Why is understanding the difference between the change in demand and the change in quantity demanded important in learning economics?

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Quantity demanded is a term used in economics to describe the total amount of a good or service that consumers demand over a given interval of time. It depends on the price of a good or service in a marketplace, regardless of whether that market is in equilibrium.

The relationship between the quantity demanded and the price is known as the demand curve, or simply the demand. The degree to which the quantity demanded changes with respect to price is called the elasticity of demand.

• In economics, quantity demanded refers to the total amount of a good or service that consumers demand over a given period of time.
• Quantity demanded depends on the price of a good or service in a marketplace.
• The price of a product and the quantity demand for that product have an inverse relationship, according to the law of demand.

The price of a good or service in a marketplace determines the quantity that consumers demand. Assuming that non-price factors are removed from the equation, a higher price results in a lower quantity demanded and a lower price results in higher quantity demanded. Thus, the price of a product and the quantity demanded for that product have an inverse relationship, as stated in the law of demand.

An inverse relationship means that higher prices result in lower quantity demand and lower prices result in higher quantity demand.

A change in quantity demanded refers to a change in the specific quantity of a product that buyers are willing and able to buy. This change in quantity demanded is caused by a change in the price.

An increase in quantity demanded is caused by a decrease in the price of the product (and vice versa). A demand curve illustrates the quantity demanded and any price offered on the market. A change in quantity demanded is represented as a movement along a demand curve. The proportion that quantity demanded changes relative to a change in price is known as the elasticity of demand and is related to the slope of the demand curve.

Say, for example, at the price of $5 per hot dog, consumers buy two hot dogs per day; the quantity demanded is two. If vendors decide to increase the price of a hot dog to $6, then consumers only purchase one hot dog per day. On a graph, the quantity demanded moves leftward from two to one when the price rises from $5 to $6. If, however, the price of a hot dog decreases to $4, then customers want to consume three hot dogs: the quantity demanded moves rightward from two to three when the price falls from $5 to $4. 

By graphing these combinations of price and quantity demanded, we can construct a demand curve connecting the three points.

Using a standard demand curve, each combination of price and quantity demanded is depicted as a point on the downward sloping line, with the price of hot dogs on the y-axis and the quantity of hot dogs on the x-axis. This means that as price decreases, the quantity demanded increases. Any change or movement to quantity demanded is involved as a movement of the point along the demand curve and not a shift in the demand curve itself. As long as consumers' preferences and other factors don't change, the demand curve effectively remains static.

Price changes change the quantity demanded; changes in consumer preferences change the demand curve. If, for example, environmentally conscious consumers switch from gas cars to electric cars, the demand curve for traditional cars would inherently shift.

The proportion to which the quantity demanded changes with respect to price is called elasticity of demand. A good or service that is highly elastic means the quantity demanded varies widely at different price points.

Conversely, a good or service that is inelastic is one with a quantity demanded that remains relatively static at varying price points. An example of an inelastic good is insulin. Regardless of price point, those who need insulin demand it at the same amount.

Quantity demanded is affected by the price of the product. If the price goes up, the demand will go down. If the price goes down, demand will go up. Price and demand are inversely related in this way.

No. Quantity demanded can apply to service products as well. For example, if a photographer offers family portrait sessions for a lower price, they should book more sessions. If they price them higher, they will book fewer sessions.

Demand and quantity demanded both pertain to purchasing but in different ways. Demand is just how many of an item a consumer is willing to buy—the sheer quantity. Quantity demanded is how many things a consumer will purchase at a specific price. Quantity demanded is a more detailed metric. Graphed out, demand is the entirety of the demand curve, whereas quantity demanded is a single point.

It’s hard to overstate the importance of understanding the difference between shifts in curves and movements along curves. Remember, when we talk about changes in demand or supply, we do not mean the same thing as changes in quantity demanded or quantity supplied.

A change in demand refers to a shift in the entire demand curve, which is caused by a variety of factors (preferences, income, prices of substitutes and complements, expectations, population, etc.). In this case, the entire demand curve moves left or right.

Figure 1. Change in Demand. A change in demand means that the entire demand curve shifts either left or right. The initial demand curve D0 shifts to become either D1 or D2. This could be caused by a shift in tastes, changes in population, changes in income, prices of substitute or complement goods, or changes future expectations.

A change in quantity demanded refers to a movement along the demand curve, which is caused only by a chance in price. In this case, the demand curve doesn’t move; rather, we move along the existing demand curve.

Figure 2. Change in Quantity Demanded. A change in the quantity demanded refers to movement along the existing demand curve, D0. This is a change in price, which is caused by a shift in the supply curve.