February 29, 2020 0

Energy inputs for tilling a hectare of land | Cosmology & Astronomy | Khan Academy

Energy inputs for tilling a hectare of land | Cosmology & Astronomy | Khan Academy


In the last video, we talked
about how humans, over time, have gotten better and better
at getting more calories out of a given unit of land. And one thing I do
want to emphasize, in the last video, these
numbers that I came up with– I picked these numbers
to make the math fairly simple– and to give you
the general idea. I don’t want you to
think that the most that a square kilometer
can support today is exactly 1,000 people. It depends, hugely, on
what the land is like. How much of the land you’re
actually using for agriculture. What crops you’re
planting, et cetera. How the people are living. How many calories they need. The whole point
of the last video was just to give you a
framework that, wow, there is this upper bound based on how
much productivity you actually get from the land. Now, what we want to think
about in this video– that was– the last video was
this axis right here, getting more and
more out of the land. What I want to think about in
this video is, how did humans, through different
technologies, how did we get by doing less
and less of the labor for getting those
calories out of the land? Obviously, you just don’t have
land and thing spontaneously grow– well, I guess that
would happen in the wild. But if you’re doing
agriculture, you need to put some
energy into the land. You’ve got to work the land. And so what we have
over here in this chart, and this chart is
derived from information from this book right over here. This is Energy and Society. And what we want
to think about here is the different
ways that humans have gone about to till soil. So we’re not even going to
think about the total process or the total energy required
to grow a certain crop. What we’re just going
to focus is one step of the agricultural process. And that is tilling the soil. And in case you’re like
me and you have never worked on a farm– which,
that’s one thing I would like to change one day is actually
go through that process– but tilling the soil is
kind of churning it up. So you get the nutrients
from the bottom layers to the surface. You bury all of the remains
from the last harvest. You bury all of the
weeds so that they die. And you also get
air in the soil. What it does is it
essentially prepares the soil for the next
agricultural cycle. So it’s a process
that humans have been doing since antiquity. And what I want to
do, in this video, is think about the
different ways to do it. And how much energy
is required to do it. And we’re going to think
about the energy in two ways. How much of that energy
comes from humans? And how much of
that energy comes from things other than humans? So just as an example, when
we talk about human power, we’re talking about
someone literally hand-plowing this field. So this woman right
over here is literally– she has this little cart that’s
digging up the soil behind her. When we talk about
oxen power, we’re talking about the oxen
doing most of the work. They’re the ones dragging
this plough which is digging up all of the soil. And this gentleman has
to be there to supervise. But this still is
fairly intense labor that this gentleman is
doing right over here. And then, when we
talk about tractors, we’re talking about
a scenario like this. Where the tractor is doing most
of the work of actually digging up, dragging this
plow behind it, and digging up all of the soil. And from this book
right over here, that’s where we
got these numbers. I’ll tell you which
numbers I got from them and which numbers I reasoned
through, because I wasn’t fully comfortable with the
numbers that they had. But these are their numbers. That, if you are human power,
to till one hectare of soil, it’ll take 400 hours. Oxen power– and this
should be a pair not a pari. That should be a pair
of oxen, 65 hours. A 6-horsepower
tractor, 25 hours. A 50-horsepower
tractor, four hours. And in case you
all are wondering, what is a hectare of soil, it
is literally a plot of land. A hectare of soil. Let me write it. A hectare of land
is a plot of land that is 100 meters
by 100 meters. And it’s roughly equal to 2 1/2
acres, not exactly 2 1/2 acres. It’s like 2.4. I think 2.47 something,
but roughly 2 1/2 acres. So we’re just thinking about
how many hours to essentially dig up all the soil for plot
of land 100 by 100 meters. So what we have over
here, so clearly, human takes a lot longer. Oxen, they can do a
little bit faster. 6-horsepower
tractor, even faster. A 50-horsepower tractor,
very powerful tractor, even faster than that. Now, this column right
over here is the amount of energy required to
actually produce and maintain the machinery used. So this is a very
unintuitive thing. Whenever you think about–
for example, whenever you think about the
amount of energy to plow this land over
here, you tend to think, OK. Well, this individual
is going to have to expend a lot of her energy. You don’t think about the
amount of energy required to actually maintain the tool. To one, build the tool that
she’s using, in this case, a hand plow. And then, to maintain
that as she does it. And so this estimate– and I
got these two from these fellows right over here, or, actually,
one of them might be a gal. But it’s about
6,000 kilocalories. And this is k. This is kilocalories
for lowercase c. And one thing I want
to emphasize here, one kcal is equal to, is the
same thing as one calorie with a capital C, which
is the same thing as 1,000 calories with a lowercase c. And we talked about
this in the last video. But when people talk
about food calories, they’re really talking about
a calorie with a capital C. Or you could say they’re
talking about kilocalories. So your candy bar,
200 calories, they’re talking about this
right over here. In chemistry class, when you
talk about the amount of energy to raise a gram of water
one degree Celsius, you’re talking about
these calories here. So in all of these numbers in
this chart right over here, they are in either–
you can either view as this unit, kcals,
kilocalories, or calories with a capital C.
They’re essentially the same units that we
used in the last video. And these are the
same numbers that you are used to from a dietary
calorie point of view. So for example, 6,000
calories, that’s about how much of a typical
male would expend in three days. So this is to maintain it over
the course of these 400 hours, in the case of the hand plow. And the total amount
of calories that were needed to make
the plow divided by the total number of hours. So whatever fraction of the
plow’s life is being used here, you use that fraction
right over here to put this 6,000 calories. But needless to say,
for at least the plow, for either the human
or the oxen scenario, this isn’t a significant
amount of the total calories. So obviously, if
you’re doing it either with human or oxen power,
you’re not using any gasoline. You’re not using any petroleum. In all of these
scenarios, we’re going to assume that someone has
10 working hours in the day. And that right over
here, this is a measure of how hard that
person’s work is. And I estimated
these numbers here. They’re slightly different than
what the original numbers were in this book right over here. But we’re saying, look. If you are actually walking
along using this hand plow, that is actually very,
very vigorous activity. So it is going to require
about 400 calories per hour to do this type of activity. You do it over 10 hours. It’s going to require
4,000 calories just to do that over 10 hours. And then, we’re assuming
that there’s some rest. That the rest of the day,
you’re going to walk around. And maybe you’re going to
cook dinner, eat breakfast. You’re going to
sleep some of it. We’re assuming that the
other 14 hours a day are going to be at about
100 calories per hour. And so this is the total. If someone were to,
using this technique, work for a total
of 10 hours, this is how many calories they
would consume in the day. And you can see, this is
the most labor-intensive. So it looks like that
they would consume the most calories per day. These two are the
least labor-intensive. You’re sitting on a
tractor although that still requires more calories than
sleeping or watching TV, and so that’s the number of
calories they would consume. Now, this right
over here– and this is the interesting
number, or one of the really interesting
numbers– based on all of these
assumptions, this is the total human
input in calories to do this task, to till
this one hectare of soil. So over here, you’re using
5,400 calories a day. If you’re working
10 hours per day and it requires 400
total hours, you’re going to be working 40
days, 400 divided by 10. 40 days times 5,400
calories per day. It’s going to take a human–
just the human part, not even thinking about
the 6,000 calories necessary to maintain and
make that plow– the human is going to spend to
216,000 calories to till, to plow that one
hectare of land. And if you add the other
6,000 in for the actual plow– and you could debate what
this number should be, but it’s not a significant
number compared to this– you get about 222,000
total calories. When you go to the
oxen situation, you’re requiring fewer hours. And each hour, it requires
a little less calories. This is still labor-intensive,
but not as labor-intensive as what this woman right
over here is doing. So on a daily basis, using
a little bit fewer calories. But since you’re only
doing 6 1/2 days of this, 65 hours divided by 10,
you’ve significantly reduced the number of calories, the
total number of calories that the human needs to
put into this task. Now, there still is
other energy being done. And now all of a sudden, the
oxen have gotten involved. And if you assume that each oxen
consumes about 20,000 calories a day, and you have two of them. So 40,000 calories per
day just to feed the oxen. And you’re going to do that for
6 1/2 days, 65 divided by 10, the oxen are going to consume
260,000 calories to do this task. So the total energy input
here, now, has gone up. So this is an
interesting phenomenon that is going on
right over here. What the human is
putting in, as we get better and
better technology, goes down substantially. 216,000 to 33,000, and
we’ll see with the tractor goes down even more. But the total energy, if you
include the amount of energy that the oxen have to
put in, or if you include the amount of energy
due to the gasoline that has to be used for the tractor,
the total amount of energy is going up to plow that field. But the human energy
goes down dramatically. Now, the last thing I
want to highlight here– and these are where my numbers
depart a little bit, or fairly significantly, from this
original study right over here, this original estimate–
is the machinery input on the tractors. So if you look this up, and
you could Google search it, they have much
larger numbers here. But I did a little research. And it looks like, for
most petroleum-based, combustion-based engines
or vehicles, roughly 20% of the total energy that’s used
in fuel, 20% of that energy is used to for the actual
production and maintenance of that vehicle over its life. So what we did over
here is, we said, OK. For a 6-horsepower tractor–
I used their numbers– where you’re going to have
to use 25 hours to do it, it’s going to use
this much petroleum, assuming that it uses 23.5
liters of gas or petroleum over 25 hours. And then, I just took
20% of that number for saying, well,
how much energy had to be used to
maintain that vehicle over that amount of time? And if you think about what
fraction of this vehicle’s life that 25 hours
represents, that fraction times the total amount of
energy required to produce that. So remember, these
things are made of metal. They had to be made in furnaces. So just producing a vehicle
requires a lot of energy. And so this right
over here is 20%. And I just use
that rule of thumb for most petroleum-based or
combustion-based vehicles. That 20% of the total
energy expenditure over the course of
that vehicle’s life is roughly equal to
the amount of energy used to produce that vehicle. But either way, you go
all the way over here. The human has to spend
less calories sitting on the vehicle. So they spend less
calories per day. And then, the total human
input right over here, for the 6-horsepower tractor,
it’s going to take them 2 1/2 days– 25 hours at
10 hours a day– is going to be 8,500 calories. But of course, you
have the petroleum used and then some estimate
of the amount of energy used to produce that tractor. And you’re just taking the
fraction over that 25 hours. You’re not taking
the entire life of that 6-horsepower tractor. To produce a
6-horsepower tractor, this number would be
much, much larger, if you talked about the
total number of energy. We’re just taking the
small fraction of its life that we’re using
it right over here. Same thing for the
50-horsepower tractor. But any way you look at
it, the human– and this is a really interesting thing. Humans, by going
from human power all the way to a
50-horsepower tractor, you’re getting almost a
factor of 200 improvement, in terms of how
little energy has to be put in by the
human to till that land. But you actually get
a total increase, if you factor in things like the
petroleum and then, definitely, the amount of energy to
actually produce that machine. So anyway, hopefully, you
found that interesting. I find this– it’s
something that you don’t think a lot about. How much energy input
has to be put in? And oftentimes, we only think
about the human energy input. But we’re not thinking about the
energy input from other things, like oxen.

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