Helping people with computers... one answer at a time.
Several components in your computer definitely use power and generate heat in proportion to how they're used. I'll review the two biggies.
Hard drives get hot because of moving parts inside. But does anything else contribute to their heat output? Does a 1TB hard drive produce more heat energy than, say, a 500GB hard drive, all other things equal. And if a 1TB hard drive, for example, is running 9 applications at once, is it producing more heat than if it were only running two applications?
In other words, is my computer burning more calories when it's thinking harder?
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Yes it does.
While strictly speaking, drive capacity doesn't have an impact, the internal characteristics of a drive certainly can. And you're right, it's all about the moving parts.
But there's more to it than just the drive.
And while I suspect you didn't mean it that way, calories is actually also an accurate term.
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Disks and Power
You can probably guess that a disk spinning generates a certain amount of heat and uses a certain amount of power. The key is that this energy consumption is constant: unless you have power saving measures like "shut down the drives after X minutes of inactivity" the drives spin constantly and at a constant speed, using a relatively constant amount of energy and producing a relatively constant amount of heat.
It's the arms that generate the heat.
Arms? Yes, arms. The arms that hold the magnetic read/write heads that move back and forth from the inside to the outside of the magnetic area of the disk.
Hard disk arm movement
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When your computer is idle and doing nothing - i.e. it's not accessing the disk at all - the arm remains stationary at wherever it last was.
On the other hand, should your computer begin some sort of disk-intensive application, perhaps defragmenting the hard disk, then the arm is moving back and forth across the hard disk faster than the eye can see.
Now, even though the arm is typically very small and very light, it still takes a lot of energy to move it that incredibly quickly, and just as quickly bring it to a stop at or near the desired track of information on the magnetic media. That, in turn, means it uses more power and generates more heat.
A busy disk is a hotter disk, there's no doubt about it.
To address your question regarding capacity: it's not typically a factor, directly. Indirectly, if the additional capacity means you're doing more, or the layout of the data on the platters within the disk is such that the arm has to move more, then yes, one drive could generate more heat than another due to a size difference. My belief is that difference is small compared to other drive-to-drive manufacturing differences.
And yes, defragging your disk somewhat regularly reduces the amount of arm movement required to access files, thus making your computer not only somewhat faster, but also slightly less hot.
The CPU and Power
The other major source of heat in your computer these days is the Central Processing Unit or CPU itself. That's the part that gets names like "Pentium", "Core 2 Duo", "Xeon", "Core i5" and the like, and the part that we often refer to as the "brain" of your computer.
It's the part that does the computing.
If you've ever looked at Task Manager or Process Explorer, you'll find that your processor is mostly doing nothing. By that I mean the processor usage measure is often below 10% or even lower. As I type this document, my processor is having a hard time getting over 8% usage, and that's mostly to update the display of CPU usage. That means that 92% of the time it's doing nothing. It's "idling" - hence the "system idle process" you often see listed.
Knowing that the processor in the average computer spends so much of its time doing nothing, this idle do-nothing state is actually optimized to use as little power as possible.
Conversely, when the processor is working hard, it uses more power. The harder it works - 10% usage, 50% usage, 90% usage to a processor fully pegged at 100% CPU usage - the more power it uses.
And the more power it uses the more heat it generates. That's why so often
what you think is the processor in your computer is really the cooling fans,
fan, piping and heat sink hiding the much smaller processor underneath. It's
important that that heat go somewhere, hence all the additional hardware to
bleed it off somehow. (And for the record, today's more powerful, faster,
denser CPUs generate more heat that their predecessors for a variety of reasons
including the fact that they're more powerful, faster and denser. 
)
So, like the disk, a busy CPU is a hotter CPU.
Of Calories and Diets
While we typically think of calories in terms of the food we eat and the weight we're not losing, the use in this context is actually quite accurate.
A calorie is a measure of energy. Heat is energy. Hence somehow, somewhere, someone could actually attach a number of calories measurement to the amount of heat generated by your CPU and hard disk.
Calorie is more commonly used as a measure of ... well, it turns out to be the same thing: energy, but as contained in food.
The more common unit of measurement for things like heat radiating from computer components would be the joule.
Article C4442 - September 7, 2010
Leo A. Notenboom has been playing with computers since he
was required to take a programming class in 1976. An 18 year career as a programmer at Microsoft soon followed.
After "retiring" in 2001, Leo started Ask Leo! in 2003 as a place for answers
to common computer and technical questions. More about Leo.
As I said before, None of the people near me follow Ken B.'s advice. I do for me and someone else's computer. One thing I keep thinking about is my Aunt's Mac. The only opening I see is a small round hole in the back, last year after her hard drive died and it was replaced by someone else.
Posted by: steven at September 12, 2010 5:58 AMAlternately, today's more powerful, faster, denser CPUs often use less power and generate less heat than their predecessors because they are engineered to do so... this is one source of longer laptop battery life (not better batteries, but better usage of the limited power they contain ;-).
Posted by: Nicholas Gimbrone at September 14, 2010 10:53 AMNot completely certain on this, but my understanding is that the calorie is a unit of heat measurement. It was picked up by dieters because the metabolism of food -- primarily a variety of slow oxidation, i.e. slow burning (!) creates heat, which is precisely commeasurate with the amount of energy the food contains. You get exactly the same amount of energy from actually burning the food as from eating it -- only when you eat the food, all that energy goes into your body rather into producing smoke and flames!
By the bye -- completely off-topic, I know -- but this explains why alchoholic beverages are always so caloric: you are drinking what is, by definition, a flammable liquid. (Note however that most beverages are dilute, and are not therefore in a flammable proportion -- the beverage must be greater than 57% abv or about 114 proof under modern measure, for that. Nevertheless, alcohol itself is, by nature, flammable.)
Posted by: Glenn P. at September 14, 2010 3:43 PMGlenn P. I love your science but it's kinda mixed up. Calorie is a measurement of energy, not heat. It can be measured in many different units - in the metric world of food for example, we use Joules (actually, kiloJoules) to describe the energy potential of the system. The system can be chemical, mechanical, electrical or thermal.
Posted by: David at September 15, 2010 9:22 PM1 kilojoule = 239.005736 calories.
Heat is measured by scales of temperature such as Celsius, Fahrenheit and Kelvin. However, in the process of expending energy (converting it into power (Watts, Horsepower, etc,) heat is created and...well, it gets very technical here and my old physics teacher was much better at explaning it LOL
Hi Glenn and David and all others reading this. I am an old physics teacher (yes!) and I'll try to explain things a bit. When you are burning food outside your body it will give a different amount of energy (=heat) from when you burn it in your body. Just try to live on gasoline and you see what I mean. The calorie is an old energy unit defined as the amount of heat (warmth) needed to raise the temperature of 1 gram of water with 1 degree Celsius (or Kelvin). Power is the amount of energy (heat, warmth) used or produced in 1 second--1 watt is the same as 1 joule/second and a metric horsepower is the energy needed to lift 75 kilogram over 1 meter in 1 second. So if I mount the staircase in my home in a little more than 2 seconds I am producing just about 1 HP. A metric HP is the same as 735.5 W. And you can use brandy to get your dishes 'flambeed'--40 vol% will do.
Posted by: Henk at September 19, 2010 12:10 PM