A2 Microeconomics: Price System and Firms

Marginal Utility and Consumer Choice

Marginal Utility

• Utility: satisfaction gained from consuming a product.
• Total utility: total satisfaction from consuming all units of a product over a period.
• Marginal utility (MU): extra satisfaction from consuming one additional unit over a period.
  • MU typically falls as consumption increases (law of diminishing marginal utility).
• Law of diminishing marginal utility:
  • As more of a good is consumed, MU from each extra unit declines.
  • Assumes limited income, rational behaviour, and a goal of maximising total utility.
• Consumers are willing to buy additional units when:
  • Price of the good ≤ marginal utility from that unit (in money terms).

Equi-Marginal Principle and Consumer Choice

• Equi-marginal principle: consumers maximise total utility when the marginal utility per unit of currency spent is equal for all goods consumed:

  [ \frac{MU_A}{P_A} = \frac{MU_B}{P_B} = \frac{MU_C}{P_C} = \dots ]

• If MU per dollar is higher for one good than another, the consumer reallocates spending towards that good until equality is restored.

• This principle underlies the derivation of an individual’s demand curve:

  • When the price of product A rises:
    • MU/P for A falls relative to other goods.
    • Consumers switch away from A toward other goods to restore equality.
    • Quantity demanded for A falls, generating a downward-sloping demand curve.

Limitations of Marginal Utility and Rational Consumer Behaviour

• Ranking difficulties:
  • Consumers cannot always rank wants clearly from most to least satisfying.
  • Different products may yield similar satisfaction; preferences may vary with mood or context.
• Measurement problems:
  • Utility is intangible; assigning precise numerical values to “satisfaction” is unrealistic.
• Behavioural assumptions:
  • Model assumes rational behaviour, stable preferences, and that “more is always better”.
  • In reality, decisions can be influenced by habits, emotions, social pressures, and imperfect self-control.

Consumer Choice: Budget Line, Substitution Effect and Income Effect

Budget Line

• Budget line: all combinations of two goods a consumer can afford given:
  • Money income.
  • Prices of the two goods.
• A change in income shifts the budget line:
  • Increase in income → parallel outward shift.
  • Decrease in income → parallel inward shift.
• A change in the price of one good pivots the budget line around the intercept of the other good.

Substitution Effect

• Substitution effect: change in consumption resulting from a change in relative prices, holding real income (utility) constant.
• When the price of a good falls:
  • It becomes relatively cheaper than substitutes.
  • Consumers substitute away from relatively more expensive goods towards the now cheaper good.
• When the price of a good rises:
  • It becomes relatively more expensive.
  • Consumers substitute towards other, now relatively cheaper goods.

Income Effect

• Income effect: change in consumption resulting from a change in real income (purchasing power) caused by a price change, with money income fixed.
  • A fall in the price of a good increases real income.
  • A rise in the price of a good reduces real income.
• For normal goods:
  • Higher real income → demand rises.
  • Lower real income → demand falls.
• For inferior goods:
  • Higher real income → demand falls (switch to better-quality goods).
  • Lower real income → demand rises (consumer is forced to buy cheaper, inferior goods).

Indifference Curves and Price Effects

Indifference Curves and Marginal Rate of Substitution

• Indifference curve (IC):
  • Shows all combinations of two goods that give the consumer the same level of satisfaction (utility).
  • Consumer is indifferent between any points on the same IC (e.g. x, y, z).
• Higher indifference curves:
  • Represent higher levels of satisfaction.
  • A curve further from the origin is more preferred.
• Marginal rate of substitution (MRS):
  • The rate at which the consumer is willing to give up one good to gain an extra unit of the other while keeping utility constant.
  • Measured by the slope of the indifference curve.
• Budget line vs indifference curve:
  • Budget line: represents what is affordable (income constraint).
  • Indifference curve: represents preferences (utility).
  • Optimal consumption occurs where the budget line is tangent to the highest achievable indifference curve:
    • At this point, MRS = price ratio.

Decomposing Price Changes: Substitution and Income Effects

• Consider a price change for good B, holding money income constant:

Price Fall for a Normal Good (B)

• Step 1: Substitution effect (E1 → E2):
  • Price of B falls; B becomes relatively cheaper than A.
  • Budget line pivots outward around A-intercept.
  • Consumer moves along initial indifference curve towards more B and less A.
• Step 2: Income effect (E2 → E3):
  • Lower price of B increases real income.
  • Budget line shifts to a higher level of attainable satisfaction (new indifference curve).
  • For a normal good, consumer buys more B (and possibly more A).
• Price effect (total effect) = substitution effect + income effect:
  • E1 → E3 is the overall change.

Price Rise for a Normal Good (B)

• Step 1: Substitution effect (E1 → E2):
  • Price of B rises; B becomes relatively more expensive.
  • Consumer substitutes away from B towards A.
• Step 2: Income effect (E2 → E3):
  • Higher price of B reduces real income.
  • For a normal good, lower real income reduces demand for both A and B.
• Overall:
  • Quantity demanded of B falls due to both effects working in the same direction.

Inferior and Giffen Goods

• Inferior goods:
  • Negative income effect: when real income rises, demand for the good falls.
  • For a price fall:
    • Substitution effect: still positive (towards the cheaper good).
    • Income effect: negative (consumer uses extra real income to switch away from the inferior good).
    • If substitution effect > income effect → overall demand increases when price falls (typical inferior good).
• Giffen goods:
  • Special type of inferior good.
  • Very strong (positive) income effect from a price rise that dominates the substitution effect.
  • For a price rise:
    • Substitution effect: negative (less of the now more expensive good).
    • Income effect: in some very low-income contexts, higher price of a staple reduces real purchasing power so much that consumers cannot afford better substitutes and must buy more of the cheaper staple (Giffen good).
    • If income effect > substitution effect (in magnitude) → quantity demanded rises when price rises.

Price Effects Summary Table

Limitations of Indifference Curve Analysis

• Behavioural assumptions:
  • Assumes consumers are rational and have consistent, transitive preferences.
  • Real choices may be affected by emotions, habits, social influence.
• Information and measurement:
  • Consumers may not know the exact level of utility they get from each bundle.
• Dimensional limitations:
  • Standard diagrams use only two (or at most three) goods; actual consumption decisions involve many goods.
• Durability:
  • Less suitable for durable goods comparisons (e.g. one-off purchases like a cake pan vs perishable goods like cake).

Efficiency and Market Failure

Types of Economic Efficiency

• Economic efficiency:
  • Scarce resources used in a way that maximises output and matches what consumers most want.
• Main forms:
  • Productive efficiency.
  • Allocative efficiency.
  • Dynamic efficiency.
  • Pareto efficiency (Pareto optimality).

Productive Efficiency

• Definition:
  • Producing goods at the lowest possible cost.
  • Achieved when a firm operates on the production possibility curve (PPC) frontier and at the minimum point on its average cost curve.
• Role of competition:
  • Competitive pressure forces firms to minimise costs to survive.
  • In perfect competition, long-run equilibrium occurs where:
    • Firms produce at the output where MC = price.
    • Price = minimum average cost (AC).
  • Result: firms are productively efficient in the long run.

Allocative Efficiency

• Definition:

  • Allocative efficiency occurs where price (P) equals marginal cost (MC).
  • At this point, the value consumers place on the last unit consumed (reflected in price) equals the cost of the resources used to produce it (MC).

• Example:

  Quantity	1	2	3	4	5
  Price per unit	6	6	6	6	6
  Marginal cost per unit	3	4	5	6	7

  • Allocative efficiency occurs at quantity 4 where P = MC = 6.

• Relationship with PPC:

  • Any point on the PPC frontier can be productively efficient.
  • To be allocatively efficient, the chosen combination must also satisfy P = MC for each good.

• How competition supports allocative efficiency:

  • Firms seek profit, so they tend to produce goods with highest demand.
  • If firms underproduce a high-demand good, price will rise above MC, signalling expansion.
  • In a competitive market, equilibrium tends toward P = MC, where the MC curve intersects the price line.

Pareto Optimality

• Pareto optimality:
  • A resource allocation where no one can be made better off without making someone else worse off.
• If an allocation is not Pareto efficient:
  • There is some reallocation that could make at least one person better off without harming others.
• Policy implications:
  • Improvements in economic efficiency often create winners and losers.
  • Compensation schemes may be needed if reforms make some groups worse off.

Dynamic Efficiency

• Dynamic efficiency:
  • Efficiency achieved over time through innovation, investment, and adapting to changing consumer preferences.
  • Involves reallocating resources so that output grows faster than input usage.
• Characteristics:
  • Firms introduce new production processes and technologies.
  • Often requires long-term investment and can be stimulated by competition.
• Effect on costs:
  • When firms become dynamically efficient, their long-run average cost (LRAC) curve shifts downwards.
  • This allows more output at lower cost in future periods.

Market Failure

• Market failure:
  • Occurs when a free market, left to itself, does not achieve productive and/or allocative efficiency.
  • Market outcomes are not socially optimal; scarce resources are misallocated.
• Causes of market failure:
  • Externalities (positive or negative).
  • Underprovision or overconsumption of merit and demerit goods.
  • Underprovision of public goods and quasi-public goods.
  • Information failure, including:
    • Asymmetric information.
    • Adverse selection.
    • Moral hazard.
  • Abuse of monopoly power (market dominance leading to high prices and low output).

Private and Social Costs and Benefits, Externalities

Social and Private Costs

• Social costs:
  • Total costs borne by society from a particular action.
  • Include both private and external costs.
  • Formula: Social Costs = Private Costs + External Costs.
• Private costs:
  • Costs directly incurred by decision-makers (e.g. firms producing a good, individuals consuming it).
• External costs:
  • Costs imposed on third parties who are not directly involved in the action.

Marginal Social Costs

• Marginal social cost (MSC):
  • The extra cost to society from producing one more unit of a good or carrying out one more unit of an activity.
• Marginal private cost (MPC):
  • The producer’s extra cost of producing one more unit.
• Marginal external cost:
  • The extra cost on third parties from the production of one more unit.
• Relationship:
  • MSC = MPC + Marginal External Cost.

Social and Private Benefits

• Social benefits:
  • Total benefits to society from a particular action.
  • Social Benefits = Private Benefits + External Benefits.
• Private benefits:
  • Benefits gained by producers and consumers directly involved.
• External benefits:
  • Benefits enjoyed by third parties who are not directly involved.
• If social benefits exceed private benefits:
  • Positive externalities exist (society gains more than the decision-maker).

Marginal Social Benefits

• Marginal social benefit (MSB):
  • The marginal satisfaction to consumers and producers plus any external (social and environmental) benefits from one more unit.
• Marginal private benefit (MPB):
  • The marginal benefit to the consumer (or producer) from consuming (or producing) one more unit.
• Marginal external benefit:
  • The extra benefit to others from the consumption or production of one more unit.
• Relationship:
  • MSB = MPB + Marginal External Benefit.

Externalities

• Externalities:
  • Situations where the actions of consumers or producers affect third parties who are not involved in the transaction.
  • Also called “spillover effects”.
• Types:
  • Negative externalities:
    • Negative production externalities (e.g. pollution from manufacturing).
    • Negative consumption externalities (e.g. second-hand smoke).
  • Positive externalities:
    • Positive production externalities (e.g. firm’s R&D benefiting other firms).
    • Positive consumption externalities (e.g. education improving society).
• Impact:
  • Firms and consumers usually consider only private costs and benefits, not social ones.
  • As a result:
    • Goods with negative externalities tend to be overproduced/overconsumed.
    • Goods with positive externalities tend to be underproduced/underconsumed.
  • Market outcome differs from the socially optimal outcome.

Deadweight Welfare Loss and Externalities Diagrams

Negative Production Externalities

• Without intervention:
  • Firms produce where MPC (their supply) = demand.
  • Market equilibrium at quantity Q and price P.
• Social optimum:
  • Takes into account external costs → MSC lies above MPC.
  • Socially efficient output is lower at Q*, with higher price P* (reflecting true social cost).
• Overproduction:
  • Q > Q* means too much of the good is produced.
  • External costs are not internalised.
• Deadweight welfare loss:
  • The triangular area between MSC and MPC over the range Q* to Q, under the demand curve.
  • Represents net loss of social welfare due to overproduction.

Positive Consumption Externalities

• Without intervention:

  • Consumers base decisions on MPB (their private demand), ignoring external benefits.
  • Market equilibrium at quantity Q and price P.

• Social optimum:

  • MSB > MPB due to positive external benefits.
  • Socially efficient output is higher at Q*, with price P*.

• Underproduction:

  • Q < Q* means too little of the good is produced and consumed.

• Deadweight welfare loss:

  • The triangle between MSB and MPB over the range Q to Q*, above the supply (MPC) curve.
  • Represents lost potential welfare from underconsumption.

• In both negative and positive externality cases:

  • Government intervention (e.g. taxes, subsidies, regulation) may be required to move the market toward Q* and reduce deadweight loss.

Short-Run Production and Cost

Short-Run Production Function

• Short run:
  • A period during which at least one factor of production is fixed.
  • Not defined by calendar time but by the presence of fixed factors.
• Short-run production function:
  • Shows the relationship between the quantity of output and the quantity of one variable factor, holding other factors constant.
• General production function:
  • ( Q = A F(K, L) )
    • Q: total output.
    • A: level of technology.
    • K: capital input.
    • L: labour input.

Measures of Production

• Total product (TP):
  • Total output produced by a firm in a given period with given inputs.
  • TP = Average Product × Labour.
• Average product (AP):
  • Output per unit of the variable factor (e.g. labour).
  • ( AP = \frac{TP}{\text{Labour}} ).
• Marginal product (MP):
  • Extra output from employing one more unit of a variable input, holding other factors fixed.
  • ( MP = \frac{\Delta Q}{\Delta \text{Input}} ).

Law of Diminishing Returns

• Law of diminishing returns (law of variable proportions):
  • When increasing amounts of a variable factor (e.g. labour) are added to a fixed factor (e.g. capital), beyond some point:
    • The marginal product of the variable factor will begin to decline.
  • Initially:
    • MP may rise due to better specialisation and coordination.
  • Later:
    • Overcrowding and overuse of fixed factors reduce additional output per extra unit of input.

Short-Run Cost Function

• Types of costs in the short run:

  • Fixed costs (FC):
    • Do not vary with output (e.g. rent, certain salaries).
    • Represented as a horizontal line when plotted against output.
  • Variable costs (VC):
    • Vary directly with output (e.g. raw materials, some labour).
    • Curve upward as output increases (due to diminishing returns).
  • Total cost (TC):
    • TC = FC + VC.
    • Starts at the level of fixed costs and follows the shape of VC as output rises.

• Average and marginal cost measures:

  • Average fixed cost (AFC):
    • ( AFC = \frac{FC}{Q} ).
    • Falls as output increases (spreading overheads).
  • Average variable cost (AVC):
    • ( AVC = \frac{VC}{Q} ).
  • Average total cost (ATC) or average cost (AC):
    • ( ATC = \frac{TC}{Q} ).
    • ATC = AFC + AVC.
  • Marginal cost (MC):
    • ( MC = \frac{\Delta TC}{\Delta Q} ).
    • Initially may fall, then rises due to diminishing marginal product.

Isoquants and Short-Run Optimum Output

• Isoquant:
  • Curve showing all combinations of two inputs (e.g. labour and capital) that produce a given level of output.
  • Analogous to an indifference curve, but for production:
    • Each point on an isoquant indicates the same total physical product.
• Optimum output in the short run:
  • Defined as output produced at the lowest possible unit cost.
  • Occurs at the point where MC intersects ATC at its minimum.
• Shape of short-run average cost curve (SRAC):
  • U-shaped because:
    • At low output:
      • Average cost falls as fixed costs are spread and inputs are used more efficiently (increasing returns).
    • At higher output:
      • Diminishing returns to the variable factor cause MC to rise, pulling ATC upwards.
  • The minimum point of SRAC indicates the most efficient scale of production in the short run.

Long-Run Production and Cost

Long-Run Production Function

• Long run:
  • All factors of production are variable.
  • Firms can adjust all inputs and scale of operation to find the most efficient combination.
• Returns to scale:
  • Increasing returns to scale:
    • Output increases proportionately more than the increase in all inputs.
    • Associated with falling long-run average costs (economies of scale).
  • Decreasing returns to scale:
    • Inputs increase proportionately more than output.
    • Associated with rising long-run average costs (diseconomies of scale).

Long-Run Average Cost (LRAC) and Minimum Efficient Scale

• LRAC curve:
  • Shows the lowest possible average cost of producing each output level when all inputs are variable.
  • Typically U-shaped or L-shaped:
    • Falling section: economies of scale dominate.
    • Flat or minimum point: lowest attainable average cost.
    • Rising section: diseconomies of scale appear as firm becomes too large.
• Minimum efficient scale (MES):
  • Smallest output level at which long-run average cost is minimised.
  • Below MES:
    • Firms have higher average costs due to not fully exploiting economies of scale.
  • Above MES:
    • No further cost advantage from increasing size; diseconomies may arise.
  • Market implications:
    • Low MES relative to market size:
      • Many firms can efficiently operate; market tends to be competitive or fragmented.
    • High MES relative to market size:
      • Only a few large firms (or a single firm) can operate efficiently, potentially leading to a natural monopoly.

LRAC as an Envelope Curve

• Relationship between SRAC and LRAC:
  • For each plant size or fixed-factor combination, there is a corresponding SRAC curve.
  • LRAC is the “envelope curve” that touches the minimum points of an infinite set of SRAC curves.
• In the long run:
  • A firm can switch between different SRAC curves by changing plant size and input mix.
  • The LRAC curve traces the lowest cost path available to the firm as it scales output.

Types of Costs, Revenue and Profit

Types of Costs (Summary)

• Fixed costs:
  • Do not vary with output in the short run.
  • Examples: rent, long-term salaries, insurance.
• Variable costs:
  • Vary directly with output.
  • Examples: raw materials, piece-rate labour, fuel.
• Total cost:
  • Sum of fixed and variable costs at any level of output.
• Average costs:
  • Fixed, variable, and total average costs defined per unit of output.
• Marginal cost:
  • Additional cost of producing one more unit of output.
• Long-run cost:
  • All costs are variable; LRAC reflects economies and diseconomies of scale.

Revenue

• Total revenue (TR):
  • TR = price × quantity sold.
• Average revenue (AR):
  • ( AR = \frac{TR}{Q} ).
  • For a single-price firm, AR is equal to price.
  • AR curve is the firm’s demand curve.
• Marginal revenue (MR):
  • ( MR = \frac{\Delta TR}{\Delta Q} ).
  • Additional revenue from selling one more unit.
• Relationship under a downward-sloping demand curve:
  • To sell more output, the firm must lower price.
  • This causes MR to lie below AR because:
    • The price reduction applies not only to the extra unit but also to previous units.

Types of Profit

• Normal profit:
  • Minimum reward required to keep an entrepreneur in the current line of business.
  • Treated as a cost of production (opportunity cost of the owner’s resources).
• Subnormal profit:
  • Profit below normal profit (actual profit < opportunity cost).
  • Occurs when price < average cost (P < AC).
  • If persistent, firms may exit the industry.
• Supernormal (abnormal) profit:
  • Profit in excess of normal profit.
  • Occurs when total revenue > total cost (TR > TC).
  • Often arises in the short run or under imperfect competition (e.g. monopoly, oligopoly).
• In competitive markets:
  • Short run:
    • Firms may earn abnormal profits or losses.
  • Long run:
    • Entry and exit push economic profit towards normal profit.

These reorganised notes cover marginal utility and consumer choice, efficiency and market failure, private and social costs and benefits, types of costs, revenue and profit, and short-run and long-run production in a concise and structured form.*