Download Demand and Supply

1
Handout #3
Demand and Supply
Prepared by Kornkarun Kungpanidchakul
September 19, 2005
perfect competitive market  buyer and seller take price as given
In the perfect competitive market, demand and supply of goods or services
determine their own price
Demand
- The quantity of goods or service that a consumer would buy for any particular
price, given other factors constant.
- Negative relationship between price and quantity
- Note that we consider only linear function in econ101.. Therefore we have
demand function in form of:
QD  a  bP
-
P
Q
Factor that shift demand curve (see more details in chapter 3, Hall and Lieberman)
1. Income
For inferior good Income increases  Demand decreases  shift left
For normal good increases  Demand increases  shift right
2. Price of related good
For substitute good, price of related good increases  demand increases
For complement good, price of related good increases  demand decreases
3. Population
4. Taste
5. Expected price
Supply
- The quantity of goods or service that a producer is willing to sell for any
particular price, given other factors constant.
- Positive relationship between price and quantity
- The supply function is in form of: QS  a  bP
2
P
Q
Factor that shift supply curve (see more details in chapter 3, Hall and Lieberman)
1. Input prices
Input prices decreases  supply increases  shift right
2. Price of an alternate good
Price of an alternate good increases  supply decreases  shift left
3. Technology
4. Number of firms
5. Expected price
6. Change in weather/natural events
Equilibrium price and quantity determination
1. Graphical approach (see more details in chapter 3, Hall and Lieberman)
P
P*
Q*
Q
2. Mathematic Approach
Ex.
QD  20  P and QS  2 P
Q D  QS
Then at the equilibrium
10  P  2P
P* = 20/3 and Q* = 40/3
3
Consumer and Producer Surplus
Consumer Surplus: The area below the demand curve but above the equilibrium price.
P
P*
Q
Q*
How to calculate?
From QD  20  P
P = 20- QD
Then CS = 1/2 (20-P*)Q*
= 1/2 (20-20/3)(40/3) = 800/9
where P* is the equilibrium price and Q* is the equilibrium quantity.
Producer Surplus: The area above the supply curve but below the equilibrium price.
P
P*
Q
Q*
PS = 1/2 P*Q* =1/2(20/3)(40/3) = 400/9
4
Horizontal Summation
- Why we call “Horizontal Summation”
To find the market demand (supply), we will add up individual quantity demanded
(supplied) at every price level.
I will show the example of horizontal summation of supply. You can apply to
horizontal summation of demand.
1. When individual quantity supplied is given. (easy case)
Example1
There are two newspaper suppliers in Madison. Find the market supply.
Supplier#1
price
Quantity supplied
8
4
6
3
4
2
2
1
0
0
Supplier#2
price
Quantity supplied
8
8
6
6
4
4
2
2
0
0
Market Supply
price
Quantity supplied
8
12
6
9
4
6
2
3
0
0
2. When individual supply function is given.
Case 1: Identical supply function
Step:
1. Rearrange the supply functions of both markets in the way that Q (quantity) is on
the LHS of the equation.
2. To do the horizontal summation, add up quantity supplied by each producers
together. Suppose that there are two producers in the market, then Qmarket = Q1
+ Q2 . (Note that from step 1, you will get Q1 and Q2 as a function of price. Then
you just need to add up these two equations to get total quantities supplied in the
market.) Now you get the market supply function and you’re done.
Quick Tip : In the case that supply function for all producers is identical. Suppose that
there are N producers in the market. The individual supply function is given by P=mQ+b.
m
Then, the market supply function is P= Q  b . (same Y-intercept, slope is divided by N)
N
5
Example2 (first midterm, 2001)
The demand for a week’s worth of newspaper delivery in Madison is given by the
equation QD= 220-20P There are two daily papers, each with supply curves given by the
equation QS =20P-10 What is the equilibrium price for a week’s worth ?
Case 2 : Individual supply function has a different slope and the different yintercept.(hard case)
In this case, the market supply is “kinked”. Suppose that the supply function for the first
producer is given by P=A-mQ. The supply function for the second producer is given by
P=B-mQ. Suppose further that A<B.
Step:
1. Find the kinked point
- The supply curve is kinked at P=B ( the y-intercept of the supply function with the
higher y-intercept)
2. Since the market supply is kinked. The market supply is composed of two functions.
When P<B and when P>B.
3. Consider when P<B. The market supply function is P=A-mQ. ( The supply function
for the first producer, with the lower y-intercept)
4. Consider when P>B. You use the same step as case 1 to find the market supply.
4.1 Rearrange the supply functions of both markets in the way that Q (quantity) is on
the LHS of the equation.
4.2 To do the horizontal summation, add up quantity supplied by each producers
together. Suppose that there are two producers in the market, then Qmarket = Q1
+ Q2 . Now you get the market supply function and you’re done.
Example 3 (The second midterm, 2001)
Consider the US automobile market. Suppose, for simplicity, there are only two
automobile
producers: Toyota and Ford. The market demand curve for automobiles is given by:
P=150-2Qd. The individual supply function for Ford is given by P=30+2Qs. The
individual supply function for Toyota is given by P=60+2Qs. Find the market supply
function.
6
7
Example 4: (first midterm: fall 2004)
Consider the city of Madison.
Let the demand for bread be described by P=10-QD
Let the supply of bread be described by P=2+QS
Suppose that the city of Madison sets a price ceiling of $3 in the bread market. Is there a surplus
or a shortage in the bread market after the imposition of the price ceiling? How much?