English:Supply and Demand
Supply and Demand
Introduction
Every day, you take part in markets. You may buy a snack, compare prices for a game, notice that concert tickets sell out, or see a store discount items that are not selling. Supply and demand is a simple economic model that helps explain why prices and quantities can change.
This aiMOOC is designed for Grades 7–8. You will learn how buyers and sellers interact, how economists use graphs to describe markets, and how changes in real life can shift supply or demand. You will also test ideas with examples, simulations, interviews, graphs, and creative projects.
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Imagine that you run a lemonade stand. How many cups would customers buy at different prices? How many cups would you be willing and able to make and sell? Those two questions lead directly to demand and supply.
Learning Goals
By the end of this course, you should be able to explain demand, supply, market equilibrium, shortages, and surpluses in your own words. You should also be able to read a basic supply-and-demand graph, tell the difference between a movement along a curve and a shift of a curve, and use the model to reason about everyday market changes.
Markets, Buyers, and Sellers
A market is any setting where buyers and sellers can exchange goods or services. A market does not have to be a physical place. A farmers' market is a market, but so are an online shop, a ticket website, and a local service business.
Consumers are buyers. They decide what they are willing and able to buy. Producers are sellers or businesses. They decide what they are willing and able to offer for sale.
A market price gives both sides information. Buyers compare the price with the value they expect to receive. Sellers compare the price with their costs and opportunities. Supply and demand does not explain every detail of every real market, but it is a useful starting model for understanding how these choices interact.
Price and Quantity
In supply-and-demand graphs, price is usually shown on the vertical axis and quantity on the horizontal axis. Quantity means a number of units, such as cups of lemonade, bicycles, tickets, or kilograms of apples.
The graph above shows two relationships. The demand curve usually slopes downward, while the supply curve usually slopes upward. The point where the curves meet is the market equilibrium.
Demand
Demand describes how much of a good or service consumers are willing and able to buy at different prices during a given period, while other relevant factors are held constant.
Quantity demanded is more specific. It is the amount consumers are willing and able to buy at one particular price.
The Law of Demand
The law of demand says that, other things being equal, when the price of a good rises, the quantity demanded tends to fall. When the price falls, the quantity demanded tends to rise.
This does not mean that every person behaves exactly the same way. It means that the demand curve describes a general relationship between price and quantity demanded while other relevant influences are kept unchanged.
For example, suppose one cup of lemonade costs $1. More students may choose to buy it than if one cup costs $5. The change in the lemonade's own price causes a movement along the demand curve, not a shift of the entire curve.
What Can Shift Demand?
Demand can change even when the product's own price has not changed. When consumers are willing and able to buy more at every price, demand shifts to the right. When they are willing and able to buy less at every price, demand shifts to the left.
Important demand shifters include:
- Income: A change in buyers' income can change demand, although the direction depends on the kind of product.
- Consumer preferences: Trends, tastes, advertising, and new information can change what people want.
- Substitutes and complements: A change in the price of a related product can affect demand.
- Expectations: What buyers think may happen in the future can affect what they buy today.
- Number of buyers: More or fewer buyers can change total market demand.
Imagine that a famous athlete starts promoting a certain type of sports shoe. If more people want that shoe at every possible price, demand shifts to the right. That is different from people buying more shoes only because the shoe's own price fell.
Supply
Supply describes how much of a good or service producers are willing and able to sell at different prices during a given period, while other relevant factors are held constant.
Quantity supplied is the amount producers are willing and able to sell at one particular price.
The Law of Supply
The law of supply says that, other things being equal, a higher price is generally connected with a higher quantity supplied, while a lower price is generally connected with a lower quantity supplied. Higher prices can give producers a stronger reason to produce and sell additional units, although real production may take time and may face limits.
If the price of lemonade rises and nothing else changes, a seller may be willing to prepare and sell more cups. This is a movement along the supply curve because the product's own price changed.
What Can Shift Supply?
Supply shifts when something other than the product's own price changes producers' willingness or ability to sell.
Common supply shifters include:
- Costs of production: Higher costs for ingredients, energy, wages, or materials can reduce supply.
- Technology: Better tools or methods can make production easier or cheaper and can increase supply.
- Weather: Weather and natural events can strongly affect farm products and some forms of transport or energy.
- Taxes and rules: Some taxes, subsidies, or regulations can change production costs and incentives.
- Number of sellers: More or fewer producers in a market can change total supply.
- Expectations: Producers may change current supply if they expect different prices or conditions later.
For example, if lemons become much more expensive, the cost of making lemonade rises. At each possible lemonade price, a seller may be willing and able to offer fewer cups. Supply shifts to the left.
Market Equilibrium
Market equilibrium occurs at the price where quantity demanded equals quantity supplied. On a standard graph, this is the point where the demand curve and supply curve intersect.
At equilibrium, the plans of buyers and sellers match for the quantity traded at that price. Equilibrium is a model, not a promise that every real market stays perfectly balanced. Real markets face delays, rules, changing information, contracts, and many other influences.
A Lemonade Market Example
Suppose a school event has the following simple market schedule:
| Price per cup | Quantity demanded | Quantity supplied |
|---|---|---|
| $1 | 100 cups | 20 cups |
| $2 | 80 cups | 40 cups |
| $3 | 60 cups | 60 cups |
| $4 | 40 cups | 80 cups |
| $5 | 20 cups | 100 cups |
At $3, quantity demanded and quantity supplied are both 60 cups, so $3 is the equilibrium price in this example and 60 cups is the equilibrium quantity.
At $2, buyers want 80 cups but sellers offer only 40. The difference is a shortage of 40 cups. At $4, sellers offer 80 cups but buyers want only 40. The difference is a surplus of 40 cups.
Shortages and Surpluses
A shortage exists when quantity demanded is greater than quantity supplied at the current price. Buyers want more units than sellers are offering.
A surplus exists when quantity supplied is greater than quantity demanded at the current price. Sellers are offering more units than buyers want at that price.
Empty shelves can be a visible sign that demand is temporarily greater than available supply, but you should be careful with the conclusion. An empty shelf may also result from delivery delays, store decisions, panic buying, or other problems. The supply-and-demand model helps you ask better questions about the cause.
In a competitive market with flexible prices, a shortage can create upward pressure on price and a surplus can create downward pressure on price. However, prices may not adjust quickly, and rules or contracts can limit changes.
Movements Along Curves and Shifts of Curves
This distinction is one of the most important ideas in the course.
A change in the product's own price causes a movement along an existing demand or supply curve. A change in another relevant factor can shift the entire curve.
Think about bicycle helmets. If the price of helmets falls, quantity demanded may rise along the same demand curve. If a new safety campaign makes more families want helmets at every price, demand itself shifts to the right.
A Simple Reasoning Routine
When you analyze a market event, ask these questions:
- What market are you studying?
- Did the product's own price change first, or did some other factor change?
- If another factor changed, did it affect buyers or sellers?
- Which curve shifts, and in which direction?
- What new pressure does that create on equilibrium price and equilibrium quantity?
Changes in Equilibrium
When demand or supply shifts, the old equilibrium may no longer match buyers' and sellers' plans. A new equilibrium can form.
If demand increases while supply stays the same, the demand curve shifts right. In the basic model, equilibrium price and equilibrium quantity both rise.
If demand decreases while supply stays the same, the demand curve shifts left. In the basic model, equilibrium price and equilibrium quantity both fall.
If supply increases while demand stays the same, the supply curve shifts right. In the basic model, equilibrium price falls and equilibrium quantity rises.
If supply decreases while demand stays the same, the supply curve shifts left. In the basic model, equilibrium price rises and equilibrium quantity falls.
These statements assume that only one curve shifts at a time. If both curves shift, you must analyze both changes carefully. Sometimes you can predict what happens to price but not quantity, or quantity but not price, without more information.
Everyday Scenarios
Concert tickets: A popular band announces only one show in a city. Stronger buyer interest can increase demand, while the limited number of seats keeps supply restricted in the short run.
Strawberries: A severe frost damages a crop. Fewer strawberries can reach the market, so supply decreases. In the basic model, this can put upward pressure on price and reduce the equilibrium quantity.
Video game consoles: A factory improves production and can make more units at a lower cost. Supply can increase, which can lower equilibrium price and raise equilibrium quantity if demand stays the same.
School canteen sandwiches: If students suddenly prefer a new sandwich, demand can increase. If the canteen does not immediately make more, a shortage may appear at the old price.
Why the Model Is Useful and Where It Has Limits
Supply and demand is powerful because it gives you a clear way to organize cause and effect. It can help explain changes in food prices, ticket prices, wages, rental markets, energy markets, and many other settings.
But models simplify reality. Real markets can include a small number of powerful sellers, government rules, taxes, subsidies, contracts, incomplete information, changing quality, unfair access, environmental effects, and goals other than profit. A supply-and-demand graph should therefore be used as a tool for reasoning, not as a complete description of every situation.
When you use the model, always state your assumptions. Economists often use the phrase other things being equal to remind us that a curve shows one relationship while other relevant influences are held constant.
Interactive Tasks
Quiz: Test Your Knowledge
What does demand describe? (How much consumers are willing and able to buy at different prices) (!How much money a business earns in one year) (!How many products a factory can make with no limits) (!How much a government chooses to spend)
What usually happens to quantity demanded when a product's price rises and other things stay the same? (It falls) (!It rises) (!It always stays the same) (!It becomes equal to supply)
What usually happens to quantity supplied when a product's price rises and other things stay the same? (It rises) (!It falls) (!It disappears) (!It becomes demand)
What is market equilibrium? (The point where quantity demanded equals quantity supplied) (!The point where every buyer gets the product for free) (!The point where all sellers leave the market) (!The highest possible price in a market)
When does a shortage occur? (When quantity demanded is greater than quantity supplied) (!When quantity supplied is greater than quantity demanded) (!When demand and supply are exactly equal) (!When all prices are unchanged)
When does a surplus occur? (When quantity supplied is greater than quantity demanded) (!When quantity demanded is greater than quantity supplied) (!When every product is sold immediately) (!When a demand curve shifts right)
Which change can shift demand for a product? (A change in consumer preferences) (!A change in the product's own price only) (!A movement along the same demand curve) (!A change in the graph's axis labels)
Which change can reduce supply? (A rise in production costs) (!A fall in the product's own price along the same curve) (!A larger quantity demanded) (!A change in consumer taste only)
What causes a movement along a demand curve? (A change in the product's own price) (!A change in the number of buyers) (!A change in consumer preferences) (!A change in expected future conditions)
If demand increases while supply stays the same, what does the basic model predict? (Equilibrium price and quantity both rise) (!Equilibrium price and quantity both fall) (!Equilibrium price falls and quantity rises) (!Equilibrium price rises and quantity falls)
Memory Game
| Demand | Amount consumers are willing and able to buy at different prices |
| Supply | Amount producers are willing and able to sell at different prices |
| Equilibrium | Point where quantity demanded equals quantity supplied |
| Shortage | Situation where buyers want more than sellers offer at the current price |
| Surplus | Situation where sellers offer more than buyers want at the current price |
| Consumer | Person or group that buys goods or services |
| Producer | Person or business that offers goods or services for sale |
Drag and Drop
| Match the correct terms. | Topic |
|---|---|
| Demand curve | Usually slopes downward |
| Supply curve | Usually slopes upward |
| Rightward demand shift | Buyers want more at every price |
| Leftward supply shift | Sellers offer less at every price |
| Market equilibrium | Quantity demanded equals quantity supplied |
...
Crossword Puzzle
| Demand | What word names consumers' willingness and ability to buy at different prices? |
| Supply | What word names producers' willingness and ability to sell at different prices? |
| Equilibrium | What word names the point where quantity demanded equals quantity supplied? |
| Shortage | What word describes a situation where buyers want more than sellers offer? |
| Surplus | What word describes a situation where sellers offer more than buyers want? |
| Producer | What word names a person or business that offers goods or services for sale? |
LearningApps
Cloze Text
Open-Ended Tasks
Easy
- Price diary: Record the prices of one everyday product in at least three places or on three days, then write two possible reasons why the prices may differ.
- Lemonade stand poster: Draw or design a one-page visual that explains demand, supply, shortage, surplus, and equilibrium using a lemonade stand.
- Demand interview: Ask three people what might make them buy more or less of the same product and sort their answers into price changes and demand shifters.
- Market photo study: Visit a shop, market, or school canteen, or use a teacher-approved photo, and write five observations about buyers, sellers, prices, and quantities.
Standard
- Classroom market simulation: Run a simple buyer-and-seller trading game, record the agreed prices, and explain whether prices appear to move toward a common range.
- Supply and demand graph: Create a demand schedule and a supply schedule for an imaginary product, graph both curves, and label the equilibrium price and quantity.
- Shift storyboard: Make a six-panel comic or digital storyboard showing one demand shift and one supply shift, including the cause and the likely effect on equilibrium.
- Business interview: Interview a local seller, craft worker, farmer, or service provider about how costs and customer interest affect decisions, then summarize the answers without sharing private information.
Advanced
- Market news analysis: Choose a teacher-approved news story about a changing price, identify possible demand and supply factors, and explain which claims are supported by evidence and which are uncertain.
- Mini market experiment: Design and carry out a safe class experiment in which students choose between two imaginary products at different prices, then graph the class results and discuss limits of the experiment.
- Price policy debate: Research a simple example of a price ceiling or price floor, prepare arguments for two viewpoints, and explain how the policy could affect shortages, surpluses, buyers, and sellers.
- Economics explainer video: Produce a two- to four-minute video that teaches younger students how a change in demand or supply can create a new equilibrium, using your own example and visuals.
Learning Assessment
- Graph reasoning: Given a market graph with one curve shifted, identify the change, predict the new direction of equilibrium price and quantity, and justify each step.
- Scenario comparison: Compare a fall in a product's own price with a rise in consumer interest and explain why one is a movement along a demand curve while the other is a demand shift.
- Evidence check: Read a short claim that an empty shelf proves demand increased, then give at least two alternative supply-side explanations and state what evidence would help decide among them.
- Market design: Create a small market schedule in which one price produces a shortage, one produces equilibrium, and one produces a surplus, then explain how you know.
- Transfer challenge: Apply supply-and-demand reasoning to a new market such as bicycle rentals, concert tickets, fruit, or tutoring and explain which assumptions make your analysis reasonable.
- Model limits: Describe one real situation where supply and demand is helpful but incomplete, and identify at least two additional factors that the basic graph does not show.
Evidence of Learning
Strong evidence of learning includes both what you know and what you can do.
Knowledge: You can accurately explain demand, supply, quantity demanded, quantity supplied, equilibrium, shortage, surplus, and curve shifts.
Graph skills: You can label price and quantity axes, read demand and supply curves, find an equilibrium, and show a shift in the correct direction.
Reasoning skills: You can distinguish a change in a product's own price from a non-price change, identify whether buyers or sellers are affected, and predict likely changes in equilibrium.
Products: Your graphs, interview summaries, posters, market simulations, storyboards, experiments, or videos communicate economic ideas clearly and use evidence responsibly.
Transfer: You can apply the model to an unfamiliar real-world example, state assumptions, recognize uncertainty, and explain where the simple model may leave out important facts.
OERs on the Topic
For further study, you can use the openly available OpenStax sections on demand, supply, and equilibrium and shifts in demand and supply.
Linked Learning Areas
The topic connects economics with mathematics through graph reading, social studies through market institutions, business through producer decisions, and media literacy through evaluating claims about changing prices.