Would running a 1500W electric resistance heater heat a room just as much as a 200W incandacent light + 50W Florencent light + 200 W refigerator + 200 watt TV + 850 Watts of other stuff just like I listed (totalling 1500W)? Assuming that the power consumption is accurate and the devices were running at the rated wattage continously.
Or put another way is it just as costly to heat with electric resistance heat as if you just turned on (non heat pump) devices to equal the same wattage?
It seems to me that everything in the 1500 watt input in both cases eventually results in heat. For example, some of the 200 W in the incadescent light originally come out as light. However, this light sooner or later gets absorbed by objects in the room which raises their temperature slightly.
The heater would heat up the room more efficiently. The other devices put some of their energy into things other than heat, like compressing the refrigerant, kinetic energy in moving parts, light that escapes the room before it heats anything up, etc. Also, much of the heat generated by the other devices is not useful as far as heating up the room. Friction and electrical resistance will heat up parts in the devices themselves, instead of the room air. Think of how warm a TV gets when it’s on - much of the energy has gone into heating the parts. A little bit of that gets transferred into the air, but not much.
But - those devices do generate some useful heat. That is, if you needed a 1500 watt heater to heat an empty room, you could use a smaller heater to heat a room containing your list of appliances.
Except that your computer, even though it has a 300 watt power supply, is likely normally running at less than 100 watts. That, in fact, is the biggest problem with the OP hypothesis. Most appliances have peak ratings and normally operate well below that value as opposed to resistance heaters which operate at rated load unless they can be dialed down.
Of course, I do know some computer hobbyists who have LOTS of hardware and claim to be able to heat a room with the equipment.
Well, my puter is just an Athlon XP (with 4 fans) and the room it’s in is generally noticeably warmer than others, especially in the summer. Currently it’s 36° inside the case.
Now, here’s a related Thermodynamics Question I once got on a test:
Consider a closed, isolated (i.e. no heat transfer with the rest of the universe) system consisting of a refrigerator within a kitchen. The inside of the refrigerator is at 5°C; the kitchen is at 20°C. If the door of the fridge is left open, what happens to the temperature within the room?
The kitchen for the sake of the problem is a closed system except for the power input. No energy can leave the system.
The refrigerator and temperatures given is a distraction to confuse the student.
Since you are injecting power into the system to run the refrigerator said power results in net heat being added to the system. Since it is a closed system the temperature of the kitchen MUST rise.
Sorry about that. I didn’t answer the OP except by inference.
Your are adding energy to the room enviornment. No matter that said energy is doing something on its way to being manifested as heat but the net result is that the room temperature will rise barring other changes is circumstances such as opening or closing windows and/or doors.
So you are saying that my computer is indeed a 100% efficient heater, regardless of the wattage it’s drawing, correct? (disregarding the negligible amount of light from six LEDs that escapes the house, of course).
Strictly speaking, a refrigerator is even more efficient than a heater depending on how you use it. It’s a heat pump which removes heat from the contents and dumps it into the room. If you continuously run the ice maker and throw all the ice out the window, for example, it becomes more than 100% efficient as a heater. The kitchen is no longer a closed system: water comes in, ice goes out. (I know, I’m neglecting the heat lost by opening the window, which might cancel out the benefit.)
spingears: The kitchen is an isolated system. Actually, I think it just had adiabatic walls. It probably had a frictionless floor, too. =) So there are no doors or windows to open/close.
I think the actual question had more distracting details, including the amounts of energy involved, the efficiency of the fridge motor, and so on. Nothing was said about the motor, but yes, the temperature will rise because some of the energy put into the system will be wasted as heat.
This was the only vaguely interesting thermodynamics question I ever had. The others were all something like ‘Prove that <thermodynamic variable> does not depend on <another thermodynamic variable>.’ Or worse.
It is still true that the most efficient way to use electricity is to use a heat pump. Think of it as air conditioning the outside world. So you get all the heating from the machinery of the pump plus a fair amount from the world outside.
What you say is generally true, but there are quite common situations where it’s possible to do better than that.
You go into the bathroom in the morning for a shower. You only expect to be in the room for 15 minutes. The room is cold. You want to feel warm for the 15 minutes you expect to be in there. Your choices are:
D’oh! should have left the central heat on in the bathroom overnight.
Turn on a 2 kW electric powered oil-filled column heater, go away for an hour or so, then come back.
Turn on a 2 kW fan heater, go away for about 15 minutes, then come back.
Turn on a 1 kW heat pump, go away for about 15 minutes, then come back.
Turn on 500 W of incandescent heat lamps, use the bathroom straightaway.
There’s no warm-up time in using the incandescent heat lamps, and they use less power, because they act in a different way. They don’t attempt to heat the air in the room, they just radiate on your skin.
In this example, the incandescent heat lamps can achieve an effiency 4 times higher than even the heat pump can, if the true nature of the problem is considered.
It’s not just the time element of the problem that makes the difference, either. The incandescent heat lamps are commonly used continuously in warehouses and factories. They can make the staff feel comfortably warm, without having to heat all the air in a drafty and poorly insulated building.
Test questions are just that. The answer to the test question as stated is that “The Temperature will rise.”
No quibbling about adiabatic walls or frictionless floors.
Take the same question to the real world with conducting walls, etc. The answer is the same. Add the qualification that some other minor losses/gains will occur, but not of such magnitude as to make the question overly complicated and impossible to solve.
Old Chinese Saying:
“Do nothing simply when a way can be found to make it complex and wonderful.”
Kanicbird, are we talking real world here or “thermodynamics class” world with “closed systems” and the like? How many of you would expect to find the room warm if you were using only non-heating appliances in the room (ie stuff that only generates heat as a biproduct rather than as the primary function)? In reality (and I think in thermodynamics as well) the work extracted by these various appliances in the form of mechanical or electrical energy is not converted efficiently into heat as it is with a heater. Nor is it efficiently conveyed to the surrounding room. The losses associated with inefficiency (and no, a refrigerator is not 100%+ efficient, there are mechanical losses associated with running the compressor and any other mechanical devices) result in colder room for the second situation in the OP.
I’m sorry but you are wrong. Energy is always conserved. If a 100W light bulb is 5% efficient, that means 5W worth of light is emitted in addition to 95W heat. If you are in a room with the curtains closed, the 5W worth of light also gets absorbed inside the room. So the light bulb is a 100% efficient 100W electric heater. Same with a TV or computer.
If the appliance stores energy in some way, it may not be a 100% efficient heater. If an elevator is used to lift something upstairs, some of the consumed energy is stored as potential energy instead of being released as heat. But unless something like that is accumulating energy in your house, all consumed energy is released as heat.
And of course I expect the room to be warmer if I use an electric appliance. They’re all warm to the touch, aren’t they? My office has 3 workstations with about 10 external hard drives total; I haven’t turned on the heat yet this winter. (OK, temperature hasn’t quite gone down to freezing yet, but I’ve stayed in my office all night/morning in T-shirts while frost is forming on the ground outside. No central A/C in this building.)
I’m afraid I have it pretty much oll korrect. I think you are treating the question in a simplistic, Physics 101 sense rather than looking at the practicalities.
First of all, as long as the refrigerator is running, there is kinetic energy in the moving parts. Part of the electrical energy used by the refrigerator is in the motion of the parts and the magnetic field of the compressor motor, not in the form of heat.
Second, it takes more energy to heat up the internal parts of 1500 watts’ worth of appliances than it does to heat up the heating coils of an electric heater. This is by design: electric heaters are meant to pump out hot air quickly, so the heating coils are designed not to store much heat. In contrast, the materials in appliances store a lot of heat. This stored heat does not heat up the air until you turn off the appliances and wait a loooong time for them to completely cool off.
Third, much of the energy emitted by, e.g. a light bulb, is radiated out the window and is not available to heat up the room air.
Using conservation of energy to say the two methods would heat the room equally requires you to cover the windows, turn off all the appliances, and wait an infinitely long time for everything to reach thermal equilibrium. There was nothing about that in the OP.
Next assume that everthing is warmed up, cooled down i.e. the refrigerator, and has been running long enough to reach equilibrium.
It makes NO difference what combination of heaters, lamps, appliances, etc. are connected to the power mains as long as the total power entering the room is 1500 watts.
The temperature will rise untill equilibriums is reached with the outside world OR if it is truly a closed system until something fails or it overheats and burns up or down!
“Do nothing simply when a way can be found to make it complex and wonderful.”
