# “A hot object has greater average kinetic energy but may not have greater total kinetic energy “

46 views
0

“A hot object has greater average kinetic energy but may not have greater total kinetic energy “

In: 1

The thing that we call “heat” ultimately refers to the motion of the atoms in the object we’re talking about. A hot object’s atoms are moving around very fast and a colder objects are moving slower. Fish swimming around like crazy in a pond, you have a lot of motion going on, but it’s not like the pond is slowly inching into the next town.

Conversely if you fired a bullet from a gun, all the atoms in the bullet are moving together, with a specific motion and energy.

It’s confusing because both explanations involve motion, but one is a random motion, confined to a space, without any “bulk movement” of the object. The other refers to actual motion of the object itself across a room.

Take two objects.

One is hot, and contains a billion particles bouncing around inside it. The average particle will have a higher kinetic energy, which we will call X.

However, the second object, while cold, contains 10 billion particles. Because they are colder, their average kinetic energy is 1/2 X.

The total kinetic energy is the number of particles times the average kinetic energy. The first would have 1,000,000,000 X (1,000,000,000 times X. The second would have 5,000,000,000 x (10,000,000,000 times 1/2 X).

Imagine the number balls in a lottery machine. They’re bouncing around in the machine like crazy. They’re knocking into each other and into the sides of the container they’re in. This is like the atoms in a hot object (specifically, this is most similar to a container of hot gas).

Now, turn the same machine off so that the balls aren’t getting bounced around anymore and drop it out of a third story window. Now the balls aren’t bouncing into each other as much anymore. Instead, they’re all going the same direction – down.

All those number balls bouncing against each other are cancelling each other’s energy out. Taken individually, each one is very energetic, but taken as a group, they’re not going anywhere or doing anything useful. The number balls falling out of the window aren’t individually very energetic since they’re not bouncing around anymore, but taken together they might damage a car parked on the street.

At the atomic level, heat is kinetic energy, but at the macro (object) level, almost all of that kinetic energy cancels out and you’re left with a hot object that’s sitting still. A cold object moving quickly may have more kinetic energy than a hot object sitting still.

A cold bullet shot at your hand is going to damage your hand (more energy) than a sizzling hot bullet sitting on your palm.

I think the key is “greater average RELATIVE kinetic energy”. It’s kind of implicit because temperature has to do with how much relative motion is happening within the particles of an object. It doesn’t mean it has more total kinetic energy because the whole object can also be moving all together with a lot more kinetic energy.

The sentence as written can be mathematically confusing if you miss the implicit “relative”.

A lot of people in here are talking about bulk movement vs temperature, but the question as posed makes me think that you might be looking for an explanation of thermal energy vs temperature. So, what’s the thermal energy of an object? It’s the total amount of energy that contributes to temperature. To illustrate the difference, lets imagine 2 rocks. They’re made of the same stuff, but one is way bigger than the other. Now we do an experiment: we place each rock separately in highly insulated box and see what happens to the temperature of the system after a period of hours. After each trial, we reset the box to a standardized temperature. Whenever we do this, so long as the small rock is not MUCH hotter than the large rock, we will find that the large rock alters the temperature of the box more than the small rock does. This is because it takes more total energy to heat the large rock to the same temperature as the small rock.