I believe there's an error in CH17a / Work Value of Gasoline

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wolfstemple's picture
wolfstemple
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I believe there's an error in CH17a / Work Value of Gasoline

One of the points I seen Chris make in his presentations and in the Crash Course is that gasoline has the potential energy of 500 human hours or 12.5-40 hour work weeks like in Chapter 17a.

I was debating with someone and I did the math and I think there is a magnitude error.

Gasoline has 1.3 x 10^8 joules of energy.  A calorie (small c) has 4.184J or a kilocal (what we see on food packaging) has 4.184x10^3J.

(1.3 x 10^8J ) / (4.184x10^3J per kcal) = 31,070 kcal

31,070kcal / 500 hours human labor only give 64 kcals for each hour hard human labor.  My body burns about 120 kcals sitting here on my computer

31,070kcal / 50 hours human labor give 640 kcals for each hour hard human labor.  This sounds more reasonable knowing what I can burn on my exercise bike going all out.

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Damnthematrix
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another way......

1L gas = 10kWh

1 Gall = 3.8L x 10kWh = 38kWh or 38,000Wh

Average human power = 100W

so...  38,000Wh / 100W = 380 hours

That's not one order of magnitude out.

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guardia
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or the 100W bulb exercise machine

There's nothing like watching someone pedal like mad just to keep a 100W bulb lit

Sure as hell implants a concrete idea in people's mind of how much energy we use

Samuel

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wolfstemple
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 Ok, I'm trying to work

Ok, I'm trying to work through the math (physics really isn't my forte), and on a wattage basis it seems a bit more reasonable (though overstated still) but on a caloric basis I just can't make it work.

I have a recumbent bike that tells me wattage, and on all the excercise places, 100W is light effort and 200W is vigorous (Lance Armstrong can sustain 500W for 20 mins).

The thing is, if I drive my car with 1 gallon on flat terrain and get 35 miles away, I can see it taking around 50 hours (6 work days) getting it back.  Or doing it with 5 other people and taking a whole day.  It taking 12 weeks (or 2 weeks with 6 people) just seems wrong.  I have pushed cars a ways single-handlely before (even vans) and just, no.  Back in WW1, when all the artillery pieces were pushed by humans/horses, it would have taken forever to get anywhere but didn't - it was slow, but it just didn't take them 3 days to go a mile.

I wish I put this topic in 17a specific chat.  This is really all I can criticize on crash course from what I checked out, and the point still stands, gas does a lot of the labor for us.  But I'm justr trying to wrap my head around this.

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admin
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 Moved the post to Chapter

Moved the post to Chapter 17a.

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HelenaV5
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wolfstemple wrote: One of

wolfstemple wrote:

One of the points I seen Chris make in his presentations and in the Crash Course is that gasoline has the potential energy of 500 human hours or 12.5-40 hour work weeks like in Chapter 17a.

I was debating with someone and I did the math and I think there is a magnitude error.

Gasoline has 1.3 x 10^8 joules of energy.  A calorie (small c) has 4.184J or a kilocal (what we see on food packaging) has 4.184x10^3J.

(1.3 x 10^8J ) / (4.184x10^3J per kcal) = 31,070 kcal

31,070kcal / 500 hours human labor only give 64 kcals for each hour hard human labor.  My body burns about 120 kcals sitting here on my computer

31,070kcal / 50 hours human labor give 640 kcals for each hour hard human labor.  This sounds more reasonable knowing what I can burn on my exercise bike going all out.

Wolfstemple, you might have gone wrong with your calculations.  I'm a dietitian and 1 calorie is 4.18 KILOjoules, not joules (1000 joules to the kilojoule) 
I've actually googled a calorie converter that got it wrong and said as you said 4.18 joules to the calorie, but that is wrong, ask for a calorie to kilojoule converter and it should say 4.2 (rounded) kilojoules to the calorie. 

Not sure if this corrects the calculation or not.

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sofistek
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Naesim wrote:Wolfstemple,

Naesim wrote:
Wolfstemple, you might have gone wrong with your calculations.  I'm a dietitian and 1 calorie is 4.18 KILOjoules, not joules (1000 joules to the kilojoule)

As wolfstemple mentioned a calorie, with a small "c", has 4.18 joules. As a dietician, you probably deal with Calories, with a capital "C", which is 4.18 kilojoules, as you say. A Calorie (capital "C") is 1000 calories (small "c"). Wolfstemple explained this in his calculation.

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