Demand and supply — elasticity and equilibrium (KCSE Business Form 4)

TL;DR

This topic explains how prices and quantities of goods are set in a market based on what people want to buy and what producers are willing to sell. You'll learn how sensitive these quantities are to price changes and how everything balances out. Understanding this helps you predict market behaviour and make smart business decisions.

1. The Mental Model

Think of a market as a constant tug-of-war between buyers and sellers. Buyers want low prices, sellers want high prices. Eventually, they meet in the middle, and that's where most deals happen.

2. The Core Material

Demand

Demand is simply how much of a good or service consumers are willing and able to buy at different prices over a specific period.

Law of Demand: This is straightforward: as the price of a good increases, the quantity demanded decreases, and vice versa, assuming everything else stays the same. People buy less of something when it's expensive.

Factors Affecting Demand (Determinants of Demand):
* Price of the good itself: As explained by the law of demand.
* Income of consumers: If you earn more, you might buy more of certain goods (normal goods). If you earn less, you might buy more of cheaper alternatives (inferior goods).
* Prices of related goods:
* Substitutes: Goods that can be used in place of another (e.g., tea and coffee). If the price of coffee goes up, demand for tea might increase.
* Complements: Goods that are used together (e.g., cars and fuel). If the price of fuel goes up, demand for cars might decrease.
* Tastes and preferences: If something becomes fashionable, demand increases.
* Consumer expectations: If you expect prices to rise in the future, you might buy more now.
* Population size: More people generally means more demand.

Supply

Supply is the quantity of a good or service that producers are willing and able to offer for sale at different prices over a specific period.

Law of Supply: This is also simple: as the price of a good increases, the quantity supplied increases, and vice versa, assuming everything else stays the same. Producers want to sell more when they can get a higher price.

Factors Affecting Supply (Determinants of Supply):
* Price of the good itself: As explained by the law of supply.
* Cost of production: If it costs more to make something (e.g., raw materials, wages), producers will supply less at any given price.
* Technology: Better technology usually lowers production costs, increasing supply.
* Government policy (taxes and subsidies): Taxes increase costs, reducing supply. Subsidies (government payments) reduce costs, increasing supply.
* Prices of other goods: If a farmer can grow maize or beans, and the price of maize goes up significantly, they might shift production towards maize, reducing the supply of beans.
* Producer expectations: If producers expect prices to fall, they might try to sell more now.
* Natural factors: Weather conditions can affect agricultural supply.

Market Equilibrium

Equilibrium is the point where the quantity demanded equals the quantity supplied. At this point, there's no pressure for the price to change. It's where the buyers and sellers agree.

• Equilibrium Price: The price at which quantity demanded equals quantity supplied.
• Equilibrium Quantity: The quantity bought and sold at the equilibrium price.

How Equilibrium is Reached:
* Surplus (Excess Supply): If the price is above the equilibrium price, the quantity supplied is greater than the quantity demanded. Producers have unsold goods, so they'll lower prices to clear stock, moving towards equilibrium.
* Shortage (Excess Demand): If the price is below the equilibrium price, the quantity demanded is greater than the quantity supplied. Consumers can't find enough goods, so they'll be willing to pay more, pushing prices up towards equilibrium.

graph TD
    A[Market Price Too High] --> B{Surplus};
    B --> C[Producers Lower Price];
    C --> D[Quantity Demanded Increases];
    C --> E[Quantity Supplied Decreases];
    D & E --> F[Move Towards Equilibrium];

    G[Market Price Too Low] --> H{Shortage};
    H --> I[Consumers Bid Up Price];
    I --> J[Quantity Demanded Decreases];
    I --> K[Quantity Supplied Increases];
    J & K --> F;

    F --> L[Equilibrium Reached];

Elasticity

Elasticity measures how responsive quantity demanded or supplied is to a change in another factor, usually price.

1. Price Elasticity of Demand (PED):
Measures how much the quantity demanded changes when the price changes.

Formula: PED = (% Change in Quantity Demanded) / (% Change in Price)

• Elastic Demand (PED > 1): A small change in price leads to a large change in quantity demanded. Consumers are very responsive to price changes (e.g., luxury goods, goods with many substitutes).
• Inelastic Demand (PED < 1): A large change in price leads to a small change in quantity demanded. Consumers are not very responsive to price changes (e.g., necessities like salt, goods with few substitutes).
• Unitary Elastic Demand (PED = 1): The percentage change in quantity demanded is equal to the percentage change in price.
• Perfectly Elastic Demand (PED = infinity): Consumers will buy an infinite amount at one price, but nothing at a slightly higher price. (Theoretical)
• Perfectly Inelastic Demand (PED = 0): Quantity demanded does not change at all, regardless of price changes (e.g., life-saving medicine). (Theoretical)

Factors Affecting PED:
* Availability of substitutes: More substitutes = more elastic.
* Necessity vs. Luxury: Necessities = more inelastic; Luxuries = more elastic.
* Proportion of income spent on the good: Goods that take up a large part of your budget = more elastic.
* Time period: Demand tends to be more elastic in the long run as people have more time to find substitutes.

2. Price Elasticity of Supply (PES):
Measures how much the quantity supplied changes when the price changes.

Formula: PES = (% Change in Quantity Supplied) / (% Change in Price)

• Elastic Supply (PES > 1): Producers can significantly increase output in response to a price rise (e.g., goods that are easy to produce).
• Inelastic Supply (PES < 1): Producers cannot easily increase output in response to a price rise (e.g., agricultural products in the short run, goods requiring complex production).
• Unitary Elastic Supply (PES = 1): Percentage change in quantity supplied equals percentage change in price.
• Perfectly Elastic Supply (PES = infinity): Producers will supply an infinite amount at one price, but nothing at a slightly lower price. (Theoretical)
• Perfectly Inelastic Supply (PES = 0): Quantity supplied does not change at all, regardless of price changes (e.g., unique artwork). (Theoretical)

Factors Affecting PES:
* Time period: Supply is generally more elastic in the long run as producers have more time to adjust production.
* Availability of resources: If resources are readily available, supply is more elastic.
* Mobility of factors of production: How easily can labour and capital be moved to produce more of a good?
* Storage capacity: If goods can be stored easily, supply can be more elastic.

3. Worked Example

Let's say the price of a 2kg packet of maize flour increases from KES 150 to KES 180. As a result, the quantity demanded by consumers falls from 10,000 packets per day to 8,500 packets per day. Calculate the Price Elasticity of Demand (PED).

Step 1: Calculate the percentage change in quantity demanded.
* Change in Quantity = New Quantity - Original Quantity = 8,500 - 10,000 = -1,500
* % Change in Quantity = (Change in Quantity / Original Quantity) * 100
* % Change in Quantity = (-1,500 / 10,000) * 100 = -15%

Step 2: Calculate the percentage change in price.
* Change in Price = New Price - Original Price = 180 - 150 = 30
* % Change in Price = (Change in Price / Original Price) * 100
* % Change in Price = (30 / 150) * 100 = 20%

Step 3: Apply the PED formula.
* PED = (% Change in Quantity Demanded) / (% Change in Price)
* PED = -15% / 20% = -0.75

Step 4: Interpret the result.
We usually take the absolute value for PED, so PED = 0.75.
Since 0.75 is less than 1, the demand for maize flour in this scenario is inelastic. This means that a 20% increase in price led to a smaller 15% decrease in quantity demanded. Consumers are not very responsive to the price change, likely because maize flour is a staple food.

4. Key Takeaways

• Demand shows how much consumers want to buy at different prices, while supply shows how much producers want to sell.
• The Law of Demand states that as price increases, quantity demanded decreases; the Law of Supply states that as price increases, quantity supplied increases.
• Equilibrium is the market clearing point where quantity demanded equals quantity supplied, setting the market price and quantity.
• Price Elasticity of Demand (PED) measures how much quantity demanded changes with a price change.
• Price Elasticity of Supply (PES) measures how much quantity supplied changes with a price change.
• Elasticity values (greater than 1, less than 1, or equal to 1) tell you if consumers/producers are very responsive or not.
• Factors like substitutes, necessity, income, and production costs significantly influence demand and supply.

Common Mistakes to Avoid:
- Confusing a "change in quantity demanded/supplied" (movement along the curve due to price) with a "change in demand/supply" (shift of the entire curve due to other factors).
- Forgetting to take the absolute value when interpreting PED, which is usually reported as a positive number.
- Mixing up the factors that affect demand with those that affect supply.
- Assuming equilibrium is always static; it constantly adjusts to new market conditions.

5. Now Try It

Imagine you are a small-scale farmer selling tomatoes. Due to heavy rains, your tomato harvest is much smaller than usual. At the same time, a popular health report praises the benefits of tomatoes, increasing consumer interest.

Exercise:
1. Describe what will happen to the