In these models of the market, we define the behavior or sellers based on the goal of profit maximization in the production and/or sale of a particular good. Higher selling prices allow a trader/seller to reap a gain over and above the price initially paid for a final good or asset. In the case of business firms, the production of additional units of a particular good involve increasing opportunity costs in drawing resource inputs away from other productive uses. Higher prices are necessary to cover these increasing costs of production. Thus, these types of behaviors on the selling side of the market typically lead to a positive relationship between market price (the dependent variable) and quantity supplied (the independent variable).

Separately, we define the behavior of buyers based on the goal of maximizing the utility gained from the purchase and consumption of this same good. As prices fall, holding income constant, the buyer finds that his/her purchasing power has increased allowing for buying greater quantities of a particular good. It is also the case that, for the consumer, additional quantities of a good consumed provide less additional satisfaction relative to previous units consumed. This notion known as diminishing marginal utility implies that the consumer is willing to pay less for these additional units as it becomes more efficient to use his/her income for the purchase of other goods. For the buyer, these types of behaviors typically lead to a negative relationship between the market price (dependent variable) and quantity demanded (another independent variable).

These relationships are demonstrated numerically in the table and graphically in the diagram in Figure 1. Press any of the 'Plot' buttons.

In these models, we assume that one unique price exists such that the Quantity Supplied by sellers is exactly equal to the Quantity Demanded by buyers. This unique price P* is defined to be the equilibrium price.

This notion of Equilibrium tends to be a rather strong assumption in these economic models.

In the physical world we often observe equilibrium conditions or situations resulting from the influence of physical laws. For example: a piece of chalk resting on a table is in equilibrium. This situation is the result of  the effects of gravity and the existence of a flat and level surface. Gravity helps to maintain and even restore this equilibrium condition if this position of rest is disturbed.

In our market models, we need to ask is: where does the gravity come from to establish and maintain an equilibrium price? The answer is in the reaction of sellers and buyers to disturbances in the market.

These sellers would react by cutting the price of their product relative to competing sellers (price-cutting is how sellers compete) and by reducing the rate of production. [Press Price Adjustment] Buyers would react to the presence of lower prices by increasing their rate of consumption. This process would be expected to continue until the excess inventories have been eliminated.

These consumers react by bidding prices up in competition with other buyers (bidding is how buyers compete) much like an auction for a single piece of art. As these prices are bid upwards, some buyers drop out of the market reducing the overall rate of consumption. [Press Price Adjustment] Sellers react to the presence of higher prices by allocating resource inputs from other uses towards production of this particular good.

Thus in our models of the market place, Competition provides the gravity to maintain or restore the equilibrium price. If surpluses exist, competition among sellers force prices downwards. If shortages exist, competition among buyers force prices upwards.

In typical market models surpluses are the result of market prices exceeding the equilibrium price such that price-cutting behavior helps restore this equilibrium price. Shortages are the result of market prices taking values below the equilibrium price such that bidding restores the equilibrium price. However, this is not always the case. For example, examine the following diagram:

In the above model, the unusual demand curve may be the result of speculative behavior by buyers. In this case, individuals are making purchasing decisions not for final consumption of this particular good, but rather in the expectation of resale of the good at an even higher price. As prices are bid upwards, these expectations are confirmed thus leading to further increases in the rate of purchase. Ultimately, prices rise to such a level that expectations of further increases are no longer realistic. At this point in time, the prices that have been inflated by these expectations (much as a bubble expands) collapse. The speculative bubble begins to burst resulting in a collapse in the market.

In reality, surpluses and shortages are caused by changes or shifts in either the demand or supply functions. These shifts are the result of shocks to other (exogenous) variables that affect supply decisions by producers or demand decisions by consumers. Typically, outward shifts in demand will lead to an increase in both the equilibrium price and quantity due to movement along an upward sloping supply curve. Inward shifts of demand will have the opposite effect (a decrease in equilibrium quantity and price). Outward shifts in supply (along a downward sloping demand curve) will lead to an increase in equilibrium quantity and a reduction in equilibrium price. You can experiment with these changes in Figure 5 below:

• Competition
• Demand
• Diminishing Marginal Utility
• Equilibrium
• Equilibrium Price
• Market
• Opportunity Costs
• Price Bidding
• Price Cutting
• Profit Maximization
• Quantity Demanded
• Quantity Supplied
• Shortage
• Speculation
• Supply
• Surplus
• Utility Maximization