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Demand Curve Definition in Economics
What is the definition of the demand curve in economics? The demand curve is a graphical illustration of the relationship between price and quantity demanded. But let's not get ahead of ourselves. What is demand? Demand is the willingness and ability of consumers to purchase a given good at any given time. It is this willingness and ability that makes one a consumer.
The demand curve is a graphical illustration of the relationship between price and quantity demanded.
Demand is the willingness and ability of consumers to purchase a given good at a given price at a given time.
Whenever you see the concept of demand in action, quantity demanded and price come into play. This is because, given that we do not have unlimited money, we can only purchase a limited quantity of goods at any given price. So, what are the concepts of price and quantity demanded? Price refers to the amount of money that consumers have to pay to acquire a given good at any given time. Quantity demanded, on the other hand, is the total amount of a given good consumers demand at different prices.
Price refers to the amount of money that consumers have to pay to acquire a given good at a given time.
Quantity demanded refers to the total amount of a given good consumers demand at different prices.
The demand curve shows the price of a good relative to its quantity demanded. We plot the price on the vertical axis, and the quantity demanded goes on the horizontal axis. A simple demand curve is presented in Figure 1 below.
Fig. 1 - Demand curve
The demand curve slopes downward because the demand curve is an illustration of the law of demand.
The law of demand contends that all other things remaining equal, the quantity demanded of a good increases as the price of that good decreases.
The law of demand states that all other things remaining equal, the quantity demanded of a good increases as the price of that good decreases.
It can also be said that price and quantity demanded are inversely related.
The Demand Curve in Perfect Competition
The demand curve in perfect competition is flat or a straight horizontal line parallel to the horizontal axis.
Why is this so?
This is because in perfect competition, as buyers have perfect information, they know who is selling the same product for a lower price. As a result, if one seller is selling the product for too high a price, the consumers will simply not buy from that seller. Rather, they will buy from a seller who sells the same product for cheaper. Therefore, all firms must sell their product at the same price in perfect competition, which leads to a horizontal demand curve.
Since the product is being sold at the same price, the consumers buy as much as they can afford to buy or until the firm runs out of product. Figure 2 below shows the demand curve in perfect competition.
Fig. 2 - Demand curve in perfect competition
Shift in the Demand Curve
Some factors can cause a shift in the demand curve. These factors are referred to by economists as the determinants of demand. The determinants of demand are factors that cause a shift in the demand curve of a good.
There is a rightward shift in the demand curve when demand increases. Conversely, there is a leftward shift in the demand curve when demand decreases at every price level.
Figure 3 illustrates an increase in demand, whereas Figure 4 illustrates a decrease in demand.
Determinants of demand are factors that cause a shift in the demand curve of a good.
Fig. 3 - Rightward shift in the demand curve
Figure 3 above depicts a demand curve shift to the right from D1 to D2 due to an increase in demand.
Fig. 4 - Leftward shift in the demand curve
As sketched in Figure 4 above, the demand curve shifts to the left from D1 to D2 due to a decrease in demand.
The main determinants of demand are income, the price of related goods, tastes, expectations, and the number of buyers. Let's explain these briefly.
- Income - After the income of consumers increases, they tend to cut down the consumption of inferior goods and increase their consumption of normal goods. This means that an increase in income as a determinant of demand causes a reduction in the demand for inferior goods and an increase in the demand for normal goods.
- Prices of related goods - Some goods are substitutes, which means that consumers can either buy one or the other. Therefore, in the case of perfect substitutes, an increase in the price of one product will result in an increase in the demand for its substitute.
- Taste - Taste is one of the determinants of demand because the tastes of people determine their demand for a given product. For instance, if people develop a taste for leather clothes, then there will be an increase in demand for leather clothes.
- Expectations - The expectations of consumers can also result in an increase or decrease in demand. For instance, if consumers hear rumors about a planned increase in the price of a given product, then the consumers will buy more of the product in anticipation of the planned price increase.
- The number of buyers - The number of buyers also increases demand by simply increasing the number of people buying a given product. Here, since the price doesn't change, and there are simply more people buying the product, demand increases, and the demand curve shifts to the right.
Read our article on Change in Demand to learn more!
Types of the Demand Curve
There are two main types of demand curves. These include the individual demand curve and the market demand curve. As the names suggest, the individual demand curve represents demand for a single consumer, whereas the market demand curve represents demand for all the consumers in the market.
The individual demand curve represents the relationship between the price and quantity demanded for a single consumer.
The market demand curve represents the relationship between the price and quantity demanded for all consumers in the market.
Market demand is a summation of all individual demand curves. This is illustrated in Figure 5 below.
Fig. 5 - Individual and market demand curves
As illustrated in Figure 5, D1 represents the individual demand curves, whereas D2 represents the market demand curve. The two individual curves are summed up to make the market demand curve.
Demand Curve with Example
Now, let's look at an example of the demand curve by showing the effect of multiple buyers on demand.
The demand schedule presented in Table 1 shows individual demand for one consumer and market demand for two consumers for towels.
| Price ($) | Towels (1 consumer) | Towels (2 consumers) |
| 5 | 0 | 0 |
| 4 | 1 | 2 |
| 3 | 2 | 4 |
| 2 | 3 | 6 |
| 1 | 4 | 8 |
Table 1. Demand Schedule for Towels
Show the individual demand curve and the market demand curve on the same graph. Explain your answer.
Solution:
We plot the demand curves with the price on the vertical axis, and the quantity demanded on the horizontal axis.
Doing this, we have:
Fig. 6 - Individual and market demand curve example
As shown in Figure 6, the market demand curve combines two individual demand curves.
Inverse Demand Curve
The inverse demand curve shows the price as a function of the quantity demanded.
Normally, the demand curve shows how the quantity demanded changes as a result of changes in price. However, in the case of the inverse demand curve, price changes as a result of changes in quantity demanded.
Let's express the two mathematically:
For demand:
\(Q=f(P)\)
For inverse demand:
\(P=f^{-1}(Q)\)
To find the inverse demand function, we simply need to make P the subject of the demand function. Let's take a look at an example below!
For instance, if the demand function is:
\(Q=100-2P\)
The inverse demand function becomes:
\(P=50-\frac{1}{2} Q\)
The inverse demand curve and the demand curve are essentially the same, hence are illustrated in the same way.
Figure 7 shows the inverse demand curve.
Fig. 7 - Inverse demand curve
The inverse demand curve presents price as a function of quantity demanded.
Demand Curve - Key takeaways
- Demand is the willingness and ability of consumers to purchase a given good at a given price at a given time.
- The demand curve is defined as a graphical illustration of the relationship between price and quantity demanded.
- Price is plotted on the vertical axis, whereas quantity demanded is plotted on the horizontal axis.
- Determinants of demand are factors other than the price that cause changes in demand.
- The individual demand curve represents demand for a single consumer, whereas the market demand curve represents demand for all the consumers in the market.
- The inverse demand curve presents price as a function of quantity demanded.
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Frequently Asked Questions about Demand Curve
What is demand curve in economics?
The demand curve in economics is defined as a graphical illustration of the relationship between price and quantity demanded.
What does the demand curve demonstrate?
The demand curve demonstrates the quantity of a product consumers will buy at different prices.
Why is the demand curve important?
The demand curve is important because it illustrates the behavior of consumers in the market.
Why is demand curve flat in perfect competition?
This is because in perfect competition, as buyers have perfect information, they know who is selling the same product for a lower price. As a result, if one seller is selling the product for too high a price, the consumers will simply not buy from that seller. Rather, they will buy from a seller who sells the same product for cheaper. Therefore, all firms must sell their product at the same price in perfect competition, which leads to a horizontal demand curve.
What is the main difference between demand curve and supply curve?
The demand curve shows the relationship between quantity demanded and price and is downward sloping. The supply curve shows the relationship between quantity supplied and price and is upward sloping.
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