What do you think you would feel if you were to touch the two bars? Click on the metal bars below to see the macroscopic effect of an object's higher or lower heat. Click on the mouse icon at left to reset the image.
The macroscopic feature of objects with a higher heat is a higher temperature. Hotter objects have a higher average kinetic energy and a higher temperature; cooler objects have a lower average kinetic energy and a lower temperature. Temperature is measured by comparing the object you want to measure to a calibrated standard, like a tube of mercury or alcohol in a thermometer. If your object and the alcohol in the thermometer have the same average kinetic energy, then they must have the same temperature.
One other important note about heat and temperature: the kinetic energies we are talking about are just averages. The atoms, molecules, and ions in objects have a range of speeds and kinetic energies: some are moving faster and others are moving slower.
A large block, a small block, and a heat sink. Just for reference, the big block is about 14 centimeters long and the smaller block is almost 4 centimeters long with the heat sink a little bit bigger than that. Of course none of these objects are cubes—but that's OK. So here's what I did. I put these in the oven for about an hour and then I took them out. Here's what they look like with an infrared camera. In case you can't tell, this shows the same three pieces of aluminum.
I'm not sure why, but it looks like the smaller block is already significantly cooler. But anyway, they are hot. After an hour of sitting out of the oven, they look like this.
Notice how you can barely see the two smaller pieces? This is because they are about the same temperature as the surroundings. Yes, my garage is sort of hot right now.
It's because large objects aren't the same as small objects. It may not be very surprising to say that hot objects eventually reach an equilibrium temperature with the surroundings. For an object like a block of aluminum, this thermal interaction can happen through thermal conduction touching other objects and thermal radiation. Let's just assume that the blocks cool off primarily through radiation.
The Stefan-Boltzman Law describes the rate of energy output the radiated power for a particular object. This power depends on both the temperature and the surface area of the object. As an equation, it looks like this assuming it is a perfect object that only radiates and does not absorb any external radiation. But of course the most important part is the area. We learn from this that a hot or cold object will cool or heat by convection at the same rate as long as the temperature difference between the object and the surrounding fluid is the same.
A fluid does not typically sustain a temperature gradient well; the gradient dissipates and the flow is by convection.
Therefore making a statement that you want to consider conduction in the surroundings is akin to saying that you define the surroundings also as a solid. The net rate of conductive flow is written as below. This equation defines the rate of temperature change of the object at any point in time.
It is a rather unwieldy equation that is typically solved by considering only one of the three cases radiation, convection, or conduction at a time. It also presumes the object heats or cools uniformly throughout. The case when you have a thermal gradient in the solid object is yet another equation.
As a general rule, the rate at which a hot object changes temperature cools down cannot be stated to be faster or slower than the rate at which a cold object changes temperature heats up.
The rate of heat transfer depends on the type of surroundings vacuum, fluid, or solid , the temperature of the surroundings, and the type of heat transfer radiation, convection, or conduction. Sign up to join this community. The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group.
Create a free Team What is Teams? Learn more. Why do cool things warm up slower than hot things cool down? Ask Question. Asked 2 years, 3 months ago. Active 2 years, 3 months ago. Viewed 2k times. The scenario I am assuming is as follows: There are two cups of water of identical volume in cups that do not affect heat exchange i.
Improve this question. Thornhill S. Thornhill 3 4 4 bronze badges. A n extreme counter example is that if I throw a glass of water on the Sun, I expect the water to heat up pretty substantially pretty fast, whilst I do not expect the temperature of the Sun to change much at all.
Add a comment.
0コメント