Please post the temps your machine is operating at. I'd recommend downloading and installing iStat Pro, you can find it using a Google search.
A normal operating iMac will get warm, saying it's hot or overheating normally means nothing. In most cases iMacs do not overheat, there are extremely unusual exceptions but most never overheat. Apple engineers them to run for years, mine is 5+ years old and run just fine and has never overheated.
Your computer has heat sensors and fans inside, if the computer senses the computer is warm the fans will promptly cool it down. So when you are feeling it hot, check 10 minutes later and it will probably be just warm again. Forget about it and enjoy using the computer.
Run Apple Hardware Test and make sure your iMac's fans are OK:
Using Apple Hardware Test
I have tried and failed to get my iMac to overheat using Geekbench stress tests. The CPU stabilizes at 65° C or so and won't go any higher. It's a 21.5" model though.
An SMC reset may also be indicated for anything fan-related.
Yes, download iStat Pro or iStat Menus. Then you'll be able to see exactly what your machine is doing at any given time and the associated temperatures and fan speeds.
I have an 2.8GHz i7 27" iMac. These are the temps I see under most conditions (per iStat Pro):
GPU Diode: 120F
GPU Heatsink: 117F
Mem Controller: 103F
Optical Drive: 85F
Power Supply2: 115F
The hottest part of the case is the top left corner most of the time.
When my machine is really cranking (when encoding video, for example) I've seen the CPU and power supply temps get up to around 130-140F.
Coming from an electronics engineering background, I can tell you most assuredly that the heat generated inside the current iMac design enclosure is not good for the electronic components over a period of time.
There are a lot of posts here of hard drive failure, CPU, GPU and even logic board failures.
I am convinced that all are heat related issues.
Heat is the enemy of all small electronic components, period!
Component failure and component "brown-outs" can be caused both by fluctuating electric current and/or heat!
Electronic components like to operate in as cool a temperature as possible. Conputer components, at one time were cooled by either water, anti-freeze-like fluids, liquid Nitrogen, etc.
In earlier iMac models, heat was vented by having a case with that has ample venting built-into the case design.
The new iMac designs IMO, do not have enough sufficient, passive venting to vent all of the heat the iMac components generate. I feel the aluminum body conducts as much heat on the inside of the enclosure as it convects outward from the enclosure with an insufficient net reduction in overall internal temperatures.
Apple has definitely NOT done long duration stress testing of its current designs.
It refreshes its product designs every couple of years or so. So why do long-term testing on a design that will be redesigned and refreshed every few years. I believe they do not.
Apple engineers were simply concerned much more with the complete quiet operation of the iMac design than with internal heat buildup issues within the current iMac enclosure designs.
I and many other users use a software fan control to ramp up the iMac's internal fans as well as use an external fan on the backs of our iMacs to further cool them down more directly.
The result is an additonal 15% to 20% reduction in the iMacs overall running temps.
As to your main question, when running OS X applications, OS X is doing things, while the apps you are also running are doing things. These things use CPU, GPU cycles, swapping commands and data in/out of RAM, maybe using hard drive cycles, etc. These processes work different hardware and software components of your iMac and, in turn, increase the components temperatures. Typically the CPU, GPU and hard drive ( and in the case of getting data on and off of discs, the optical drive) do heat up appreciably and heat up even more when the application tasks get more and more demanding of the Mac hardware.
I think you guys are misunderstanding what I'm asking. Unlike the many threads where posters claim that their iMacs are overheating, I'm certain that mine isn't. It does get very warm to the touch but I am not concerned about this.
More concerning/annoying to me is that the computer seems to get warm at unpredictable times, like when Activity Monitor is showing no CPU or disk activity and there's nothing special going on on the screen.
I have a degree in electrical engineering. Of course I understand that electronics can overheat but you're not providing any evidence that iMacs actually overheat. Of course there are many posts here about iMac hardware failures since this is a message board for Apple product support. It doesn't mean the iMacs are overheating. As someone in electronics surely you understand about temperature ratings for chips, e.g., obviously non-milspec TTL logic will operate indefinitely at 85C, etc. I don't know what the components of an iMac are rated at but none of my temperatures are anywhere close to values that would worry me, even though the top feels uncomfortably warm to the touch.
It's naive to think that Apple doesn't stress test their designs. Every company that sells millions of any given product does extensive testing. Actually I live very close to a company that does test outsourcing and I know Apple uses them to test some computers although I don't know about iMacs in particular. They do all the standard electronics testing: 4 corner, vibration, EMI, etc.
I think I've come up with an answer to this question. Turns out it's the monitor brightness. (!!)
The reason why my iMac case temperature seemed to vary throughout the day randomly is because I had "Automatically adjust brightness" turned on, so sometimes the screen was at maximum brightness and at other times it was fairly dim depending on weather and time of day.
I have turned off that setting and set the monitor to medium brightness and so far I have not caught the iMac with anything more than a slightly warmish case.