# How is temperature transferred?

Since temperature is a function of movement, how does the lack of movement in one molecule contribute the the lessening of movement in an adjacent molecule?

To put it another way, if I have an ice cube and drop it into a cup of water, how does the lack of movement in the ice slow down the movement of molecules in water? Or if I blew air onto hot soup, why doesnt the movement of air excite the soup molecules and transfer energy into them, thus making it hotter?

Heat always flows from an area of high concentration (hot place) to an area of low concentration (cold place). The water heats up the ice, giving up some of its heat in the process.

Instead of one very localized area of 32[sup]o[/sup] and a more generalized area of, say, 77[sup]o[/sup], you wind up with a more or less homogenous area of, say, 68[sup]o[/sup].

If one person with no money came into a room with 9 people who all had \$10 each, and every one of those nine people gave the new person \$1, then the room would have 10 people each with \$9. Same total amount of money (heat energy) spread among more people (molecules).

I think you have your understanding of heat transfer slightly backwards. Cold is not transferred, only heat, from a warmer source to a colder destination. Someone else will doubtless be along to provide a more technical explanation, but simplistically, the molecules in the warm object hit the molecules in the cold object, making them speed up and correspondingly making the warm molecule cool down a bit. The overall movement of the molecules as a group, e.g. the air in your soup example, is not as important as the individual molecular movement. Your breath is cooler than the soup, and so the soup molecules impart their kinetic energy to the air molecules you send at it.

Valete,
Vox Imperatoris

ETA: Like KneadToKnow said, eventually everything will be the same temperature. This also applies to the universe as a whole, by the way.

Heat is transferred via three separate mechanisms:

1. Conduction
2. Convection

I’ll touch on your example about blowing on the soup. This is an example of convection between the soup and the air. Heat transfer by convection is directly proportional to three values: the temperature difference between the two fluids (soup and air in this case), the exposed surface area, and the convective heat transfer coefficient. Let’s consider the soup in two situations: 1) Sitting alone on a table, and 2) Still sitting on a table, but with you blowing over the top. In both cases, the area and temperature difference is the same. However, blowing over the top increases the convective heat transfer coefficient, so the soup cools faster.

I think YogSosof asks a very perceptive question that isn’t getting answered.

It has to do with randomness. Atoms and molecules are always somewhat massive and have somewhat elastic force and distance relationships with each other, primarily with their neighbors. It is a fairly accurate and useful model to imagine a three dimensional network of springs that are joined together at weights. The weights jiggle around, and carry kinetic energy by virtue of their motion. The springs are compressed or stretched and carry potential energy for this reason. Molecules are doing the same thing in a solid (liquids and gasses work a little differently but no need to get into that yet).

If you have a jiggling network and a quiet network, and you suddenly join them together at multiple points, the quiet network will start moving because it is disturbed by the jiggling network. And, because it is doing work to the quiet network, the jiggling network will lose some of its energy and will jiggle less vigorously.

It is possible to configure networks to be jiggling in such a way that when they get connected together, they act no differently. But this would require very special combinations of motions in each. Given that the motions are random in a certain sense in any real networks, all that matters is the ability of each network to absorb or release energy when it contacts another network. When the motions are at the molecular scale, this is still true, and the ability is referred to as temperature.

The field of thermodynamics deals with the meaning of the energies and flows and the potential to transfer energy. More specifically, the field of statistical mechanics deals with the motions themselves, as discrete motions and as statistical distributions.

The ability of things to glow when they get hot is a direct result of this motion. Atoms or molecules at the surface of an object generally have unevenly distributed charge, and when this charge is moving around rapidly it transmits electromagnetic radiation.

Because the molecules and their parts have masses and characteristic elasticities in their bonds, they vibrate preferentially at certain frequencies. This is the basis of infrared spectroscopy.

How does a game of pool work?

KneadToKnow’s money example is actually pretty good if it’s developed a little more.

The rules are as follows…
Every time you bump into someone, you have to give them half your money.
The more money you have, the faster you have to walk around the room.

In the ice cube example, you have a dozen people standing around with \$0 and a dozen people with \$64 running around the room. If two people with \$64 bump into each other, they swap \$32, still have \$64, and so continue on at their same pace. If a \$64 person bumps into the “ice cube” he gives them \$32 and gets nothing in return, He’s got \$32 and slows to a walk. The \$0 person now has \$32 and starts walking. Now you’ve got two \$32 people walking, 11 \$0 standing around, and 11 \$64 running. Because of the way he was facing, the former \$0 (now \$32) immediately walks into another \$0… so they both end up strolling off with \$16. A 64 runs into one of the 16s and they part their ways speed walking with \$40 each. You’ve got 10 \$0s standing around, 1 \$16 strolling along, 1 \$32 going for a walk, 2 \$40s speedwalking, and 10 \$64s running. Keep doing this long enough and you’ll eventually end up with two dozen people walking around with \$32.

How do you cool off soup? Well… those soup simulation guys think they’re all real smart and note that the rules say “when you bump into someone give them half your money” and insist that just because some other knucklehead bumps into them it doesn’t mean they’ve bumped into someone. They have all these really arcane rules for when people collide to determine who bumped into who and by how much and how much money each party has to hand over. The main jist is that if somebody hits you from behind they bumped into you

Anyway. You’ve got a room of soup simulation guys who, on average, have \$64, but because of their interpretation of rule 1, some are sprinting like crazy and some are just tottering along. Luckily for the sprinters they collide with people more often and since they’re running so fast almost nobody can catch up to hit them from behind, so it’s almost always the case that they bump into someone else and can slow down to rest after they hand over the money.

Doors open on either end of the soup simulation room and a line of guys with \$16 start strolling through from one door to the other. Since they’re all going the same speed and direction, they tend not to bump into each other. In fact most of them get to the other door without colliding with anyone at all.

Since the really fast soup guys bump into people more often, they’re usually the ones that hit the strollers. Occasionally a stroller might run over a soup simulator just tottering along, or a stroller might hit a soup guy from behind and the soup guy just runs off with his money, but for the most part you have a soup bumping into a stroller (who then starts walking and bumps to the stroller in front of him, then they’re both walking a little faster until the front guy bumps into the next guy and slows down and so on). It’s a real mess, but they keep walking towards the other door and out of the room. The soup guy that started it all is, of course, off running around the room again.

Net effect is that the richest/fastest soup guys tend to bump the strollers and the strollers just take any money they may get with them out the other door. Because the average soup guy is has \$64 and the average stroller has \$16, it’s pretty rare for a stroller to hit a soup simulator from behind, but it still happens from time to time with some of the slower soup guys.

I really like this money analogy.