1. Law of Demand
Meaning of Demand
In economics, demand means the quantity of a good or service that a consumer is willing and able to buy at a particular price during a particular period of time.
Demand is not simply a desire to buy something.
For example, Rahul may want to buy a ₹1,00,000 laptop, but if he does not have the money to buy it, this is only a desire, not effective demand.
Law of Demand
The Law of Demand states that:
Other things remaining constant, when the price of a good rises, its quantity demanded falls, and when its price falls, its quantity demanded rises.
Therefore, there is generally an inverse relationship between price and quantity demanded.
Simple Example
Suppose the price of a pizza changes:
| Price of Pizza | Quantity Demanded |
| ₹400 | 1 pizza |
| ₹300 | 2 pizzas |
| ₹200 | 3 pizzas |
| ₹100 | 5 pizzas |
When the pizza becomes cheaper, people are willing to buy more.
When it becomes expensive, people usually buy less.
Real-Life Example: Ice Cream
Imagine that an ice-cream shop sells a particular ice cream for ₹100.
At this price, a student may buy one ice cream.
If the shop reduces the price to ₹50, the student may buy two ice creams or may buy ice cream more frequently.
If the price increases to ₹150, the student may decide to buy a cheaper snack instead.
Thus:
Price ↑ → Quantity Demanded ↓
Price ↓ → Quantity Demanded ↑
Why does this happen?
There are several reasons:
1. Substitution Effect
When the price of a good rises, consumers may shift to a cheaper substitute.
Example:
If the price of tea increases considerably, some people may start drinking more coffee or another beverage.
2. Income Effect
When the price of a product falls, the consumer’s purchasing power effectively increases.
For example, suppose you have ₹500.
- At ₹100 per burger, you can buy 5 burgers.
- At ₹50 per burger, you can buy 10 burgers.
The lower price allows you to buy more with the same money.
3. Diminishing Marginal Utility
Usually, consumers get less additional satisfaction from consuming additional units of the same product.
For example, the first glass of cold drink on a hot day may give you a lot of satisfaction. The second may give you less, and the third even less.
Therefore, consumers generally want additional units only at lower prices.
2. Law of Supply
Meaning of Supply
Supply means the quantity of a good or service that producers are willing and able to sell at different prices during a particular period of time.
Law of Supply
The Law of Supply states that:
Other things remaining constant, when the price of a good rises, its quantity supplied rises, and when the price falls, its quantity supplied falls.
Thus, there is generally a direct relationship between price and quantity supplied.
Simple Example
Suppose a farmer sells tomatoes.
| Price per kg | Quantity Supplied |
| ₹20 | 50 kg |
| ₹30 | 70 kg |
| ₹40 | 100 kg |
| ₹50 | 130 kg |
As the price increases, the farmer has a greater incentive to bring more tomatoes to the market.
Real-Life Example: Vegetable Seller
Suppose a vegetable seller normally earns a small profit by selling potatoes at ₹20 per kg.
If the market price rises to ₹35 per kg, selling potatoes becomes more profitable.
The seller may therefore:
- purchase more potatoes from wholesalers,
- bring more potatoes to the market,
- keep larger stocks, if practical.
Therefore:
Price ↑ → Quantity Supplied ↑
Price ↓ → Quantity Supplied ↓
Why does supply usually increase with price?
The main reason is profit incentive.
A higher selling price can make production more profitable. Producers therefore have greater motivation to supply more.
However, the ability to increase supply depends on factors such as:
- availability of raw materials,
- production capacity,
- technology,
- time,
- labour availability.
3. Change in Price and Quantity Demanded
It is important to understand that a change in the price of the good itself normally causes a change in quantity demanded, not a change in demand itself.
Example
Suppose the price of a notebook falls from ₹60 to ₹40.
Students buy more notebooks.
This is called:
Increase in quantity demanded due to a fall in price.
It is represented by a movement along the same demand curve.
Similarly:
Price rises → Quantity demanded falls
Price falls → Quantity demanded rises
Important distinction
Do not confuse:
Change in quantity demanded
with
Change in demand.
A change in the product’s own price causes movement along the demand curve.
A change in other factors, such as income, preferences or population, can shift the entire demand curve.
4. Change in Price and Quantity Supplied
The same idea applies to supply.
If the price of a product increases, producers generally supply more.
If the price decreases, producers generally supply less.
This is a change in quantity supplied and is represented by movement along the same supply curve.
Example: Handmade Bags
Suppose a producer sells handmade bags.
At ₹500 per bag, the producer supplies 100 bags.
If the price increases to ₹700, the producer may supply 150 bags because the higher price provides greater incentive to produce.
But if the price falls to ₹300, the producer may reduce production.
5. Equilibrium Price and Equilibrium Quantity
The market has two important forces:
- Demand comes mainly from buyers.
- Supply comes mainly from sellers.
The price at which the quantity demanded equals the quantity supplied is called the equilibrium price.
The quantity bought and sold at this price is called the equilibrium quantity.
Simple Example
Suppose the market for school notebooks is:
| Price | Quantity Demanded | Quantity Supplied |
| ₹20 | 100 | 40 |
| ₹30 | 80 | 60 |
| ₹40 | 60 | 60 |
| ₹50 | 40 | 80 |
At ₹40:
Quantity Demanded = 60 notebooks
Quantity Supplied = 60 notebooks
Therefore:
Equilibrium Price = ₹40
Equilibrium Quantity = 60 notebooks
Why is equilibrium important?
At equilibrium, there is no immediate tendency for the market price to change because:
Quantity demanded = Quantity supplied
Buyers can purchase the quantity they want at the prevailing market price, while sellers can sell the quantity they are willing to supply.
6. Surplus
A surplus occurs when:
Quantity supplied is greater than quantity demanded.
Example
Suppose the equilibrium price of a product is ₹100.
But sellers charge ₹150.
At ₹150:
- Consumers want to buy 50 units.
- Producers want to sell 90 units.
Therefore:
Surplus = 90 − 50 = 40 units
There are 40 units that producers want to sell but consumers do not want to buy.
What happens next?
Sellers may find it difficult to sell all their products.
They may respond by:
- reducing prices,
- offering discounts,
- providing special offers.
As prices fall, quantity demanded tends to increase and quantity supplied tends to decrease.
The market therefore tends to move back toward equilibrium.
Real-Life Example
Imagine a shopkeeper expects strong demand for winter jackets and stocks 1,000 jackets.
If winter is unusually warm, customers may buy only 500 jackets.
The shopkeeper now has excess stock.
To clear the stock, the shopkeeper may offer:
“30% OFF — Winter Clearance Sale.”
This is an example of a market surplus encouraging sellers to reduce prices.
7. Shortage
A shortage occurs when:
Quantity demanded is greater than quantity supplied.
Example
Suppose a product normally sells for ₹100.
If the price is kept at ₹60:
- Consumers want 100 units.
- Producers supply only 60 units.
Therefore:
Shortage = 100 − 60 = 40 units
There are 40 units that consumers want but cannot obtain at that price.
Real-Life Example: Umbrellas
Suppose heavy rainfall suddenly begins.
Many people want to buy umbrellas at the same time.
If shops have only a limited stock, customers may find that umbrellas are unavailable.
Demand becomes greater than available supply.
This creates a shortage.
8. How Changes in Demand Affect Equilibrium
Equilibrium does not remain fixed forever.
Changes in market conditions can shift demand or supply.
Increase in Demand
Suppose the demand for electric fans increases because summer becomes unusually hot.
At the old price:
- consumers want more fans,
- existing producers cannot immediately supply enough fans.
This creates upward pressure on price.
As the market adjusts:
Equilibrium price generally rises.
Equilibrium quantity generally rises.
Example
Before a heat wave:
- Equilibrium price = ₹2,000
- Equilibrium quantity = 1,000 fans
During an unusually hot summer:
- Demand increases.
- New equilibrium might be ₹2,300.
- Quantity traded might rise to 1,300 fans.
The exact change depends on market conditions.
9. How a Decrease in Demand Affects Equilibrium
Suppose demand for umbrellas falls sharply after the rainy season ends.
Consumers now want fewer umbrellas.
This creates downward pressure on price.
Generally:
Demand ↓ → Equilibrium price ↓
Demand ↓ → Equilibrium quantity ↓
Example
During the monsoon, umbrella demand is high.
After the rainy season, demand decreases.
Stores may offer discounts to sell remaining stock.
10. How Changes in Supply Affect Equilibrium
Supply can also change because of:
- changes in production costs,
- technology,
- weather,
- taxes,
- availability of raw materials,
- number of producers.
Increase in Supply
Suppose improved technology allows a factory to produce smartphones at a lower cost.
More smartphones can now be supplied at each price.
Generally:
Supply ↑ → Equilibrium price ↓
Supply ↑ → Equilibrium quantity ↑
Real-Life Example
Suppose a farmer uses improved irrigation technology and gets a much larger crop of tomatoes.
The increased supply of tomatoes can put downward pressure on their market price, assuming demand does not change significantly.
11. Decrease in Supply
Suppose severe floods destroy a large part of a vegetable crop.
The quantity available in the market falls.
Therefore:
Supply ↓ → Equilibrium price generally rises
Supply ↓ → Equilibrium quantity generally falls
Example
Suppose normally 10,000 kg of tomatoes reach a market every week.
After severe crop damage, only 6,000 kg arrive.
If demand remains strong, tomato prices may rise.
12. Price Ceiling
A price ceiling is a legally fixed maximum price that sellers are allowed to charge for a particular good or service.
The government introduces a price ceiling when it wants to make an essential product or service affordable to consumers.
Example
Suppose the market price of a particular essential medicine would normally be ₹100.
The government may decide that sellers cannot charge more than ₹80.
Therefore:
₹80 = Price Ceiling
The seller cannot legally charge ₹100 if the ceiling is binding.
When does a price ceiling create a shortage?
A price ceiling creates a shortage when it is set below the equilibrium price.
For example:
- Equilibrium price = ₹100
- Government maximum price = ₹70
At ₹70:
- Consumers want 1,000 units.
- Producers supply only 600 units.
Therefore:
Shortage = 400 units
13. Why Can Price Ceilings Create Shortages?
At a low controlled price:
Consumers
Because the product is cheaper, more consumers want to buy it.
Quantity demanded increases.
Producers
Because the selling price is lower, producers may have less incentive to supply the product.
Quantity supplied decreases.
Therefore:
Quantity demanded > Quantity supplied
This creates a shortage.
14. Black Markets
A shortage caused by a binding price ceiling can sometimes encourage an illegal or unofficial market.
A black market is a market in which goods are sold illegally or outside the legally regulated system, often at prices higher than the permitted price.
Example
Suppose the legal maximum price of a scarce product is ₹50.
Because of the price ceiling:
- Consumers want 1,000 units.
- Sellers supply only 600 units.
The product becomes difficult to obtain.
Some people may secretly offer ₹100 to obtain it.
A seller may illegally agree to sell at ₹100.
This creates a black-market transaction.
Important point
A price ceiling does not automatically create a black market in every situation.
A black market is more likely when:
- the price ceiling creates a significant shortage,
- the product is highly demanded,
- enforcement is weak,
- consumers are willing to pay more than the legal price.
15. Market Failure
Meaning
A market failure occurs when the normal operation of markets does not lead to an efficient or socially desirable allocation of resources.
In simple words:
A market failure occurs when the market mechanism by itself does not produce the best outcome for society.
This does not necessarily mean that markets are always bad.
Markets are very useful for allocating many resources, but under certain conditions they may produce problems that require government action or other solutions.
16. Main Types of Market Failure
Important examples include:
- Externalities
- Public goods
- Information problems / information asymmetry
- Market power
Let’s understand them one by one.
17. Externalities
An externality occurs when an economic activity affects people who are not directly involved in the buying and selling transaction.
There are two major types:
- Negative externality
- Positive externality
A. Negative Externality
A negative externality occurs when an activity creates a harmful effect on third parties.
Example: Factory Pollution
Suppose a factory produces clothes.
The factory sells clothes to customers and earns revenue.
But the factory also releases smoke into the surrounding air.
People living nearby may suffer from:
- polluted air,
- unpleasant smell,
- health problems,
- reduced quality of life.
These people are not part of the transaction between the factory and its customers.
Therefore, pollution is a negative externality.
Another Example: Loud Music
Suppose a restaurant plays extremely loud music late at night.
The restaurant and its customers may enjoy the music, but nearby residents may lose sleep.
The disturbance imposed on residents is a negative external effect.
18. Positive Externality
A positive externality occurs when an activity provides benefits to people who are not directly involved in the transaction.
Example: Education
Suppose a student receives a good education.
The student benefits through:
- better knowledge,
- better employment opportunities,
- higher income potential.
But society can also benefit because educated people may:
- become more productive,
- make informed decisions,
- contribute to innovation,
- participate more effectively in society.
Therefore, education can generate positive external benefits.
Another Example: Vaccination
When a person gets vaccinated, that person receives protection.
But vaccination can also reduce the spread of certain infectious diseases to other people.
Thus, others may benefit even though they did not directly purchase the vaccination.
19. Market Power
A market works best when there is meaningful competition.
But sometimes one firm or a small number of firms can have significant control over the market.
This is called market power.
Example
Suppose only one company provides a particular service in a remote area.
Because consumers have very few alternatives, the company may have considerable power to influence:
- prices,
- output,
- terms of service.
This can lead to outcomes that are less favourable to consumers.
20. Information Asymmetry
Sometimes buyers and sellers do not have equal information.
This is called information asymmetry.
Example: Used Car Market
Suppose a person wants to buy a second-hand car.
The seller knows that the car has had serious engine problems, but the buyer does not know.
The seller has more information than the buyer.
The buyer may pay a high price for a car that is actually worth much less.
This difference in information can cause market problems.
Another Example: Health Services
A doctor generally has much more technical knowledge about a medical treatment than a patient.
The patient depends on the doctor’s information to make decisions.
This is another situation where information is not equally distributed.
21. Public Goods
A public good is a good or service that has two important characteristics:
1. Non-excludable
It is difficult or impossible to prevent people from using the good, even if they have not directly paid for it.
2. Non-rival
One person’s use of the good does not significantly reduce the amount available for others.
Therefore:
Public goods are generally non-excludable and non-rival.
22. Example: Street Lighting
Street lighting is a simple example.
Imagine a streetlight illuminating a public road.
Non-excludable
It is difficult to prevent one particular pedestrian from benefiting from the light.
Even if that person did not directly pay for the streetlight, they can still walk under its light.
Non-rival
If one person walks under the light, it does not normally prevent another person from also benefiting from the same illumination.
Therefore, street lighting has public-good characteristics.
23. Example: Public Park
A public park can also provide public-good benefits, although real parks can become crowded and may therefore not be perfectly non-rival at all times.
Suppose a city maintains a public green space.
Many people can enjoy:
- walking,
- fresh surroundings,
- greenery,
- recreation.
If one person enjoys the scenery, that does not necessarily prevent another person from enjoying it.
However, if the park becomes extremely crowded, one person’s use can interfere with another person’s enjoyment.
So public goods are often best understood through their degree of non-excludability and non-rivalry, rather than assuming that every public service is perfectly both.
24. The Free-Rider Problem
One important problem associated with public goods is the free-rider problem.
A free rider is a person who receives the benefit of a good or service without directly paying for it.
Example
Suppose a city installs streetlights.
Every resident benefits from better lighting.
It would be difficult to make one person pay every time they walk past a streetlight.
Some people may therefore enjoy the benefits without making a direct payment.
This is called the free-rider problem.
25. Why Does the Government Often Provide Public Goods?
Because private firms may find it difficult to earn enough revenue from goods that are non-excludable.
Suppose a private company installs streetlights.
How will it charge every person who walks under the light?
It may be extremely difficult to collect payment from each user.
As a result, private markets may provide too little of some public goods.
Governments can finance such goods through:
- taxes,
- public budgets,
- government programmes.
Examples include:
- street lighting,
- public roads,
- national defence,
- certain public safety services.
26. Quick Comparison: Demand and Supply
| Basis | Demand | Supply |
| Main actor | Consumer | Producer |
| Meaning | Quantity consumers are willing and able to buy | Quantity producers are willing and able to sell |
| Relationship with price | Generally inverse | Generally direct |
| Price rises | Quantity demanded generally falls | Quantity supplied generally rises |
| Price falls | Quantity demanded generally rises | Quantity supplied generally falls |
27. Quick Comparison: Surplus and Shortage
| Situation | Meaning | Result |
| Surplus | Supply > Demand | Sellers may reduce price |
| Shortage | Demand > Supply | Price tends to rise if prices are flexible |
| Equilibrium | Demand = Supply | Market clears at that price and quantity |
28. Quick Comparison: Market Failure Types
| Type | Simple Meaning | Example |
| Negative externality | Activity harms others | Factory pollution |
| Positive externality | Activity benefits others | Education |
| Public goods | Difficult to exclude users and generally non-rival | Street lighting |
| Information asymmetry | One side has more information | Used-car market |
| Market power | A firm/firms have substantial control | Dominant supplier |
29. Key Concepts to Remember
Students can remember the entire topic through these simple relationships:
Demand
Price ↑ → Quantity Demanded ↓
Price ↓ → Quantity Demanded ↑
Supply
Price ↑ → Quantity Supplied ↑
Price ↓ → Quantity Supplied ↓
Equilibrium
Quantity Demanded = Quantity Supplied
Surplus
Supply > Demand
Shortage
Demand > Supply
Price Ceiling
Maximum legal price
If it is below equilibrium price → shortage may occur.
Market Failure
Market mechanism does not produce an efficient or socially desirable outcome.
Public Goods
Non-excludable + Non-rival
Free Rider
Receives the benefit without directly paying for it.
30. One Complete Real-Life Example
Let’s combine almost everything using the example of vegetables.
Suppose the normal market price of tomatoes is ₹40 per kg.
At this price:
- Consumers want 1,000 kg.
- Farmers and sellers supply 1,000 kg.
Therefore:
₹40 = Equilibrium Price
1,000 kg = Equilibrium Quantity
Situation 1: Price rises to ₹60
Consumers may buy less.
Farmers and sellers may want to supply more.
So:
Quantity demanded ↓
Quantity supplied ↑
This can create a surplus.
Situation 2: Heavy rainfall destroys crops
Supply decreases.
Now fewer tomatoes reach the market.
If demand remains strong:
Supply ↓ → Price tends to rise
A new equilibrium is established.
Situation 3: Government fixes a maximum price below equilibrium
Suppose the equilibrium price is ₹50 but the government fixes the maximum legal price at ₹30.
At ₹30:
- Consumers may want 1,500 kg.
- Sellers may supply only 800 kg.
Therefore:
Shortage = 700 kg
If the shortage becomes severe, some sellers and buyers may try to trade outside the legal system, creating a black market.
Situation 4: Pollution
Suppose a tomato-processing factory releases untreated waste into a nearby river.
The factory and its customers conduct a normal transaction, but nearby residents and farmers may suffer from polluted water.
This is a:
Negative externality.
Situation 5: Public facilities
Suppose the government provides street lighting near the market.
People can benefit from the lighting without being individually charged every time they use it.
This illustrates the characteristics of a:
Public good.
Final Concept Map
Demand + Supply
↓
Market Interaction
↓
Equilibrium Price + Equilibrium Quantity
↓
Changes in market conditions can cause:
- Shortage
- Surplus
- New Equilibrium
Government may intervene through:
Price Ceiling
↓
May create Shortage
↓
Possible Black Market
Markets can also fail because of:
Externalities + Public Goods + Information Problems + Market Power
This is why understanding demand and supply is important not only for examinations but also for understanding prices, shortages, government policies and everyday economic decisions.