February 22, 2018 

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SHORT-RUN SUPPLY CURVE: For a perfectly competitive firm, the marginal cost curve that lies above the average variable cost curve. This segment of the marginal cost guides a perfectly competitive firm's profit maximizing production as it equates price to marginal cost. Because the marginal cost curve is positively sloped (due to the law of diminishing marginal returns), each firm's supply curve and the market supply curve are also positively sloped. The law of diminishing marginal returns thus provides an explanation for the law of supply. However, this only works for firms with NO market control. Monopoly, monopolistic competition, and oligopoly, with market control, do not achieve the same result.

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A disruption of consumer equilibrium identified with utility analysis caused by changes in the price of a good, which likely results in a change in the quantities of the goods consumed. The change in the price alters the marginal utility-price ratio and forces a reevaluation of the rule of consumer equilibrium.
Utility analysis can be used to illustrate how a change in the price of a good alters the consumer equilibrium combination of goods consumed. According to the law of demand, a change in the demand price of one good results in an opposite change in the quantity demanded of that good.

Moreover, according to the other prices demand determinant, a change in the price of one good is also likely to affect the purchase of another good. As such, this particular utility analysis of consumer equilibrium can provide insight into both the law of demand and the other prices demand determinant.

A Review of Consumer Equilibrium

Pretzels and Sundaes
Consumer Equilibrium
First, a review of consumer equilibrium is in order.
  • Two Goods: The table at the right presents utility information for Duncan Thurly, a snack-hunger consumer who is purchasing two goods, Max Mulroney's Pretzel-on-a-Stick and Hot Momma Fudge Bananarama Ice Cream Sundae.

    The left half of the table summarizes the utility numbers for Duncan's pretzel consumption. The utility story for Duncan's hot fudge sundae consumption is in the right half of the table.

  • Income and Prices: Duncan's initial choice of pretzels and hot fudge sundaes is based on a snack purchasing income of $20, a pretzel price of $2, and a hot fudge sundae price of $4.

  • The Choice: Under these circumstances, Duncan selects 4 pretzels and 3 hot fudge sundaes, spending $8 on pretzels and $12 on hot fudge sundaes.

  • Consumer Equilibrium: This combination satisfies the rule of consumer equilibrium and equates the marginal utility-price ratio for each good. The marginal utility of the last pretzel is 6 utils, giving a marginal utility-price ratio of 3 utils per dollar. The marginal utility of the last hot fudge sundae is 12 utils, also resulting in a marginal utility-price ratio of 3 utils per dollar. Consumer equilibrium is achieved.

  • Utility Maximization: With consumer equilibrium, utility is maximized at a total of 84 utils. The purchase of 4 pretzels generates 36 utils of satisfaction and the consumption of 3 hot fudge sundaes adds another 48 utils. No other combination of pretzels and hot fudge sundaes purchased with $20 of income and the existing prices generates a greater level of utility.

A Sundae Price Change

The main point of analysis is to pose the question: What happens if there is a change in the price of one good? In particular, suppose that the price of hot fudge sundaes declines from $4 to $2. How might the consumer equilibrium purchase of pretzels and hot fudge sundaes change?

To answer this question, the original consumer equilibrium must be reevaluated. If Duncan continues to consume 3 sundaes and 4 pretzels after the sundae price reduction, his total expenditures are only $14 ($6 spent on 3 sundaes and $8 spent on 4 pretzels). In effect, Duncan has an extra $6 of income to spend.

With the lower $2 hot fudge sundae price, Duncan could purchase up to 3 more sundaes. However, in that the pretzel price is also $2, Duncan could purchase up to 3 additional pretzels, too. In fact, because both goods now carry a $2 price, Duncan could purchase any combination of extra pretzels and sundaes that sums up to 3.

Pretzels and Sundaes
Consumer Equilibrium

The key to determining the extra quantity of each good purchased is the same as that for the initial consumer equilibrium--the marginal utility-price ratio. Because the price of hot fudge sundaes changes, the marginal utility-price ratio for each quantity also changes. To reveal the new ratios, click the [New Ratios] button in the exhibit to the right. Note that for each hot fudge sundae, Duncan now receives a greater level of satisfaction per dollar spent.

This new set of marginal utility-price ratios reveals that the original purchase of 3 sundaes generates 6 utils per dollar, compared to the initial ratio of 3 utils per dollar. Moreover, this new ratio means the rule of consumer equilibrium is now out of balance. The fourth pretzel generates 3 utils per dollar, but the third sundae generates 6 utils per dollar.

Because Duncan receives a greater amount of satisfaction per dollar spent on sundaes, it seems reasonable for him to purchase more.

  • Suppose Duncan opts for a fourth sundae. If so, his total expenditure rises to $16, and the marginal utility-price ratio for sundaes falls to 4 utils per dollar. While this moves him closer to consumer equilibrium, the rule of consumer equilibrium remains out of balance. His sundae marginal utility-price ratio (4 utils per dollar) is still greater than his pretzel marginal utility-price ratio (3 utils per dollar).

  • A fifth sundae seems to be in order. This purchase raises his total expenditure to $18 and drives the sundae marginal utility-price ratio down to 2 utils per dollar. This ratio, however, is now below the 3 utils per dollar pretzel marginal utility-price ratio.

  • What about buying another pretzel? A fifth pretzel does the trick of restoring consumer equilibrium. This purchase raises total expenditure to $20. It also drives the marginal utility-price ratio for pretzels down to 2 utils per dollar, achieving equality with the marginal utility-price ratio for 5 hot fudge sundaes.
The purchase of 5 hot fudge sundaes and 5 pretzels achieves consumer equilibrium, given the new hot fudge sundae price. This combination satisfies the rule of consumer equilibrium and maximizes the utility achieved with his $20 budget. Click the [New Equilibrium] button to highlight this combination.

Summing Up

What can be concluded from this analysis?
  • Law of Demand: First, consider how the change in the price of hot fudge sundaes affects the quantity of hot fudge sundaes purchased. The price decreases and the quantity increases. This reflects the basic inverse relation between demand price and quantity demanded that is the law of demand.

  • Other Prices Demand Determinant: Second, consider a change in the price of hot fudge sundaes affects the quantity of pretzels purchased. The price of sundaes decreases and the quantity of pretzels increases. This particular example suggests a complementary relation between pretzels and sundaes.


Recommended Citation:

PRICE CHANGE, UTILITY ANALYSIS, AmosWEB Encyclonomic WEB*pedia,, AmosWEB LLC, 2000-2018. [Accessed: February 22, 2018].

Check Out These Related Terms...

     | income change, utility analysis | preferences change, utility analysis |

Or For A Little Background...

     | Utility analysis | price | consumer equilibrium | law of demand | demand price | quantity demanded | demand determinant | rule of consumer equilibrium | marginal utility-price ratio | satisfaction |

And For Further Study...

     | utility | total utility | consumer demand theory | utility measurement | cardinal utility | ordinal utility | util | utilitarianism | total utility curve | marginal utility curve | diamond-water paradox | elasticity |

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