Interesting argument between friends

In summary: Yes, a lot of energy is used in putting up a Christmas tree. A lot of energy is used to move ornaments, to buy lights, to install them, to power the tree stand, and to power the lights themselves.
  • #36
rbj said:
why not assume it has negative resistance?

the issue is, yung, that when that cap discharges, the energy stored in the cap ends up somewhere. where do you think it goes?

better clip on some heat sinks to the MOSFETs.

My name is Yungman or Alan

I am using an ideal case, there is no ohmic loss as a start to look at whether there is power involve in an ideal case.

For real case, you can use a MOSFET with very low output impedance to sink or source large current with very little heat. eg, if on channel resistance is 10mΩ, 1A only generate 10mV and power dissipation is very low even if charging a cap to 10V.

In computer, almost all circuits are MOSFETs, the biggest biggest problem is to charge and discharge the input and the line capacitance to achieve the logic level. Input capacitance and drive is THE single major issue of the speed limit. They get away with higher and higher speed inside the CPU only when they shrink to transistor down to lower the input capacitance. That's the reason they manage to run GHz inside the chip. But any external bus is still slow because of the trace and input capacitance need to be driven. The output has to be buffered over and over so the speed goes down. I want to see how driving a cap don't consume power.
 
Last edited:
Engineering news on Phys.org
  • #37
Alan, no matter what you want to assume, when the transistors are ON, their resistance is at least as much as a wire of short length. when Q2 discharges the capacitor, if there is no other resistance in the loop, that energy will be dissipated in whatever resistance that current sees. in the wires, even those inside the capacitor, the soldered nodes, and in any other conductive element in the loop (i.e. Q2).

specifically, getting back to the original question, a computer consuming 1 kW is rather unlikely, but a computer left on that continually draws W watts will dissipate nearly the very same amount of heat and heat the room just as well as a space heater that also consumes the same W watts. only energy that somehow crosses the boundary between what is inside the room and what is outside (sound leaking out, light going out a glass window), only that energy will not heat the room.

a battery charger (drawing W watts) will heat the room less if energy is going into and stored in the battery. an electric motor (drawing W watts) with a shaft going through the wall to the outside will heat the room less than a space heater consuming the same W watts.
 
Last edited:
  • #38
I know your argument, I am just confused about the current need to charge and discharge the cap. I just take everything ideal and look at one part. Assuming there is no ohmic loss, is there power deliver into the box?
 
  • #39
If there are no resistances anywhere in that circuit. The circuit will not consume any energy.

The charge stored in the capacitor will "return" to the flow when the capacitor is discharges and then charged again. We we were talking about positive charges moving, they would first move onto the positive part of the capacitor (charging it). Then they would move around loop 2 and end up on on the bottom (discharging the capacitor).

When the capacitor is charged again, these positive charges would flow from the ground node and through the voltage source.

So the circuit would be a perfect, forever circuit (if your voltage source was ideal also).
 
  • #40
I don't see how, the current that charge the cap is from the battery with voltage, the current discharge the cap is by Q2, nothing to do with the battery. Far as the battery concern, it only see the charging cycle time after time.

I need to be convinced that there is no power transfer from the battery into the box using an idea case. Then worry about the real resistance that cause power loss and heat.
 
  • #41
Actually it's not so easy at all. You would have infinite currents somewhere and its generally stupid to think about it this way.

There WOULD be resistances everywhere, and because of this, the energy is dissipated in these resistances.

As Russ has stated several times, almost all energy in a circuit will be dissipated as HEAT in that same circuit. A little bit of energy will perhaps be radiated as radiowaves, but again, these will turn into heat when they hit the walls of the house.

So for all practical purposes, the energy consumed by a computer will be dissipated in that computer as heat because of resistances in that circuit.
 
  • #42
As soon as you add a resistance in the wires all these problems dissapear, and all power equations will be satisfied.

The power in supplied by the voltage source, will be dissipated by the resistances.

By wanting to have no resistances you present an impossible scenario with infinite current. Infinite current means the electrons move infinitely fast. They will bang into stuff when they reach the end of the wire and be dissipated as heat.

But as soon as you add just 1 Ohm of resistance in the wires everything becomes apparent = The power is dissipated in the resistances.
 
  • #43
yungman said:
I know your argument, I am just confused about the current need to charge and discharge the cap. I just take everything ideal and look at one part. Assuming there is no ohmic loss, is there power deliver into the box?
Setting aside the issue of infinite current, if the capacitor is ideal and there is no resistance elsewhere, there us no power either used or dissipated.

But how does that help us?
 
  • #44
yungman said:
I don't see how, the current that charge the cap is from the battery with voltage, the current discharge the cap is by Q2, nothing to do with the battery. Far as the battery concern, it only see the charging cycle time after time.

I need to be convinced that there is no power transfer from the battery into the box using an idea case. Then worry about the real resistance that cause power loss and heat.
If you write a conservation of energy statement to calculate that, it reduces to 1=0 or gives a divide by zero error, so that's why assuming no resistance doesn't clarify things:

I^2R=VI

Set R=0...
 
Last edited:
  • #45
With non-zero R value in the cap charge/discharge network, the power lost is the same regardless of value, as long as the RC time constant is short compared to the period of switching. An Rdson in the FET of 0.10 ohm, 0.010 ohm, or 0.001 ohm results in the same power dissipation.

If the Rdson is actually 0 ohm, the current is still limited, as it cannot be infinite due to the inherent inductance present. Since the energy stored in the cap is not returned to the battery it is either dissipated or radiated. In the R=0 case it is radiated. The parasitic inductance value of the FET, herein "L", forms an LC network with the capacitor. When the cap is fully charged w/ the FET off there is energy W = C*V^2/2. Then the FET turns on. The cap discharges into L, the FET inductance, which results in L storing energy. When the cap energy is depleted, V=0, but I continues through L charging C negatively. This is a resonant LC network.

Oscillation takes place & energy is radiated. The small value of L results in a high resonant frequency. If the energy is not dissipated as heat, as in the nonzero R case, then it is radiated due to LC resonance. It does not recharge the battery because there looks to be no path for that to happen.

A real world FET will have nonzero Rdson, so I don't think we should dwell on radiated power. The energy in the cap is dissipated in Rdson on a per cycle basis. Anything unclear can be explained if needed.

Claude
 
  • #46
i agree with russ

thought experiment:

place that computer (tv, stereo, whatever) inside a black box in center of the room.

Whatever energy goes in (1kw of electrical) must come out into room

light will impinge on walls of black box and warm them
sound will echo around inside till it is converted to heat warming the air(recall Newton's error in calculating speed of sound)
RF will be absorbed by the computer's internal shielding and warm it

the heater FEELS warmer because it radiates heat at higher temperature than the others

but recall temperature is not heat.

my 2cents

old jim
 
  • #47
This seems to be the apex of the argument. While I agree that the "black box" concept is true, recall that our goal is to HEAT the room. To do this we must add thermal energy to the room faster than the surroundings can take it away. While at the end of the day all the energy will go to thermal energy (as everything does in the universe), a heater turns "nearly" all of it into thermal energy, while with something like a computer or a television the energy gets "sloshed around" via other means and does not contribute to heating the room when the heat is needed
 
  • #48
But again that goes back to the difference in heat and temperature. I don't think anyone would argue that a heater is more efficient at changing the temperature in a room than a computer, but that doesn't mean that the two aren't producing the same amount of heat.

The biggest difference there is a heater is designed to transfer thermal energy to the room very rapidly.

In a computer the primary source of heat transfer is heat sinks. Heat sinks are designed to have a higher thermal mass, meaning they transfer thermal energy slowly (a high thermal mass can store a lot of thermal energy). This means they aren't going to quickly change the temperature of a room, even though the same amount of heat is involved in the two scenarios because the heat sink stores the thermal energy and releases it over a longer period of time.
 
  • #49
JustNobody said:
This seems to be the apex of the argument. While I agree that the "black box" concept is true, recall that our goal is to HEAT the room. To do this we must add thermal energy to the room faster than the surroundings can take it away. While at the end of the day all the energy will go to thermal energy (as everything does in the universe), a heater turns "nearly" all of it into thermal energy, while with something like a computer or a television the energy gets "sloshed around" via other means and does not contribute to heating the room when the heat is needed
While strictly speaking you are correct, in practical terms the difference is negligible since the only portion that 'sloshes around' for more than a few seconds, is the heat absorbed by the mass of the computer. That's a small amount and it only causes a few minutes of lag for that small amount of heat it holds.
 
  • #50
I have just laboured my way through this thread, reading as much as I could bring myself to and scanning through the rest of it. It amazes me that otherwise sensible people seem to be arguing about the principle of Conservation Of Energy in the specific case of someone's front room! How can it not apply here, same as in the nucleus and everywhere else?
Unless the energy leaves the room / data centre in the form of sound (I don't think so) or EM radiation (a few Watts at the most) it must end up by heating up the air, walls or people in there.
Why are you wasting time trying to show that energy in a circuit magically disappears because Mosfets and Capacitors are involved (are the REALLY that ideal?). It's crazy; isn't this supposed to be a Physics and not a Magic forum?

Where heating is concerned, there are a lot of details involved in where the heat ends up in the room and how our body actually becomes aware of it. Cold feet in an otherwise warm room is not comfortable, for instance. But that is an issue that heating engineers and serious environmental engineers know all about. A watt hour is a watt hour and it's not how you produce it that counts - it's how you use it.
End of rant. :smile:
 
  • #51
If we are talking about purely heat, then yes, I guess they would give off the same amount of heat.

But if some guy is trying to argue that using a 1000W computer will keep you as warm as a 1000W heater will, I'd say that guy definitely needs to switch majors.

I guarantee a heater gives off more heat per second than a computer.
 
  • #52
LawRooney said:
If we are talking about purely heat, then yes, I guess they would give off the same amount of heat.

But if some guy is trying to argue that using a 1000W computer will keep you as warm as a 1000W heater will, I'd say that guy definitely needs to switch majors.

I guarantee a heater gives off more heat per second than a computer.

A large portion of this thread has had discussion that disagrees with you.
 
  • #53
sophiecentaur said:
I have just laboured my way through this thread, reading as much as I could bring myself to and scanning through the rest of it. It amazes me that otherwise sensible people seem to be arguing about the principle of Conservation Of Energy

I totally agree. :rolleyes: I've given up on this thread already. Russ probably needs to post the facts one final time and lock this.

Maybe I could suggest that many of the responders here could then head on over to the general math forum and start a new topic on [itex]0.\dot{9} \neq 1[/itex]. Hurkyl would just love that. :wink:
 
  • #54
I guarantee a heater gives off more heat per second than a computer.

Of course it won't. A heater might be more effective at circulating the air in the room but that doesn't change the fact that all but a few watts of the power the computer draws is heat transfer to its surroundings. At least argue why you think otherwise.
 
  • #55
LawRooney said:
If we are talking about purely heat, then yes, I guess they would give off the same amount of heat.

But if some guy is trying to argue that using a 1000W computer will keep you as warm as a 1000W heater will, I'd say that guy definitely needs to switch majors.

I guarantee a heater gives off more heat per second than a computer.
That is rubbish, I'm afraid. How is one KiloWatt different from another Kilowatt? The only possible difference in the energy supplied to the room would be in the light power that escapes through the window from the monitor (perhaps a Watt).

We have already been into the fact that supplying 1kW to a room is not the whole story when it comes to making people feel warm but that is not the issue.

Hmm - fools rush in, you know.
 
  • #56
All I can say is that a 1000W computer - is it's rated value, full power. So just because the rating is 1000W, does not mean you are using all of that 1000W - If you max out the power supply - then ALL of the 1000W it consumes is left as residual heat.

I would also like to add that Sound and EM are not the only ways to consume the energy - think of all of the ways you can do work ( affect a change in a system) and not give off heat. For example all of the ways to convert to Potential Energy - for example a 1000W Battery charger does NOT give off 1000W of heat.
 
  • #57
LawRooney said:
If we are talking about purely heat, then yes, I guess they would give off the same amount of heat.

But if some guy is trying to argue that using a 1000W computer will keep you as warm as a 1000W heater will, I'd say that guy definitely needs to switch majors.

I guarantee a heater gives off more heat per second than a computer.
That is self-contradictory: either the numbers are equal or they aren't.
 
  • #58
Windadct said:
All I can say is that a 1000W computer - is it's rated value, full power. So just because the rating is 1000W, does not mean you are using all of that 1000W - If you max out the power supply - then ALL of the 1000W it consumes is left as residual heat.

I would also like to add that Sound and EM are not the only ways to consume the energy - think of all of the ways you can do work ( affect a change in a system) and not give off heat. For example all of the ways to convert to Potential Energy - for example a 1000W Battery charger does NOT give off 1000W of heat.

Obviously 'rating' is not what counts and a battery charger is a valid exception but that's only finding loopholes in an argument which was dealing with the notion that dissipated energy is somehow different, according to what's written on the front panel label of the equipment.

And, before questioning just how much heat a computer can generate, just consider the vast amount of trouble they have in getting rid of all the heat in a big Server Room. It's a serious problem.
 
  • #59
LawRooney said:
If we are talking about purely heat, then yes, I guess they would give off the same amount of heat.

But if some guy is trying to argue that using a 1000W computer will keep you as warm as a 1000W heater will, I'd say that guy definitely needs to switch majors.

I guarantee a heater gives off more heat per second than a computer.

if the room is windowless and if the room is completely sound insulated (note neither of these conditions are about either the heater or the computer) and if both the heater and the computer have watt-meters attached and both are drawing the same power of W watts, then I guarantee that you're incorrect, Rooney. they will both heat the room exactly the same.
 
  • #60
rbj said:
if the room is windowless and if the room is completely sound insulated (note neither of these conditions are about either the heater or the computer) and if both the heater and the computer have watt-meters attached and both are drawing the same power of W watts, then I guarantee that you're incorrect, Rooney. they will both heat the room exactly the same.

Yeah, I don't see how I would be incorrect. I made no statement whatsoever about an "ideal room" and simply stated that a heater would obviously warm you up better, even if the total amount of heat given off ended up being the same.
 
  • #61
LawRooney said:
Yeah, I don't see how I would be incorrect. I made no statement whatsoever about an "ideal room" and simply stated that a heater would obviously warm you up better, even if the total amount of heat given off ended up being the same.

I suppose you could say that if you are sitting in front of the heater.
-
Let me ask you this: Take 4 incandescent light bulbs of 250 watts each mounted on a board. Then take a 1000 watt radiant heater of the same area as the board. Which one do you think will warm you up better?
 
  • #62
LawRooney said:
Yeah, I don't see how I would be incorrect. I made no statement whatsoever about an "ideal room"

well, i just wanted to plug leaks. photons or phonons that leave the room won't heat it. so i wanted to make clear that none were.

and simply stated that a heater would obviously warm you up better, even if the total amount of heat given off ended up being the same.

and that's incorrect. they're both giving off the same amount of heat.
 
  • #63
LawRooney said:
I guarantee a heater gives off more heat per second than a computer.

Your intuition is likely right but your terminology is wrong. What I suspect you're thinking is if I place a computer in one cold room and a (well designed) space heater in another, turn them both on, return in 30 minutes, then the room with the computer will have a lower ambient temp rise.

I suspect this is right because the thermal resistance of the space heater is likely lower than that of the computer case. Or to put it another way, the computer needs to heat itself more than a space heater before it can heat the room so its startup time is longer.

This effect really only matters on short time scales (there needs to be a point where the computer is releasing all the heat it produces or it will continue to rise in temp) and the initial assumption in the thread told us to ignore this effect.

Also, a real computer connected to a 1KW supply likely does not draw constant 1KW as the power supply is sized for the worst, not typical, case. But again, the initial assumption stated in this case the computer does.

http://en.wikipedia.org/wiki/Thermal_resistance
 
  • #64
es1 said:
What I suspect you're thinking is if I place a computer in one cold room and a (well designed) space heater in another, turn them both on, return in 30 minutes, then the room with the computer will have a lower ambient temp rise.

I suspect this is right because the thermal resistance of the space heater is likely lower than that of the computer case.

Actually, I suspect that a rack of processors consuming at 1kW will need to be very well cooled and the fans will soon be pumping out the heat at the full 1kW rate because they do not want the internal temperature to get more than the optimum. They have to be as good to the stuff inside the boxes as you r heating system needs to be at home - and with a similar thermal time constant.
 
  • #65
JustNobody said:
...
My EE friend has a theoretical heater which runs on 1000 watts of power. In his apartment he also has another electronic device (ie. a computer, a television ect) which also requires 1000 watts to run. He argues that, due to conservation of energy, 1000 watts is 1000 watts and the same amount of heat will be produced from his "device" as from his heater (and thus if his theoretical device is always running at 1000 watts he will never need to turn out his heater to heat his house).
I agree with your friend. I have a macbook, and when I installed the SETI* software, the fan came on. Sounded like a jet engine inside my laptop. I uninstalled the SETI software. Nothing I did before, nor since, has made my fan work that hard.

I have two spare tower PC's that I no longer use, and have considered starting them up in the winter months running the SETI software. I'm a very big fan of cogeneration, (also known as combined heat and computing, CHC :tongue2:)

hmmm...

Om's Macbook said:
Battery Information:
...
Full Charge Capacity (mAh): 4565
Voltage (mV): 12492
...

I think my battery routinely lasts for 6 hours, so it would appear to be consuming:
4.565Ah * 12.492V / 6 hours = 9.5 watts

Time my laptop battery would survive if my laptop were drawing 1000 watts:

57.0 wh/1000 watts = 3.42 minutes

:bugeye:



Thank you,

You're welcome.



---------------------------------
* I do not mean to imply in any way that I am promoting the History Channel.
 
  • #66
Puts me in mind of Crocodile Dundee, comparing a laptop and an Internet Server "That's not a computer - this is a computer" (knife).
 
  • #67
sophiecentaur said:
Puts me in mind of Crocodile Dundee, comparing a laptop and an Internet Server "That's not a computer - this is a computer" (knife).

I've never seen a real Internet Server.

But this is starting to remind me of Thanksgiving, and the fact that I'm a metaphor whore...


Borek said:
Happy Thanksgiving!

Matherole?

I so want to use that in the "How do magnets work" thread.

Meta Crazy Om said:
So you've got these green beans, and through some poly-dimensional gearing mechanism, do something quite extraordinary...


----------------------------------
/me hides from the donderbolts.
 
  • #68
OmCheeto said:
I've never seen a real Internet Server.

But this is starting to remind me of Thanksgiving, and the fact that I'm a metaphor whore...




Matherole?

I so want to use that in the "How do magnets work" thread.




----------------------------------
/me hides from the donderbolts.
Never heard the word Materhole before but, when googling, I found this, totally randomly, on YouTube_Nerdy or what?

Do you think she could be a recruit for PF?
 
  • #69
sophiecentaur said:
Never heard the word Materhole before but, when googling, I found this, totally randomly, on YouTube_Nerdy or what?

Do you think she could be a recruit for PF?

Borek posted 4 of her videos:

Borek said:
Happy Thanksgiving!

Fractal Thanksgiving Turduckenen-Duckenen
Mathed Potatoes (with a visual display of a brute force algorithm)
Green Bean Vector Matherole
Borromean Onion Rings

I suppose PF is one of these places where this approach to Thankgiving will be appreciated!

It's not often I will finish one video, much less 4. I would very much like to invite her to the forum. She could make videos for us!

"How to cook a 15 lb. Fractal Thanksgiving Turduckenen-Duckenen with a pair of 1000 watt Personal Computers in under X* hours."

----------------------
* Does anyone know the specific heat capacity of turkey?
Oh never mind. Google apparently now knows everything:
– thanks to Ciaran for this great link from the Engineering ToolBox that lists the specific heat capacity of a turkey as 2.81 x 103J/(kg C).

But I have to go to work now. I'll solve for "X" tomorrow.
 
<h2>1. What is an interesting argument between friends?</h2><p>An interesting argument between friends is a debate or disagreement between two or more individuals who have a close relationship. The argument is usually centered around a topic or issue that both parties have strong opinions about. It can also involve challenging and questioning each other's beliefs or perspectives.</p><h2>2. Why do friends argue with each other?</h2><p>Friends argue with each other for a variety of reasons, such as differences in opinions, misunderstandings, or conflicting values. Arguments can also arise from a desire to prove a point or to express frustration. However, it's important to remember that arguments between friends can also be healthy and can lead to better understanding and communication.</p><h2>3. How can an argument between friends be resolved?</h2><p>The best way to resolve an argument between friends is through open and honest communication. Both parties should listen to each other's perspectives and try to understand where the other person is coming from. It's also important to remain respectful and avoid personal attacks. Finding a compromise or agreeing to disagree can also help resolve the argument.</p><h2>4. Is it normal for friends to argue?</h2><p>Yes, it is normal for friends to argue. Friends are individuals with their own thoughts, opinions, and beliefs, so disagreements are bound to happen. However, it's important for friends to handle arguments in a healthy and respectful manner to maintain a strong and positive relationship.</p><h2>5. Can arguments between friends strengthen their relationship?</h2><p>Yes, arguments between friends can strengthen their relationship if they are handled in a healthy way. When friends are able to communicate openly and resolve conflicts, it can lead to a deeper understanding and appreciation for each other. It also shows that both parties are willing to work through difficulties and value the friendship.</p>

1. What is an interesting argument between friends?

An interesting argument between friends is a debate or disagreement between two or more individuals who have a close relationship. The argument is usually centered around a topic or issue that both parties have strong opinions about. It can also involve challenging and questioning each other's beliefs or perspectives.

2. Why do friends argue with each other?

Friends argue with each other for a variety of reasons, such as differences in opinions, misunderstandings, or conflicting values. Arguments can also arise from a desire to prove a point or to express frustration. However, it's important to remember that arguments between friends can also be healthy and can lead to better understanding and communication.

3. How can an argument between friends be resolved?

The best way to resolve an argument between friends is through open and honest communication. Both parties should listen to each other's perspectives and try to understand where the other person is coming from. It's also important to remain respectful and avoid personal attacks. Finding a compromise or agreeing to disagree can also help resolve the argument.

4. Is it normal for friends to argue?

Yes, it is normal for friends to argue. Friends are individuals with their own thoughts, opinions, and beliefs, so disagreements are bound to happen. However, it's important for friends to handle arguments in a healthy and respectful manner to maintain a strong and positive relationship.

5. Can arguments between friends strengthen their relationship?

Yes, arguments between friends can strengthen their relationship if they are handled in a healthy way. When friends are able to communicate openly and resolve conflicts, it can lead to a deeper understanding and appreciation for each other. It also shows that both parties are willing to work through difficulties and value the friendship.

Similar threads

Replies
1
Views
1K
Replies
2
Views
3K
Replies
14
Views
2K
Replies
1
Views
2K
  • Sticky
  • Electrical Engineering
20
Replies
684
Views
91K
Replies
26
Views
8K
Replies
1
Views
1K
  • Electrical Engineering
Replies
12
Views
3K
  • Thermodynamics
Replies
4
Views
7K
  • Mechanical Engineering
Replies
15
Views
650
Back
Top