This is your brain on power

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Coppersmith said:
:? I might be confused but ....

The Watt is a unit of power therefore a quantity. Watts per hour would be a rate.
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Watts per hour makes no sense, becuase, like you guys said, a watt is a rate. Watts per hour would be analogous to an acceleration. We all know Watt-hours is a quantity of energy, As are Joules, one watt-hour is equal to 3600 Joules.

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Coppersmith said:
:? I might be confused but ....

The Watt is a unit of power therefore a quantity. Watts per hour would be a rate.
Click to expand...
No. The Joule is a unit of energy and the watt is one Joule per second.
A 100 watt light bulb uses energy at a specific rate rather than containing a specific total amount of energy (like, for example, a battery).
An amount of power per unit of time is not a particularly useful quantity. Example: "His energy use increased by 1kW per hour. He used 1kwh the first hour, 2kWh the second hour, etc."

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Im reading this stuff to avoid doing my thermo homework and only end up talking about thermo. Go figure.

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JFletcher said:
I remember reading somewhere before the human body generates about a hundred watts per hour. I also remember going to York Power Station as a kid and riding the bike they had hooked to a little Dynamo. you could just about power a hundred watt bulb peddling as fast as possible. then they said that at that rate you would have to pedal for something like 3.6 days to generate the amount of energy contained in a gallon of gas or maybe it was a pound of coal
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There's a youtube video of this guy, Olympic cyclist I think. You should see the legs on this guy. He was able to do about 700 watts for almost a minute. He was on the floor completely spent after. Its crazy how much energy is in a gallon of gas compared to what us puny humans can do.
 
Coppersmith said:
:? I might be confused but ....

The Watt is a unit of power therefore a quantity. Watts per hour would be a rate.
Click to expand...

Would it make more sense to say the human body generates about the same amount of energy that a hundred watt light bulb uses?

Perhaps I'm misremembering what I read. Heck is bad as my memory is, I might be quoting you something out of The Matrix. LOL

Watts per hour is indeed a rate. 100 watt light bulb uses 100 watts per hour or 2.4 kilowatts per day if were on continuously.
 
electrofelon said:
There's a youtube video of this guy, Olympic cyclist I think. You should see the legs on this guy. He was able to do about 700 watts for almost a minute. He was on the floor completely spent after. Its crazy how much energy is in a gallon of gas compared to what us puny humans can do.
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That is very impressive for a human. Almost 1 horsepower for 1 minute.

At some point though, speed is the limitation, and torque comes into play. I would think that somebody on a rowing machine could generate far more torque via using their arms and legs, and not be limited by their body weight.
 
JFletcher said:
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Watts per hour is indeed a rate. 100 watt light bulb uses 100 watts per hour or 2.4 kilowatts per day if were on continuously.
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No!
A 100W light bulb uses 100 Watt-hours (Wh) per hour.

Looking at it slightly differently, 100Wh/h = 100W.
There is a fundamental difference between using 100W for an hour, which makes sense, and using 100W per hour which makes no sense.
 
JFletcher said:
Would it make more sense to say the human body generates about the same amount of energy that a hundred watt light bulb uses?

Perhaps I'm misremembering what I read. Heck is bad as my memory is, I might be quoting you something out of The Matrix. LOL

Watts per hour is indeed a rate. 100 watt light bulb uses 100 watts per hour or 2.4 kilowatts per day if were on continuously.
Click to expand...

You remember correctly. The average human consumes 2,000 calories per day. Those are large calories; it is the same as 2,000,000 thermodynamic calories. That is the same as 2,326 watt-hours. For a 24-hour day, that would be a rate of 2,326/24 = 96.9 watts. Obviously you pump out more during the day when active so 100 watts is a reasonable ballpark. It's the easy way to estimate HVAC loads for a given space. How many people x 100 watts. This also means that 20 watts is not an outrageous estimate for what keeps the ole' noggin ticking away.
 
gadfly56 said:
You remember correctly. The average human consumes 2,000 calories per day. Those are large calories; it is the same as 2,000,000 thermodynamic calories. That is the same as 2,326 watt-hours. For a 24-hour day, that would be a rate of 2,326/24 = 96.9 watts. Obviously you pump out more during the day when active so 100 watts is a reasonable ballpark. It's the easy way to estimate HVAC loads for a given space. How many people x 100 watts. This also means that 20 watts is not an outrageous estimate for what keeps the ole' noggin ticking away.
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He remembers correctly that the human body dissipates about 100W (while awake and active, or averaged over the whole 24 hour cycle? It also varies with ambient temperature because of the need to maintain body temp in a cold environment.)
But the rest of his remembrance is flat out wrong.
 
GoldDigger said:
He remembers correctly that the human body dissipates about 100W (while awake and active, or averaged over the whole 24 hour cycle? It also varies with ambient temperature because of the need to maintain body temp in a cold environment.)
But the rest of his remembrance is flat out wrong.
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His line 1 is correct.

His line 3 is not correct. As pointed out, Watts are already a rate, it is incorrect to add "per hour".

I think a major source of confusion is the fact that we use BTU's as a unit of energy and blithely use it interchangeably with Watts and forget that one is a rate, the other is a quantity.
 
GoldDigger said:
No!
A 100W light bulb uses 100 Watt-hours (Wh) per hour.

Looking at it slightly differently, 100Wh/h = 100W.
There is a fundamental difference between using 100W for an hour, which makes sense, and using 100W per hour which makes no sense.
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You are correct. A watt is a unit of power just like horsepower, and it would not make any sense to say like your car makes 300 horsepower per hour.

Could one use watts per hour if discussing a rate of change, such as a hundred watt light bulb dimmed down to 10W over the course of a minute?
 
JFletcher said:
... Could one use watts per hour if discussing a rate of change, such as a hundred watt light bulb dimmed down to 10W over the course of a minute?
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Yes.
But using multiple different units (seconds and hours) for the same thing in a single expression would be confusing.
Watts per second would be preferred.
 
Besoeker said:
Not in my opinion.
A watt is already a rate.

1W = 1J/s

What did I miss?
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Please reconsider. The rate of change of wattage is also a rate.
As in the example of a light dimming from 100W to zero Watts over 10 seconds is *dimming* at 10 Watts per second.

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Watts per second, or Joules per second-squared, could be correctly used to describe rate of change of power.

Watts per hour, or Joules per second per hour, could also be used, but it would be confusing.

But rate of change of power is something that almost never comes up, and it's a lame attempt to find a gnat's penis worth of redeeming value in an earlier post that was actually just a mistake.

Another place it might be used is to describe how quickly a powerplant can change its output to match a changing load.
 
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