Which of the following explains why the supply curve is upward sloping and the demand curve is downward sloping?

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The supply-and-demand framework is the most fundamental framework in economics. It explains both the price of a good or a service and the quantity produced and purchased.

The market supply curve comes from adding together the individual supply curves of firms in a particular market. A competitive firm, taking prices as given, will produce at a level such that

Marginal cost usually increases as a firm produces more output. Thus an increase in the price of a product creates an incentive for firms to produce more—that is, the supply curve of a firm is upward sloping. The market supply curve slopes upward as well: if the price increases, all firms in a market will produce more output, and some new firms may also enter the market.

A firm’s supply curve shifts if there are changes in input prices or the state of technology. The market supply curve is shifted by changes in input prices and changes in technology that affect a significant number of the firms in a market.

The market demand curve comes from adding together the individual demand curves of all households in a particular market. Households, taking the prices of all goods and services as given, distribute their income in a manner that makes them as well off as possible. This means that they choose a combination of goods and services preferred to any other combination of goods and services they can afford. They choose each good or service such that

Marginal valuation usually decreases as a household consumes more of a product. If the price of a good or a service decreases, a household will substitute away from other goods and services and toward the product that has become cheaper—that is, the demand curve of a household is downward sloping. The market demand curve slopes downward as well: if the price decreases, all households will demand more.

The household demand curve shifts if there are changes in income, prices of other goods and services, or tastes. The market demand curve is shifted by changes in these factors that are common across a significant number of households.

A market equilibrium is a price and a quantity such that the quantity supplied equals the quantity demanded at the equilibrium price (Figure 31.5 "Market Equilibrium"). Because market supply is upward sloping and market demand is downward sloping, there is a unique equilibrium price. We say we have a competitive market if the following are true:

• The product being sold is homogeneous.
• There are many households, each taking the price as given.
• There are many firms, each taking the price as given.

A competitive market is typically characterized by an absence of barriers to entry, so new firms can readily enter the market if it is profitable, and existing firms can easily leave the market if it is not profitable.

• Market supply is upward sloping: as the price increases, all firms will supply more.
• Market demand is downward sloping: as the price increases, all households will demand less.
• A market equilibrium is a price and a quantity such that the quantity demanded equals the quantity supplied.

Figure 31.5 Market Equilibrium

Figure 31.5 "Market Equilibrium" shows equilibrium in the market for chocolate bars. The equilibrium price is determined at the intersection of the market supply and market demand curves.

If we let p denote the price, qd the quantity demanded, and I the level of income, then the market demand curve is given by

where a, b, and c are constants. By the law of demand, b > 0. For a normal good, the quantity demanded increases with income: c > 0.

If we let qs denote the quantity supplied and t the level of technology, the market supply curve is given by

where d, e, and f are constants. Because the supply curve slopes upward, e > 0. Because the quantity supplied increases when technology improves, f > 0.

In equilibrium, the quantity supplied equals the quantity demanded. Set qs = qd = q* and set p = p* in both equations. The market clearing price (p*) and quantity (q*) are as follows:

and

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The law of supply is the microeconomic law that states that, all other factors being equal, as the price of a good or service increases, the quantity of goods or services that suppliers offer will increase, and vice versa.

The law of supply says that as the price of an item goes up, suppliers will attempt to maximize their profits by increasing the number of items for sale.

• The law of supply says that a higher price will induce producers to supply a higher quantity to the market.
• Because businesses seek to increase revenue, when they expect to receive a higher price for something, they will produce more of it.
• Meanwhile, if prices fall, suppliers are disincentivized from producing as much.
• Supply in a market can be depicted as an upward-sloping supply curve that shows how the quantity supplied will respond to various prices over a period of time.
• Together with demand, it forms half of the law of supply and demand.

The chart below depicts the law of supply using a supply curve, which is upward sloping. A, B, and C are points on the supply curve. Each point on the curve reflects a direct correlation between quantity supplied (Q) and price (P). So, at point A, the quantity supplied will be Q1 and the price will be P1, and so on.

Investopedia / Julie Bang

The supply curve is upward sloping because, over time, suppliers can choose how much of their goods to produce and later bring to market. At any given point in time, however, the supply that sellers bring to market is fixed, and sellers simply face a decision to either sell or withhold their stock from a sale; consumer demand sets the price, and sellers can only charge what the market will bear.

If consumer demand rises over time, the price will rise, and suppliers can choose to devote new resources to production (or new suppliers can enter the market), which increases the quantity supplied. Demand ultimately sets the price in a competitive market; supplier response to the price they can expect to receive sets the quantity supplied.  

The law of supply is one of the most fundamental concepts in economics. It works with the law of demand to explain how market economies allocate resources and determine the prices of goods and services.

British economist Alfred Marshall (1842-1924), a specialist in microeconomics, contributed significantly to supply theory, especially in his pioneering use of the supply curve. He emphasized that the price and output of a good are determined by both supply and demand: The two curves are like scissor blades that intersect at equilibrium.

The law of supply summarizes the effect price changes have on producer behavior. For example, a business will make more video game systems if the price of those systems increases. The opposite is true if the price of video game systems decreases. The company might supply 1 million systems if the price is $200 each, but if the price increases to $300, they might supply 1.5 million systems.

To further illustrate this concept, consider how gas prices work. When the price of gasoline rises, it encourages profit-seeking firms to take several actions: expand exploration for oil reserves; drill for more oil; invest in more pipelines and oil tankers to bring the oil to plants where it can be refined into gasoline; build new oil refineries; purchase additional pipelines and trucks to ship the gasoline to gas stations; and open more gas stations or keep existing gas stations open longer hours.

The law of supply is so intuitive that you may not even be aware of all the examples around you:

• When college students learn that computer engineering jobs pay more than English professor jobs, the supply of students with majors in computer engineering will increase.
• When consumers start paying more for cupcakes than for donuts, bakeries will increase their output of cupcakes and reduce their output of donuts in order to increase their profits.
• When your employer pays time and a half for overtime, the number of hours you are willing to supply for work increases.

The law of supply summarizes the effect price changes have on a producer's behavior. For example, a business will make more of a good (such as TVs or cars) if the price of that product increases. So, if the price of TVs increases, TV producers are incentivized to produce more of them. Likewise, other companies may be induced to start producing TVs. This will increase the overall supply of televisions in the market. At some point, the abundant supply will tend to cause prices to moderate and fall.

There are five types of supply—market supply, short-term supply, long-term supply, joint supply, and composite supply. Meanwhile, there are two types of supply curves—individual supply curves and market supply curves. Individual supply curves graph the individual supply schedule, while market supply curves represent the market supply schedule.

Supply is influenced by prices and consumer demand. In addition, the number of suppliers available, the level of competition, the state of technology, and the presence of government support or restriction will play important roles. For certain products like agricultural commodities, supply is also impacted by things like weather and crop yields.

The law of demand is a fundamental principle of economics that states that at a higher price consumers will demand a lower quantity of a good, and vice-versa.

The law of supply and demand outlines the interaction between a buyer and a seller of a resource. Supply and demand says that sellers will supply less of a product or resource as price decreases, while buyers will buy more, and vice versa, until an equilibrium price and quantity is reached. It incorporates both the law of supply and the law of demand.