Supply and Demand

Scarcity, Choice, and Opportunity Cost

The Fundamental Economic Problem

Scarcity is the central problem of economics: human wants are infinite, but the resources available to Satisfy those wants are finite. Because resources are scarce, individuals, firms, and governments must Make choices about how to allocate them. Every choice involves a trade-off: choosing one alternative Means forgoing another.

The four factors of production are:

Land: all natural resources (land, minerals, water, forests)
Labour: the physical and mental effort contributed by workers
Capital: manufactured goods used to produce other goods and services (machinery, tools, factories). Capital is distinct from financial capital (money), which is not itself a factor of production
Entrepreneurship: the ability to organise the other factors of production and take risks in pursuit of profit

Opportunity Cost

The opportunity cost of a decision is the value of the next best alternative foregone. It is not The sum of all alternatives, but only the single most valuable one that was rejected.

Opportunity cost applies at every level:

Individual: a student choosing to attend university forgoes the full-time salary they could have earned
Firm: a factory producing cars cannot simultaneously use the same factory floor to produce buses
Government: spending on healthcare means less spending available for education or defence

Economic vs. Accounting Profit

Accounting profit $= \text{Total Revenue} - \text{Explicit Costs}$
Economic profit $= \text{Total Revenue} - \text{Explicit Costs} - \text{Implicit Costs}$

Implicit costs include the opportunity cost of the owner”s time and capital. A firm may earn a Positive accounting profit but a negative economic profit if it could earn more by deploying its Resources elsewhere.

Production Possibilities Frontier (PPF)

Definition and Interpretation

The PPF is a curve showing the maximum possible combinations of two goods or services an economy can Produce when all resources are fully and efficiently employed, given the current state of technology.

Assume an economy produces two goods, capital goods ( $K$ ) and consumer goods ( $C$ ). The PPF curves Outward (concave to the origin) because resources are not perfectly adaptable: as more of one good is Produced, increasingly larger sacrifices of the other good are required. This reflects the law of Increasing opportunity cost.

Points on, inside, and beyond the PPF

On the PPF: the economy is productively efficient — all resources are fully employed and allocated to their best use
Inside the PPF: the economy is productively inefficient — resources are unemployed or underemployed (e.g., during a recession)
Beyond the PPF: currently unattainable given the existing resources and technology

Shifts of the PPF

The PPF can shift outward (economic growth) due to:

Increases in the quantity of factors of production (more labour, capital, land)
Improvements in the quality of factors of production (better education, technological progress)
Institutional improvements (property rights, reduced corruption)

The PPF can shift inward due to:

Natural disasters, war, or disease that destroy resources
Resource depletion (exhaustion of non-renewable resources)

An asymmetric shift (where one axis shifts more than the other) occurs when a technological Improvement or resource increase is specific to one industry.

PPF and Opportunity Cost

The slope of the PPF at any point represents the marginal opportunity cost of producing one more Unit of the good on the x-axis, measured in terms of the good on the y-axis:

$\text{Slope of PPF} = -\frac{\Delta C}{\Delta K}$

A linear PPF implies constant opportunity costs (resources are equally suited to producing both Goods). A concave PPF implies increasing opportunity costs (resources are not perfectly Transferable between uses).

Supply and Demand

The Law of Demand

The law of demand states that, ceteris paribus, as the price of a good rises, the quantity Demanded falls. The relationship is captured by a downward-sloping demand curve. A movement along The curve reflects a change in price (change in quantity demanded). A shift of the curve reflects A change in a non-price determinant of demand.

Non-price determinants of demand:

Income (normal vs. Inferior goods)
Price of related goods (substitutes and complements)
Tastes and preferences
Population and demographics
Expectations of future prices or income

A substitute has a positive cross-price elasticity of demand: if the price of coffee rises, Demand for tea shifts rightward. A complement has a negative cross-price elasticity: if the Price of petrol rises, demand for cars shifts leftward.

The Law of Supply

The law of supply states that, ceteris paribus, as price rises, quantity supplied increases. The Supply curve slopes upward because higher prices increase the profitability of production, Incentivising firms to expand output.

Non-price determinants of supply:

Costs of production (wages, raw materials, energy)
Technology
Indirect taxes and subsidies
Number of firms in the market
Expectations of future prices
Supply shocks (natural disasters, political instability)

Market Equilibrium

Equilibrium occurs where quantity demanded equals quantity supplied. The equilibrium price $P^*$ and Quantity $Q^*$ are found at the intersection of the demand and supply curves.

If the market price is above $P^*$ A surplus (excess supply) exists, placing downward pressure On price. If the market price is below $P^*$ A shortage (excess demand) exists, placing upward Pressure on price.

The rationing function of price allocates scarce resources to those willing and able to pay. The signalling function communicates information about scarcity and consumer preferences to Producers.

Consumer and Producer Surplus

Consumer surplus is the difference between the maximum price a consumer is willing to pay and The actual market price. Graphically, it is the area below the demand curve and above the Equilibrium price.

$\mathrm{CS} = \int_{0}^{Q^*} D(Q) \, dQ - P^* \cdot Q^*$

For a linear demand curve $P = a - bQ$ :

$\mathrm{CS} = \frac{1}{2}(a - P^*) \cdot Q^*$

Producer surplus is the difference between the actual market price and the minimum price a Producer is willing to accept. Graphically, it is the area above the supply curve and below the Equilibrium price.

$\mathrm{PS} = P^* \cdot Q^* - \int_{0}^{Q^*} S(Q) \, dQ$

For a linear supply curve $P = c + dQ$ :

$\mathrm{PS} = \frac{1}{2}(P^* - c) \cdot Q^*$

At equilibrium, total welfare (consumer surplus + producer surplus) is maximised. Any deviation from Equilibrium creates a deadweight loss (DWL):

\mathrm{DWL} = \frac{1}{2} \times (\text{change in quantity}) \times (\text{difference in marginal benefit and marginal cost at the new quantity})

Functions of Price

Prices serve several critical functions in a market economy:

Rationing: prices allocate scarce goods to those who value them most (measured by willingness and ability to pay)
Signalling: prices convey information about scarcity, consumer preferences, and production costs
Incentive: high prices incentivise producers to increase supply and consumers to reduce demand; low prices do the reverse

Common Pitfalls

Confusing a movement along a demand curve (caused by a price change) with a shift of the demand curve (caused by a change in a non-price determinant). Always state the cause .
Forgetting that PED is expressed as an absolute value. A PED of $-0.5$ should be described as inelastic (since $|-0.5| < 1$ ), not as “negative elastic.”
Confusing public goods with merit goods. Public goods are defined by their characteristics (non-excludable, non-rivalrous); merit goods are defined by their positive externalities and information failure.
Stating that subsidies “increase supply” without specifying that the supply curve shifts rightward. An increase in supply is a shift; an increase in quantity supplied is a movement along the curve.
Neglecting to discuss deadweight loss when analysing taxes and subsidies. Always identify the welfare loss triangle on the diagram.
Confusing negative externalities of production with negative externalities of consumption. Pollution from a factory is a production externality; second-hand smoke from cigarettes is a consumption externality.
Confusing economic profit with accounting profit. Economic profit includes implicit costs (opportunity costs).
Stating that a monopolist charges “the highest possible price.” A profit-maximising monopolist charges the price on the demand curve at the quantity where MR $=$ MC, not the highest price consumers would pay.
Confusing the shutdown condition with the exit condition. A firm shuts down in the short run if $P <$ AVC; a firm exits the industry in the long run if $P <$ ATC.
Drawing the MR curve incorrectly. For a linear demand curve, MR has the same vertical intercept but twice the slope (MR falls twice as fast as AR).
Confusing diminishing returns with diseconomies of scale. Diminishing returns occur in the short run (one factor is fixed); diseconomies of scale occur in the long run (all factors are variable).

Practice Problems

Problem 1: PPF and Opportunity Cost

An economy can produce the following combinations of capital goods ( $K$ ) and consumer goods ( $C$ ):

Point	K (units)	C (units)
A	0	100
B	10	95
C	20	85
D	30	70
E	40	50
F	50	0

(a) Calculate the opportunity cost of increasing capital goods production from 20 to 30 units.

(b) Does this PPF show constant or increasing opportunity costs? Explain.

(a) Moving from point C (20 $K$ 85 $C$ ) to point D (30 $K$ 70 $C$ ):

$\text{Opportunity cost} = \frac{\Delta C}{\Delta K} = \frac{85 - 70}{30 - 20} = \frac{15}{10} = 1.5 \; C \text{ per } K$

Producing 10 additional units of capital goods requires sacrificing 15 units of consumer goods.

(b) The opportunity costs are increasing:

From A to B: $\frac{100 - 95}{10 - 0} = 0.5 \; C$ per $K$
From B to C: $\frac{95 - 85}{20 - 10} = 1.0 \; C$ per $K$
From C to D: $\frac{85 - 70}{30 - 20} = 1.5 \; C$ per $K$
From D to E: $\frac{70 - 50}{40 - 30} = 2.0 \; C$ per $K$
From E to F: $\frac{50 - 0}{50 - 40} = 5.0 \; C$ per $K$

The opportunity cost of each additional 10 units of $K$ increases as more $K$ is produced, confirming Increasing opportunity costs (a concave PPF).

(c) Point D is on the PPF (30 $K$ 70 $C$ ), so yes, the economy can produce this combination if all Resources are fully and efficiently employed. However, to move from point B to point D, the economy Must reallocate resources from consumer goods to capital goods, which requires time and adjustment.

Problem 2: PED, YED, and XED Calculations

The demand for good $X$ is given by $Q_{d,X} = 200 - 2P_X + 0.5Y + 0.8P_Y$ Where $P_X = 20$ $Y = 400$ (income), and $P_Y = 30$ (price of related good $Y$ ).

(a) Calculate the quantity demanded of $X$ .

(b) Calculate PED at this point.

(d) Calculate XED at this point. Are $X$ and $Y$ substitutes or complements?

(a) $Q_{d,X} = 200 - 2(20) + 0.5(400) + 0.8(30) = 200 - 40 + 200 + 24 = 384$

(b) $\mathrm{PED} = \frac{\partial Q}{\partial P_X} \times \frac{P_X}{Q} = (-2) \times \frac{20}{384} = -0.104$

$\|\mathrm{PED}\| = 0.104 < 1$ : demand is inelastic at this point.

YED $> 0$ : $X$ is a normal good. Since $0 <$ YED $< 1$ It is a necessity (income inelastic).

(d) $\mathrm{XED} = \frac{\partial Q}{\partial P_Y} \times \frac{P_Y}{Q} = 0.8 \times \frac{30}{384} = 0.0625$

XED $> 0$ : $X$ and $Y$ are substitutes. The positive but small value suggests they are weak Substitutes.

Problem 3: Negative Externality with Tax Correction

The market for a chemical product has the following characteristics:

Demand: $P = 100 - Q$ (MPB $=$ MSB)
Private supply: $P = 20 + Q$ (MPC)
Marginal external cost: MEC $= 10$ per unit at all output levels

(a) Find the market equilibrium (private equilibrium) price and quantity.

(b) Find the socially optimal price and quantity.

(d) What specific tax would correct the market failure?

(a) Market equilibrium: set demand $=$ supply:

$100 - Q = 20 + Q \implies 2Q = 80 \implies Q_{\text{private}} = 40$ $P_{\text{private}} = 100 - 40 = 60$

(b) Social optimum: MSC $=$ MPC $+$ MEC $= (20 + Q) + 10 = 30 + Q$ . Set MSB $=$ MSC:

$100 - Q = 30 + Q \implies 2Q = 70 \implies Q_{\text{social}} = 35$ $P_{\text{social}} = 100 - 35 = 65$

$\mathrm{DWL} = \frac{1}{2} \times \mathrm{MEC} \times (Q_{\text{private}} - Q_{\text{social}}) = \frac{1}{2} \times 10 \times (40 - 35) = \frac{1}{2} \times 10 \times 5 = 25$

(d) The specific tax should equal the marginal external cost: a tax of USD 10 per unit. This shifts The supply curve from $P = 20 + Q$ to $P = 30 + Q$ (which is MSC), leading to the socially optimal Quantity of 35 units.

Problem 4: Monopoly Profit Maximisation and Welfare Analysis

A monopoly faces the demand curve $P = 150 - 2Q$ and has a total cost function $\mathrm{TC} = 100 + 10Q + Q^2$ .

(a) Find the profit-maximising price and quantity.

(b) Calculate the monopolist’s economic profit.

(d) Calculate the deadweight loss of monopoly.

(a) $\mathrm{MR} = 150 - 4Q$ (twice the slope of demand). $\mathrm{MC} = \frac{d\mathrm{TC}}{dQ} = 10 + 2Q$ .

Set MR $=$ MC:

$150 - 4Q = 10 + 2Q \implies 6Q = 140 \implies Q_m = \frac{140}{6} = 23.33$ $P_m = 150 - 2(23.33) = 150 - 46.67 = 103.33$

(b) $\mathrm{TR} = P_m \times Q_m = 103.33 \times 23.33 = 2411.1$

$\mathrm{TC} = 100 + 10(23.33) + (23.33)^2 = 100 + 233.3 + 544.3 = 877.6$

$\pi = 2411.1 - 877.6 = 1533.5$

$150 - 2Q = 10 + 2Q \implies 4Q = 140 \implies Q_c = 35$ $P_c = 150 - 2(35) = 80$

(d) Deadweight loss:

$\mathrm{DWL} = \frac{1}{2} \times (P_m - P_c) \times (Q_c - Q_m) = \frac{1}{2} \times (103.33 - 80) \times (35 - 23.33)$ $\mathrm{DWL} = \frac{1}{2} \times 23.33 \times 11.67 = 136.1$

Problem 5: Tax Incidence and Welfare

The demand for a good is given by $Q_d = 120 - P$ and the supply is $Q_s = 2P - 40$ . The government Imposes a specific tax of USD 15 per unit.

(a) Find the pre-tax equilibrium price and quantity.

(b) Find the post-tax equilibrium price paid by consumers and price received by producers.

(d) Determine the proportion of the tax borne by consumers and producers.

(a) Pre-tax: $Q_d = Q_s$ :

$120 - P = 2P - 40 \implies 3P = 160 \implies P_0 = 53.33$ $Q_0 = 120 - 53.33 = 66.67$

(b) With a specific tax of $t = 15$ The supply curve shifts upward. The new supply (price received by Producers) is $P_s = P_d - 15$ . Setting $Q_d = Q_s$ :

$120 - P_d = 2(P_d - 15) - 40 = 2P_d - 70$ $190 = 3P_d \implies P_d = 63.33$ $P_s = 63.33 - 15 = 48.33$ $Q_t = 120 - 63.33 = 56.67$

Deadweight loss $= \frac{1}{2} \times t \times (Q_0 - Q_t) = \frac{1}{2} \times 15 \times (66.67 - 56.67) = \frac{1}{2} \times 15 \times 10 = 75$

(d) Consumer burden $= P_d - P_0 = 63.33 - 53.33 = 10$

Producer burden $= P_0 - P_s = 53.33 - 48.33 = 5$

Consumer share $= \frac{10}{15} = 66.7\%$ . Producer share $= \frac{5}{15} = 33.3\%$ .

Consumers bear a larger share because demand is less elastic than supply at the equilibrium.

Problem 6: Monopolistic Competition Long-Run Equilibrium

A firm in monopolistic competition has the demand curve $P = 200 - Q$ and total cost function $\mathrm{TC} = 500 + 40Q + Q^2$ . In the long run, entry and exit ensure zero economic profit.

(a) Find the long-run equilibrium quantity and price.

(b) Calculate the excess capacity.

(a) Zero economic profit: $\mathrm{TR} = \mathrm{TC}$ .

$\mathrm{TR} = P \times Q = (200 - Q)Q = 200Q - Q^2$

$\mathrm{TC} = 500 + 40Q + Q^2$

Set TR $=$ TC:

$200Q - Q^2 = 500 + 40Q + Q^2$ $2Q^2 - 160Q + 500 = 0$ $Q^2 - 80Q + 250 = 0$

Using the quadratic formula:

$Q = \frac{80 \pm \sqrt{6400 - 1000}}{2} = \frac{80 \pm \sqrt{5400}}{2} = \frac{80 \pm 73.48}{2}$

$Q = 76.74$ (the other root, $Q = 3.26$ Gives a higher ATC).

$P = 200 - 76.74 = 123.26$

(b) ATC is minimised where MC $=$ ATC. $\mathrm{MC} = 40 + 2Q$ . $\mathrm{ATC} = 500/Q + 40 + Q$ .

$40 + 2Q = 500/Q + 40 + Q \implies Q = 500/Q \implies Q^2 = 500 \implies Q_{\min} = 22.36$

Excess capacity $= 76.74 - 22.36 = 54.38$ units. The firm operates well below its minimum efficient Scale.

(c) No. Allocative efficiency requires $P = \mathrm{MC}$ . Here, $P = 123.26$ while $\mathrm{MC} = 40 + 2(76.74) = 193.48$ . Since $P < \mathrm{MC}$ This suggests an issue. In practice, the firm should also satisfy the MR $=$ MC Condition. $\mathrm{MR} = 200 - 2Q = 200 - 153.48 = 46.52$ . Setting MR $=$ MC: $46.52 = 193.48$ is not Satisfied. The correct approach recognises that in long-run equilibrium for monopolistic competition, The demand curve is tangent to ATC where MR $=$ MC, and $P > \mathrm{MC}$ . The firm produces less than The socially optimal quantity.

Problem 7: Subsidy Welfare Analysis

The government provides a USD 5 per unit subsidy on solar panels. Before the subsidy, the Equilibrium price was USD 100 and the equilibrium quantity was $10000$ units. After the subsidy, the Quantity increases to $14000$ units. Calculate the total cost of the subsidy to the government and Discuss potential government failures.

Total subsidy cost $= \$ 5 \times 14000 = $70000$.

Potential government failures include:

The subsidy may lead to overproduction if the new quantity exceeds the socially optimal level, creating a deadweight loss.
Firms may become dependent on the subsidy and fail to innovate or reduce costs independently.
The subsidy is funded by taxpayers, representing an opportunity cost — the funds could have been spent on other public services.
If solar panel production has its own negative externalities (e.g., manufacturing pollution), the subsidy could exacerbate those.
Administrative costs of implementing and monitoring the subsidy reduce its net benefit.

Consumer Choice Theory (HL Extension)

Budget Constraints

A consumer’s budget constraint represents all combinations of two goods they can afford given their Income and the prices of the goods. If a consumer has income $M$ The price of good $X$ is $P_X$ And The price of good $Y$ is $P_Y$ :

$P_X \cdot X + P_Y \cdot Y = M$

The budget line has:

Slope $= -P_X / P_Y$ (the opportunity cost of one more unit of $X$ in terms of $Y$ )
X-intercept $= M / P_X$ (maximum quantity of $X$ affordable)
Y-intercept $= M / P_Y$ (maximum quantity of $Y$ affordable)

The budget constraint shifts outward when income increases or when the price of both goods falls Proportionally. A change in the price of one good rotates the budget line around the intercept of The other good.

Indifference Curves

An indifference curve shows all combinations of two goods that give the consumer the same level of Utility (satisfaction). The consumer is indifferent between any two points on the same curve.

Properties of indifference curves:

Downward-sloping: to maintain the same utility, consuming more of one good requires consuming less of the other
Convex to the origin: due to the diminishing marginal rate of substitution
Cannot intersect: if two curves intersected, a point would simultaneously represent two different utility levels, which is a logical contradiction
Higher curves represent higher utility: the further from the origin, the greater the combination of goods consumed and therefore the higher the utility

Marginal Rate of Substitution (MRS)

The MRS measures the rate at which a consumer is willing to trade one good for another while Maintaining the same utility level:

$\mathrm{MRS}_{XY} = -\frac{\Delta Y}{\Delta X}$

At any point on the indifference curve, the MRS equals the absolute value of the slope of the Indifference curve. As the consumer moves down along the curve, the MRS diminishes: the more of $X$ And the less of $Y$ consumed, the less $Y$ the consumer is willing to give up for an additional unit Of $X$ .

The MRS also equals the ratio of marginal utilities:

$\mathrm{MRS}_{XY} = \frac{MU_X}{MU_Y}$

Consumer Equilibrium

A rational consumer maximises utility subject to their budget constraint. The optimal consumption Bundle occurs where the indifference curve is tangent to the budget line:

$\mathrm{MRS}_{XY} = \frac{P_X}{P_Y}$

Or equivalently:

$\frac{MU_X}{P_X} = \frac{MU_Y}{P_Y}$

This condition states that the marginal utility per dollar spent must be equal across all goods. If $\frac{MU_X}{P_X} > \frac{MU_Y}{P_Y}$ The consumer should buy more $X$ and less $Y$ until the Ratio equalises.

Income and Substitution Effects

When the price of a good changes, the total effect on quantity demanded can be decomposed into:

Substitution effect: the change in consumption due to the change in relative prices, holding utility constant. The substitution effect is always negative (price and quantity move in opposite directions)
Income effect: the change in consumption due to the change in real purchasing power (real income). The direction depends on whether the good is normal or inferior

For a normal good:

Both income and substitution effects work in the same direction
When price falls, both effects increase quantity demanded
The demand curve slopes downward

For an inferior good:

The substitution effect increases quantity demanded when price falls
The income effect decreases quantity demanded (because the consumer is effectively richer and switches to superior goods)
If the income effect is smaller than the substitution effect, the demand curve still slopes downward (most inferior goods)
If the income effect outweighs the substitution effect, the demand curve slopes upward: this is a Giffen good (extremely rare; requires the good to be a staple that dominates the budget of very poor consumers)

Numerical example:

A consumer has income $M = 100$$P_X = 5$$P_Y = 2$ . The consumer’s utility function is $U = X^{0.5} \cdot Y^{0.5}$ .

Budget constraint: $5X + 2Y = 100$

$\mathrm{MRS} = \frac{MU_X}{MU_Y} = \frac{0.5 X^{-0.5} Y^{0.5}}{0.5 X^{0.5} Y^{-0.5}} = \frac{Y}{X}$

Setting $\mathrm{MRS} = P_X / P_Y$ :

$\frac{Y}{X} = \frac{5}{2} \implies Y = 2.5X$

Substituting into the budget constraint:

$5X + 2(2.5X) = 100 \implies 10X = 100 \implies X^* = 10, \quad Y^* = 25$

Common Pitfalls in Consumer Choice

Drawing indifference curves that slope upward or that intersect. Indifference curves must be downward-sloping and non-intersecting.
Confusing the MRS with the budget line slope. At equilibrium they are equal, but they represent different concepts (subjective trade-off vs. Market trade-off).
Assuming all inferior goods are Giffen goods. A Giffen good is a special case where the income effect dominates the substitution effect. Most inferior goods have a normally-shaped demand curve.

Worked Examples

Example 1: Shift in Demand

The market for electric vehicles (EVs) is initially in equilibrium at price $40,000 and quantity 100,000. A government subsidy for EV purchases shifts demand rightward. The new equilibrium is$ 45,000 and 150,000 units.

The increase in equilibrium price ( $+\$ 5,000 $) and quantity ($ +50,000$) is consistent with a rightward demand shift
Consumer surplus increases if the demand shift is driven by a change in tastes/preferences, but the effect depends on the magnitude of the shift relative to the price increase

Example 2: PPF and Opportunity Cost

An economy can produce the following combinations of food ( $F$ ) and machines ( $M$ ):

Combination	Food (units)	Machines (units)
A	0	100
B	40	90
C	70	70
D	90	40
E	100	0

Moving from B to C, the opportunity cost of 30 additional units of food is 20 machines ( $20/30 = 0.67$ machines per unit of food). Moving from C to D, the opportunity cost of 20 additional units of food is 30 machines ( $30/20 = 1.5$ machines per unit of food). The increasing opportunity cost reflects the concavity of the PPF.

Summary

Scarcity forces all economic agents to make choices; every choice has an opportunity cost (the value of the next best alternative foregone)
The PPF illustrates maximum output combinations, productive efficiency, and economic growth (outward shifts)
Supply and demand determine equilibrium price and quantity; non-price factors shift the curves, while price changes cause movements along them
Consumer choice theory uses indifference curves and budget constraints to model rational decision-making
Key diagram skills: drawing and shifting supply/demand curves, interpreting PPF shifts, calculating opportunity cost from PPF data

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