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- Now that we understand a firm's cost
curves and its entry and exit decisions

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we're able to show how supply curves are
actually derived from these more

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fundamental considerations.
Let's take a closer look.

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- The supply curve
is built upon firm

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entry and exit decisions and the
effect of these decisions on industry

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costs and the key question is this as
industry output increases, what happens to

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costs? There are three possibilities.
First, an increase in cost industry. That

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is industry costs increase with greater
output. Second, constant cost industry

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industry costs are flat. They don't change
with greater or lesser output and

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finally a decreasing cost industry,
industry cost falls with greater output.

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As we'll see the first and second are
quite common the third is quite uncommon

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but is nevertheless important and
interesting in order to understand

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economic geography which we'll come to a
bit later. Let's show how the industry

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supply curve is derived from the entry and
exit and cost curves of individual firms.

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We can do this for an increase in cost
industry very easily with just a two firm

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example. Suppose that firm one is a
producer of oil where its oil is very

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close to the surface so it has a quite low
average cost curve. It's pretty cheap for

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this firm to produce oil. On the other
hand, firm two has a much higher average

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cost curve because for firm two it's
located in a part of the world where it

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has to drill much deeper in order to get
the oil. Now given these figures what's

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the industry supply curve of oil if the
price of oil is below $17? Well, if the

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price of oil is below $17 neither of
these firms can make a profit.

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That's below the minimum point of the
average cost curve for both of these

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firms. So neither of these firms is going
to want to be in the industry. So if the

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price of oil is below $17 the industry
supply is just going to be zero, right

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here, zero. Now what happens at $17? Well,
at $17 firm one just breaks even. So

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we'll say firm one will just enter the
industry. So at $17 the industry output

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is the same as the output of firm one
namely four units. Notice that at $17

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firm two doesn't enter the industry
because the price is still too low. Firm

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two is not going to make a profit. We'll
take a loss at that price. Indeed as the

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price of oil increases the output from
firm two will increase as it moves along

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its marginal costs curve. That will
continue to happen so industry output will

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increase along with the output of firm one
until we reach a price of $29. At the

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price of $29 firm two just breaks even
and it enters the industry. So at $29

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total industry output is 6 units from
firm 1 and 5 units from firm 2 for

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a total of 11 units from the industry. As
the price goes above $29 both firm 1

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and firm 2 expand along their marginal
cost curves so the industry output is then

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the sum of the output from both firms. So
what we see here is that the industry

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supply curve is upward sloping because the
cost curves of these firms are different

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because in order to attract more firms
into this industry the only way we can do

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that is by attracting higher cost firms.
So the industry supply curve is upward

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sloping. Any industry where its difficult
to exactly duplicate the productive inputs

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is going to be an increase in cost
industry. I've already mentioned oil but

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copper, gold, silver all the mining
industries are very similar. We can't just

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duplicate another gold mine. If we want
another gold mine we're going to have to

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dig deeper, we're going to have to look
elsewhere, it's going to be more expensive

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to produce it than it is now. Coffee is
another example because there's really

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only a limited number of places in the
world where we could produce great coffee.

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If we want coffee from other places than
Brazil or Guatemala it's going to be

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lower quality. We're going to have to go
down further on the mountain. It's going

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to require more inputs. Nuclear engineers,
very hard to expand the supply of nuclear

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engineers. There's a limited number of
people who can be a nuclear engineer. If

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we want more nuclear engineers, we're
really going to have to pull them from

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other industries where they have very high
opportunity cost. So it's hard to expand

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the supply of nuclear engineers without
pushing up the wages of nuclear engineers.

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That's an increasing cost industry.
Moreover, any industry that buys a large

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fraction of the output of an increasing
cost industry will also be an increasing

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cost industry. So pretty obviously
gasoline is an increasing cost industry

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because if we want more gasoline that
requires more oil and oil is an increasing

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cost industry. Electricity will primarily
be an increasing cost industry to the

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extent that we generate our electricity
from coal. So if we want a lot more

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electricity we're going to require more
coal and that's going to push the price of

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coal up which is going to push
the cost of producing electricity up.

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- So what we just showed is that for an
increasing cost industry you can derive a

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upward sloped supply curve.
We're now going to do a constant cost

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industry for which we'll show you actually
get a flat supply curve and then a

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decreasing cost industry, which as you
might expect will give you now a

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downward-sloped supply curve. We'll
do these in separate lectures. Thanks.

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- If you want to test yourself,
click, "Practice Questions," or if you're

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ready to move on,
just click, "Next Video."

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