind of). The ice cream is incredible, too.

Answer 2

You've got your laws mixed up. Its the second law that leads to heat flowing.

Though you can percieve the second law as leading to flow of energy, as the first law says energy cannot be created or destroyed this is not a particularly instructive way to look at things. What the second law really says is that entropy tends to increase. And this is another way of saying that the available energy gets shared out as much as possible.

Now for you ice cream, the contents of the bag are cold. Hence they represent a lack of energy. This means that more energy is located outside the bag. However large the reservoir outside the bag - even if it is infinite - it will contain less energy and the bag more if heat flows.

The bag itself, of course, will change temperature in this process. The key point about the bag is that it is a poor conductor of heat energy. This means the temperature on one side is higher than the other, so there is a temperature gradient within the walls of the bag.

Because the bag is a poor conductor it will feel warm - because it will not draw much heat from you hand whether it is hot or cold.

Once all of the ice cream melts there will be no temperature gradient, and the temperature of the ice cream, bag and outside will all be the same.

Answer 3

Because, at the same time that heat is being transferred to the ice cream, the cold is being transferred to the air. In nature everything tries to find it's equilibrium and so water finds it's own height and temperatures try to come to an average.

The cold from the ice cream is conducted through the bag to the air, and the heat from the air is passed through the bag to the ice cream. It goes both ways.

Answer 4

dude...at first i was about to second Jens answer (the "huh?").Because even I didnt understand it.but after a lot to head scratching like a monkey i think i understood your question.

You want to ask that since heat flows (only) from hot place to cold place.The Bag should get hot.But it is becoming cold instead.By this u imply that the first law of thermodynamics is wrong because it is defying the common everyday observation.
Well...I have got bad news for you.Your First Law is yours alone.No one else in the world will agree with you.First Law states that "Internal energy of system tends to increase if energy is added as heat and decrease if energy is lost as work done by the system."

dont ask me what system,Internal Energy..etc are.Find them out yourself for a good physics text book.The concept that should be applied here is that the heat keeps flowing either way until both of them(the hot body and the cold body) attain the same temperature.This state where they attain common temp is called "equilibrium state" or "equilibrium condition".
This is called the "Principle of Caloriemetry"(check spelling plz).It states that heat is countinously exchanged between two bodies(systems) until they reach equilibrium.
We have to apply this principle here.We also have to apply the "Zeroth Law OF thermodynamics" which is as follows:
A,B,C are three objects.A,B are independently in thermal equilibrium,without the effect of C.Now B,C are also independently in thermal equilibrium.This means A,C are also in thermal equilibrium.

So your ice cream,bag,air all are countinously exchanging heat until all three come to a common temp or equilibrium.Since ice cream is at 0 and air is at 35 . Heat will go to ice cream and since air looses heat,the air around the ice cream(and bag) will cool down.Till all three reach the same temp.But this wll never happen because air a massive source of heat and ice cream is nothing compared to it.If you wait long enough.All the 3 (ice cream,bag,air) will reach the room temp.ie your delicious ice cream will melt.So eat it before this happens.

Answer 5

Well, the bag does heat up a bit, but as soon as it gains heat from the air it transfers it to the ice cream. Furthermore, when you feel the bag as cold, it is simply transferring heat from your hand to the ice cream! It's not really a difficult concept.

Answer 6

I decide to take my dogs to the community creek the place the water is often chilly as a results of fact it comes nicely suited out of the mountain. whilst this is one hundred levels out it fairly is often 20 cooler interior the coloration of the trees and this isn't any longer too deep so she will splash around and not totally submerge herself (she does not like no longer being waiting to the touch the backside). She loves it there so taking her to a public pool does not be her thought of exciting. although this is effective to work out others doing so.

Answer 7

surface area of the bag allows its heat/ cold to dissipate into the air

Answer 8

It's when you eat ice cream your body gets warm but if eat chili your body gets cold.

Answer 9

Excellent Question. I'm going for a sleep

Answer 10

Huh?