# Economics 3rd ch03

Economics
THIRD EDITION

By John B. Taylor
Stanford University

1

Chapter 3
The Supply and
Demand Model

2

Overview
The supply and demand model is fundamental to

economics. Supply and demand are discussed as
representing the interests of sellers and buyers,
respectively. Next, supply and demand curves are
shown. Shifts of the curves compared to
movements along the curves are discussed. The
model is then used to show the determination of
price. The causes of shortages and surpluses are
explained. Finally, market interference is discussed
within the issues of price ceiling and price floors.

3

Teaching Objectives
1. Develop the demand curve. Differentiate between
changes in demand and changes in quantity
demanded.
2. Develop the supply curve. Differentiate between
changes in supply and changes in quantity
supplied. List the factors that shift the supply
curve.
3. Show how the interaction of supply and demand
determines the equilibrium price.

4

Teaching Objectives (Cont.)
4. Use supply and demand analysis to
explain things students are interested in.
5. Show how price controls interfere with the
market and create surpluses or shortages.
6. Discuss how shifts in the demand and/or
the supply curve will change the
equilibrium price. Shift curves
simultaneously. Again, the "shifts" versus

"movements along" distinction is very
important but difficult for some students.

5

Demand

1a. Demand is a relation between the price
of a good or service and the quantity of that
good or service consumers are willing to
buy at that price per unit of time, ceteris
paribus. Demand curves relate price and
quantity demanded

6

Demand
1b. The law of demand states that price and
quantity demanded are negatively related.
Price is the independent variable and
quantity demanded is the dependent
variable. Price determines quantity.

7

Figure 3.1
The Demand Curve

8

Demand
1c. Demand is shown graphically by the
demand curve – see Figure 3.1.
We relate the negative slope to the inverse
relationship in the law of demand.
1d. Price affects quantity because changes
in price create an incentive to substitute.

9

Shifts in the Demand Curve
1e. Demand curves shift around, indicating
changes in other variables. A shift is called a
change in demand. See Figure 3.2.
1e.1. The main determinants of demand are
consumer preferences, information, income,
price expectations, population, and the
prices of related goods.

10

Figure 3.2
A Shift in the
Demand Curve

11

Shifts versus movements

1e.2. It is important to distinguish between shifts
of the demand curve and movements along the
demand curve. Use Figure 3.3.
Note: Changes in a variable not identified on the
vertical axis shift the function, whereas changes in
the listed variable cause movement along the
given function.

12

Figure 3.3
Shifts versus
Movements Along
the Demand Curve

13

2. Supply
2a. Supply is a relation between the price of
a good or service and the quantity of the
good or service a supplier is willing to sell
at that price, ceteris paribus.
2b. The law of supply states that price and
quantity supplied are positively related. This
is shown in Table 3.2.

14

Figure 3.4
The Supply Curve

15

Supply
2c. As with demand, price creates an incentive to
increase the quantity supplied as price increases.
Again, price is the independent variable and quantity
supplied is the dependent variable. Relate this to the
issue of causality discussed earlier. Figure 3.4 gives the
supply curve
The positive slope is due to the positive relationship in
the law of supply.

16

Shifts in Supply Curve
2d. Supply curves shift around, indicating changes
in other variables. See Figure 3.5. A shift is called
a change in supply.
2d.1. The principal determinants of supply are the
state of technology, the price of inputs, price
expectations, the number of sellers, and other
things that affect costs such as taxes, subsidies,
and regulations.

17

Figure 3.5
A Shift in the Supply
Curve

18

Shifts versus Movements
• 2e. Figure 3.6 to shows the difference
between movements along the supply curve
and shifts in it.
• Movements are due to changes in non-price
determinants of supply while movements
are due to price changes (variable on the
vertical axis).

19

Figure 3.6
Shifts versus Movements
Along the Supply Curve

20

Figure 3.7
Overview of
Supply and
Demand

21

3. Market Equilibrium: Combining Supply and Demand

• 3a. The model of supply and demand is used to
explain the determination of market price. Use
Table 3.3 and Figure 3.7.

3b. Price adjusts to differences in quantity
demanded and quantity supplied. A shortage
indicates a willingness on the part of consumers to
pay sellers higher prices. A surplus indicates a
willingness on the part of sellers to lower prices.
3c. Price reaches equilibrium when quantity
demanded equals quantity supplied, ceteris
paribus.

22

Figure 3.8
Equilibrium Price and
Equilibrium Quantity

23

Equilibrium

movements along the curves and not shifts
of the curves.
3d. Shifts in the demand and/or the supply
curve may result in a price adjustment.
Figures 3.8 and 3.9 will be useful.

24

4. Using the Supply and Demand Model: A Case Study

• 4a. Figures 3.11, 3.12, and 3.13 show the
effects of shifts in the demand and supply of
peanuts on the price of peanuts. This
example shows the confusion between
movements along the demand curve and
shifts in the demand curve. See Figure 3.10. 