What a buyer pays for a unit of the specific good or service is called the price. The total number of units purchased at that price is called the quantity demanded. A rise in the price of a good or service almost always decreases the quantity of that good or service demanded. Conversely, a fall in price will increase the quantity demanded. When the price of a gallon of gasoline goes up, for example, people look for ways to reduce their consumption by combining several errands, commuting by carpool or mass transit, or taking weekend or vacation trips closer to home. Economists call this inverse relationship between price and quantity demanded the law of demand. The law of demand assumes that all other variables that affect demand are held constant. An example from the market for gasoline can be shown in the form of a table or a graph. A table that shows the quantity demanded at each price, such as Table 1, is called a demand schedule. Price in this case is measured in dollars per gallon of gasoline. The quantity demanded is measured in millions of gallons over some time period (for example, per day or per year) and over some geographic area (like a state or a country).
The demand curve shows how much of a good people are willing to buy at different prices. Watch this video to see an example of the demand for oil. When oil prices are high, fewer people are willing to pay the hefty price tag but some consumers, like airliners, depend so heavily on using oil for fuel, they are willing to pay a lot. Other low-value consumers will be less likely to pay for expensive oil, as they could find substitutes or alternatives. Demand curves will look somewhat different for each product. They may appear relatively steep or flat, or they may be straight or curved. Nearly all demand curves share the fundamental similarity that they slope down from left to right. In this way, demand curves embody the law of demand: As the price increases, the quantity demanded decreases, and conversely, as the price decreases, the quantity demanded increases. In economic terminology, demand is not the same as quantity demanded. When economists talk about demand, they mean the relationship between a range of prices and the quantities demanded at those prices, as illustrated by a demand curve or a demand schedule. When economists talk about quantity demanded, they mean only a certain point on the demand curve, or one quantity on the demand schedule. In short, demand refers to the curve and quantity demanded refers to the (specific) point on the curve. These factors matter both for demand by an individual and demand by the market as a whole. Exactly how do these various factors affect demand, and how do we show the effects graphically? To answer those questions, we need the ceteris paribus assumption. A demand curve or a supply curve (which we'll cover later in this module) is a relationship between two, and only two, variables: price on the vertical axis and quantity on the horizontal axis. The assumption behind a demand curve or a supply curve is that no relevant economic factors, other than the product's price, are changing. Economists call this assumption ceteris paribus, a Latin phrase meaning "other things being equal." Any given demand or supply curve is based on the ceteris paribus assumption that all else is held equal. Therefore, a demand curve or a supply curve is a relationship between two, and only two, variables when all other variables are held equal. If all else is not held equal, then the laws of supply and demand will not necessarily hold.Ceteris paribus is applied when we look at how changes in price affect demand or supply, but ceteris paribus can also be applied more generally. In the real world, demand and supply depend on more factors than just price. For example, a consumer's demand depends on income, and a producer's supply depends on the cost of producing the product. How can we analyze the effect on demand or supply if multiple factors are changing at the same time—say price rises and income falls? The answer is that we examine the changes one at a time, and assume that the other factors are held constant. For example, we can say that an increase in the price reduces the amount consumers will buy (assuming income, and anything else that affects demand, is unchanged). Additionally, a decrease in income reduces the amount consumers can afford to buy (assuming price, and anything else that affects demand, is unchanged). This is what the ceteris paribus assumption really means. In this particular case, after we analyze each factor separately, we can combine the results. The amount consumers buy falls for two reasons: first because of the higher price and second because of the lower income. Watch this video to review the theory of demand. Remember that, according to the law of demand and all other things being equal (ceteris paribus):
ceteris paribus: When changing one variable in a function (e.g. demand for some product), we assume everything else held constantdemand: the relationship between the price of a certain good or service and the quantity of that good or service someone is willing and able to buydemand curve: a graphic representation of the relationship between price and quantity demanded of a certain good or service, with price on the vertical axis and quantity on the horizontal axisdemand schedule: a table that shows the quantity demanded for a certain good or service at a range of priceslaw of demand: the common relationship that a higher price leads to a lower quantity demanded of a certain good or service and a lower price leads to a higher quantity demanded, while all other variables are held constantprice: what a buyer pays for a unit of the specific good or servicequantity demanded: the total number of units of a good or service consumers wish to purchase at a given price CC licensed content, Shared previouslyAll rights reserved content
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The original demand curve D0, like every demand curve, is based on the ceteris paribus assumption that no other economically relevant factors change. Now imagine that the economy expands in a way that raises the incomes of many people, making cars more affordable. How will this affect demand? How can we show this graphically? Return to Figure 1. The price of cars is still $20,000, but with higher incomes, the quantity demanded has now increased to 20 million cars, shown at point S. As a result of the higher income levels, the demand curve shifts to the right to the new demand curve D1, indicating an increase in demand. Table 1, below, shows clearly that this increased demand would occur at every price, not just the original one.
When a demand curve shifts, it does not mean that the quantity demanded by every individual buyer changes by the same amount. In this example, not everyone would have higher or lower income and not everyone would buy or not buy an additional car. Instead, a shift in a demand curve captures a pattern for the market as a whole: Increased demand means that at every given price, the quantity demanded is higher, so that the demand curve shifts to the right from D0 to D1. And, decreased demand means that at every given price, the quantity demanded is lower, so that the demand curve shifts to the left from D0 to D2. We just argued that higher income causes greater demand at every price. This is true for most goods and services. For some—luxury cars, vacations in Europe, and fine jewelry—the effect of a rise in income can be especially pronounced. A product whose demand rises when income rises, and vice versa, is called a normal good. A few exceptions to this pattern do exist, however. As incomes rise, many people will buy fewer generic-brand groceries and more name-brand groceries. They are less likely to buy used cars and more likely to buy new cars. They will be less likely to rent an apartment and more likely to own a home, and so on. A product whose demand falls when income rises, and vice versa, is called an inferior good. In other words, when income increases, the demand curve shifts to the left. A change in price does not shift the demand curve. It only shows a difference in the quantity demanded. The demand curve will move left or right when there is an underlying change in demand at all prices.
Other goods are complements for each other, meaning that the goods are often used together, because consumption of one good tends to enhance consumption of the other. Examples include breakfast cereal and milk; notebooks and pens or pencils, golf balls and golf clubs; gasoline and sport utility vehicles; and the five-way combination of bacon, lettuce, tomato, mayonnaise, and bread. If the price of golf clubs rises, since the quantity of golf clubs demanded falls (because of the law of demand), demand for a complement good like golf balls decreases, too. Similarly, a higher price for skis would shift the demand curve for a complement good like ski resort trips to the left, while a lower price for a complement has the reverse effect. While it is clear that the price of a good affects the quantity demanded, it is also true that expectations about the future price (or expectations about tastes and preferences, income, and so on) can affect demand. For example, if people hear that a hurricane is coming, they may rush to the store to buy flashlight batteries and bottled water. If people learn that the price of a good like coffee is likely to rise in the future, they may head for the store to stock up on coffee now. These changes in demand are shown as shifts in the curve. Therefore, a shift in demand happens when a change in some economic factor (other than the current price) causes a different quantity to be demanded at every price.A shift in demand means that at any price (and at every price), the quantity demanded will be different than it was before. Following is a graphic illustration of a shift in demand due to an income increase. Step 1. Draw the graph of a demand curve for a normal good like pizza. Pick a price (like P0). Identify the corresponding Q0. An example is shown in Figure 5. Step 2. Suppose income increases. As a result of the change, are consumers going to buy more or less pizza? The answer is more. Draw a dotted horizontal line from the chosen price, through the original quantity demanded, to the new point with the new Q1. Draw a dotted vertical line down to the horizontal axis and label the new Q1. An example is provided in Figure 6. Step 3. Now, shift the curve through the new point. You will see that an increase in income causes an upward (or rightward) shift in the demand curve, so that at any price, the quantities demanded will be higher, as shown in Figure 7. complements: goods or services that are used together because the use of one enhances the use of the othersubstitutes: goods or services that can be used in place of one anotherinferior good: good or service whose demand decreases when a consumer's income increases and demand increases when income decreasesnormal good: good or service whose demand increases when a consumer's income increases and demand decreases when income decreases CC licensed content, Original
CC licensed content, Shared previouslyPage 3These questions allow you to get as much practice as you need, as you can click the link at the top of the first question (“Try another version of these questions”) to get a new set of questions. Practice until you feel comfortable doing the questions and then move on. Note that you will use the information provided in the first question for all of the questions on this page. |