A-Level Economics · ⁨A-Level 경제학⁩

A free market often gives a good result, but not always. market failure 市场失灵 is when the market does not allocate 配置 (share out) resources efficiently — too much or too little of a good is made. Then the government may step in.

There are several reasons for market failure.

Externalities

An externality 外部性 is a cost or a benefit that falls on a third party 第三方 — someone who is not the buyer or the seller.

• a negative externality 负外部性 is an external cost 外部成本 paid by others, like the pollution from a factory. The social cost 社会成本 (the cost to all of society) is bigger than the private cost 私人成本 (the cost to the producer). When people ignore the external cost, the market makes too much of the good.
• a positive externality 正外部性 is an external benefit 外部收益 enjoyed by others, like the protection that vaccination gives to everyone. Here the market makes too little, because buyers ignore the benefit to others.

Public goods

A public good 公共物品 (like street lighting or national defence) has non-rivalry 非竞争性 and non-excludability 非排他性. Because non-payers cannot be shut out, there is a free-rider problem 搭便车问题, so private firms make no profit and supply none. The market fails completely.

Merit and demerit goods

A merit good 优值品 (like education) is under-consumed; a demerit good 劣值品 (like cigarettes) is over-consumed. People misjudge them because of information failure 信息失灵 — they do not know, or do not use, the true benefit or harm.

Monopoly power

monopoly 垄断 means one firm dominates a market. A firm with monopoly power 垄断势力 can cut output and raise the price above the competitive level. This wastes resources and harms consumers.

Inequality and asymmetric information

• the market rewards people who own resources, so it can produce great inequality 不平等 — some people are left with too little to live on.
• asymmetric information 信息不对称 is when one side of a deal knows more than the other (a used-car seller knows the faults; the buyer does not). This leads to poor decisions and unfair deals.

Indirect taxes

An indirect tax 间接税 is a tax on spending. It raises a firm's costs, shifts the supply curve to the left, and so raises the price and cuts the quantity. Governments tax demerit goods (like tobacco) and goods with negative externalities to make people use less.

The incidence of a tax 税收归宿 is who really pays it. This depends on elasticity:

• if demand is inelastic 缺乏弹性, consumers pay most of the tax (the price rises by nearly the full tax).
• if demand is elastic 富有弹性, producers pay most of the tax (they cannot pass it on).

Subsidies

A subsidy 补贴 is a government payment to producers. It lowers their costs, shifts the supply curve to the right, and so lowers the price and raises the quantity. Subsidies are used for merit goods and goods with positive externalities.

Maximum and minimum prices

• a maximum price 最高限价 is a legal top price, set below equilibrium (for example, rent control). Because the price is held down, quantity demanded is greater than quantity supplied, so there is a shortage 短缺.
• a minimum price 最低限价 is a legal bottom price, set above equilibrium (for example, a minimum wage, or a floor for farm prices). Because the price is held up, quantity supplied is greater than quantity demanded, so there is excess supply 超额供给.

Buffer stocks

A buffer stock 缓冲库存 is used to steady the price of farm goods. The government buys and stores the good when the price is low, and sells from the store when the price is high. This keeps the price inside a target band.

Regulation, provision and permits

• regulation 管制 means rules and laws — bans, age limits, safety standards, or pollution limits.
• state provision 政府提供 means the government 政府 supplies a good directly, often free at the point of use (public goods and merit goods like schools and hospitals).
• tradable permits 可交易许可证 set a total cap on pollution and give firms permits to pollute. Firms that pollute less can sell spare permits to firms that pollute more. This caps total pollution at the lowest cost.

Government failure

Intervention does not always help. government failure 政府失灵 is when the government's action makes the use of resources worse, not better. Common causes:

• poor information, so the government sets the wrong tax or subsidy.
• high cost of running the policy.
• unintended results — for example, a maximum price can create an illegal black market 黑市.
• decisions made for political reasons rather than for efficiency.

Worked example. A government puts a £2 per-packet tax on cigarettes, whose demand is very price-inelastic. Who really pays it, and what does that mean for the policy's two aims? The incidence falls mainly on whichever side is less able to respond - the more inelastic one. Because smokers keep buying almost the same quantity as the price rises, producers can pass most of the tax on, so consumers bear most of the incidence: the price might rise by about £1.80, with producers absorbing only about £0.20. That creates the tension the question is really testing. As a revenue raiser the tax succeeds: quantity barely falls, so revenue is large and steady. As a way of cutting consumption it largely fails - for exactly the same reason. And because poorer smokers spend a larger share of their income on it, the tax is regressive. Argue from elasticity to incidence, then judge each aim separately.

It helps to separate two ideas:

• income 收入 is a flow 流量 — money received over a period, such as wages, rent and interest.
• wealth 财富 is a stock 存量 — the value of assets owned at a point in time, such as houses, shares and savings.

Wealth produces income (savings earn interest), and income can be saved to build wealth, so the two are linked. Inequality has many causes: differences in skills and wages, the ownership of wealth, inheritance, and unemployment.

Policies to reduce inequality

• a progressive tax 累进税 takes a larger percentage 百分比 from higher incomes than from lower incomes. This narrows the gap after tax.
• benefits 福利金 (also called transfer payments 转移支付) are money paid by the government to the poor, the old, and the unemployed.
• state provision of free services, like education and health care, raises the real living standards of poorer people.

Together these policies bring about redistribution 再分配 — moving income and wealth from richer to poorer people. But there is a trade-off 取舍: very high taxes and large benefits may weaken the incentive 激励 to work and to invest.

• Define market failure as a misallocation of resources, then name the type (externality, public good, information gap).
• On an externality diagram, mark the divergence of private and social cost/benefit and shade the welfare loss.
• Evaluate each intervention (tax, subsidy, regulation, tradable permits) with one advantage and one drawback.
• Show that tax incidence depends on elasticity — the more inelastic side bears more of the tax.

To judge how an economy is doing, we measure its national income 国民收入 — the total value of everything it produces in a year.

• gross domestic product 国内生产总值 (GDP) is the value of all goods and services made inside a country in a year.
• gross national income 国民总收入 (GNI) is GDP plus the income a country's people and firms earn abroad, minus income foreigners earn inside it.

Two important differences:

• nominal 名义 values are measured at current prices. real 实际 values are adjusted to remove the effect of rising prices, so they show the true change in output. Always compare real values over time.
• total values cover the whole country. per capita 人均 values are divided by the population, so they show the average per person.

Uses and limitations

National income is used to measure the standard of living 生活水平 and to compare countries. But it has limits:

• it ignores the distribution 分配 of income — a high average can hide great inequality.
• it leaves out unpaid work (housework) and the informal economy 非正规经济 (hidden, untaxed activity).
• it says nothing about pollution, leisure time, or health.

So a higher GDP per capita usually means a better life, but not always.

The circular flow of income 收入循环流 shows money moving around the economy. Households 家庭 supply factors of production to firms 企业 and receive income (wages, rent, interest, profit). They spend that income on goods and services, and the money flows back to firms.

Money also leaves and enters this flow:

• leakages 漏出 take money out of the flow: saving 储蓄, taxation 税收, and spending on imports 进口.
• injections 注入 put money into the flow: investment 投资, government spending 政府支出, and exports 出口.

The flow is in equilibrium 均衡 when total injections equal total leakages. If injections are greater, the economy grows; if leakages are greater, it shrinks.

A closed economy 封闭经济 has no trade with other countries. An open economy 开放经济 trades, so it has exports and imports.

Aggregate demand

aggregate demand 总需求 (AD) is the total spending on a country's goods and services at each price level. It has four parts:

• C is consumption 消费 — spending by households.
• I is investment — spending by firms on capital.
• G is government spending.
• (X − M) is net exports 净出口 — exports minus imports.

The AD curve slopes downward: a lower price level means higher real spending, so a greater real output is demanded. AD shifts when any of its four parts changes (for example, a tax cut raises C).

Aggregate supply

aggregate supply 总供给 (AS) is the total output firms are willing to make at each price level.

• short-run aggregate supply 短期总供给 (SRAS) slopes upward. In the short run, higher prices make production more profitable, so firms supply more. SRAS shifts when costs change (wages, raw materials, taxes).
• long-run aggregate supply 长期总供给 (LRAS) shows the economy's full productive capacity 生产能力. It shifts only when the quantity or quality of factors changes.

Macroeconomic equilibrium

macroeconomic equilibrium 宏观经济均衡 is where AD crosses AS. It sets the price level 价格水平 and the real output 实际产出 of the economy. A shift in AD or AS changes both.

economic growth 经济增长 is a rise in a country's real output (real GDP) over time.

• actual growth 实际增长 is the rise in output that really happens (a movement to a point further out, or from inside the LRAS towards it).
• potential growth 潜在增长 is the rise in the economy's capacity (the LRAS shifting outward).

Growth is caused by more or better factors of production: a bigger or more skilled workforce, more investment in capital, new technology, and higher productivity 生产率 (output per worker).

The business cycle

Real output does not grow smoothly. The business cycle 经济周期 (also called the trade cycle) is the up-and-down pattern of output over the years. It has four phases: boom 繁荣 (fast growth), recession 衰退 (falling output), slump 萧条 (the low point), and recovery 复苏.

Costs and benefits of growth

unemployment 失业 means people who are able and willing to work, and are looking for a job, but cannot find one.

Measuring unemployment

• the claimant count 申领救济人数 counts only people claiming unemployment benefits. It is cheap but misses those who do not claim.
• the labour force survey 劳动力调查 asks a large sample whether they are working or seeking work. It follows an international standard, so it is better for comparing countries. The labour force 劳动力 is everyone working or looking for work.

Types and causes

• frictional unemployment 摩擦性失业 — short-term, while people move between jobs.
• structural unemployment 结构性失业 — when an industry declines and workers' skills no longer match the jobs available.
• cyclical unemployment 周期性失业 — caused by low AD in a recession. This is usually the largest type.
• seasonal unemployment 季节性失业 — when work is only available at certain times of the year (such as farming or tourism).

Consequences

Unemployment wastes output (the economy produces inside its PPC), lowers incomes, and costs the government (more benefits, less tax). It also harms families and can raise crime and ill health.

Inflation, deflation and disinflation

• inflation 通货膨胀 is a sustained rise in the general price level. Money then buys less, so its purchasing power 购买力 falls.
• deflation 通货紧缩 is a sustained fall in the general price level.
• disinflation 反通货膨胀 is when inflation is still positive but is slowing down (prices rise, but more slowly).

The price level is measured with the consumer price index 消费者价格指数 (CPI). A typical household's spending is used to build a "basket" of goods. Each good is given a weight 权重 for how much is spent on it, and the change in the basket's cost from year to year is the inflation rate:

Worked example. A country's CPI rises from $100$ to $104$ over one year, then to $107$ the year after. Find the inflation rate in each year.

Year 1: $\dfrac{104 - 100}{100} \times 100\% = 4\%$. Year 2: $\dfrac{107 - 104}{104} \times 100\% = 2.9\%$.

Notice that prices are still rising in year 2, just more slowly — this fall in the inflation rate (from $4\%$ to $2.9\%$) is disinflation, not deflation.

Worked example. A worker's wage rises from $\text{\pounds} 500$ to $\text{\pounds} 520$ a week (a $4\%$ rise) while the CPI rises by $4\%$. What happens to the real wage?

Money (nominal) pay is up $4\%$, but prices are also up $4\%$, so the real wage — what the pay can actually buy — is unchanged. A pay rise only raises living standards if it beats inflation.

Causes of inflation

• demand-pull inflation 需求拉动型通货膨胀 — AD rises faster than AS, so "too much money chases too few goods".
• cost-push inflation 成本推动型通货膨胀 — firms' costs rise (wages or raw materials), so they raise prices, shifting SRAS to the left.

Consequences

Inflation lowers purchasing power, especially for people on fixed incomes. It creates uncertainty, can make a country's exports dearer, and helps borrowers but harms savers. Deflation sounds good but can be harmful: people delay buying (waiting for lower prices), so AD falls, firms cut output, and unemployment rises.

• Distinguish real vs nominal (real removes inflation) and calculate an inflation rate from a price index.
• Explain AD $= C + I + G + (X - M)$ and what shifts each part; distinguish an AD shift from an AS shift.
• Distinguish the types of unemployment (frictional, structural, cyclical) and how each is tackled.
• Distinguish demand-pull vs cost-push inflation on an AD/AS diagram.

A government usually has five main aims:

• price stability 物价稳定 — low and steady inflation 通货膨胀 (often a target of about 2% a year).
• full employment 充分就业 — jobs for almost everyone who wants to work.
• economic growth 经济增长 — a rising real output over time.
• balance of payments 国际收支 stability — not importing far more than the country exports for years on end.
• redistribution of income 收入再分配 — narrowing the gap between rich and poor.

Conflicts between objectives

The aims can conflict 冲突 — reaching one can hurt another:

• growth versus inflation: fast growth raises aggregate demand 总需求, which can pull prices up.
• growth versus the balance of payments: as incomes rise, people buy more imports, worsening the trade balance.
• unemployment versus inflation: cutting unemployment by raising demand can cause more inflation.
• growth versus the environment: more output often means more pollution.

The government has three sets of tools. Fiscal and monetary policy are demand-side policies 需求侧政策 (they work on AD). Supply-side policy works on aggregate supply 总供给.

fiscal policy 财政政策 is the use of government spending and taxation to influence the economy.

The budget 预算 compares the two:

• a budget deficit 预算赤字 is when spending is greater than tax revenue. The government must borrow, adding to the national debt 国债.
• a budget surplus 预算盈余 is when tax revenue is greater than spending.

Types of tax

• a direct tax 直接税 is paid straight to the government on income or profit (income tax, corporation tax).
• an indirect tax 间接税 is a tax on spending (added to the price of goods).

Taxes are also grouped by how the rate changes with income:

Using fiscal policy

• expansionary 扩张性 fiscal policy raises government spending or cuts taxes. This raises AD, to fight a recession and unemployment.
• contractionary 紧缩性 fiscal policy cuts spending or raises taxes. This lowers AD, to fight inflation.

Effectiveness. Fiscal policy can act fast on AD, but it has problems: time lags (a budget is set once a year), large deficits and debt, and crowding out 挤出效应 — when heavy government borrowing pushes up interest rates and reduces private investment.

monetary policy 货币政策 is run by the central bank 中央银行. Its main tool is the interest rate 利率, but it can also change the money supply 货币供给 or influence the exchange rate 汇率.

The transmission mechanism

The transmission mechanism 传导机制 is how a change in the interest rate reaches the wider economy. If the central bank raises the interest rate:

• borrowing becomes dearer, so households and firms spend and invest less.
• saving is rewarded, so people spend less.
• the currency tends to rise, so exports fall.

All of these lower AD, which slows inflation. A cut in the interest rate does the opposite, raising AD.

Effectiveness. Monetary policy is flexible (the rate can change at any time), but it works with a time lag of a year or more. In a deep slump, even very low rates may not make worried firms and households borrow.

supply-side policy 供给侧政策 tries to raise the economy's productive capacity 生产能力 — to shift long-run aggregate supply to the right. It aims to make markets work better and raise productivity 生产率.

There are two kinds.

Market-based policies let markets work more freely:

• cutting income tax and benefits to strengthen the incentive 激励 to work.
• privatisation 私有化 — selling state firms to private owners, who run them for profit.
• deregulation 放松管制 — removing rules that block competition.

Interventionist policies use the government directly:

• spending on education and training 培训 to raise workers' skills.
• spending on infrastructure 基础设施 — roads, ports and broadband.
• support for research and new technology.

Effectiveness. Supply-side policy is powerful because it can raise growth and lower inflation at the same time, with no trade-off. But it is slow (education takes years to pay off), costly, and some measures, like cutting benefits, can widen inequality.

Worked example. An economy is in a deep recession with a large output gap, and interest rates are already near zero. Which policy should the government use, and why is the other one weak here? Monetary policy is weak: with the base rate already near zero there is almost no room to cut further, and even cheap credit will not make firms invest when they expect no demand. Fiscal policy is stronger: government spending raises aggregate demand directly, without waiting for anyone to choose to borrow, and the multiplier means each pound spent raises national income by more than a pound as it is re-spent. Weigh the costs against it - a bigger budget deficit and rising debt, plus time lags on large projects. Match the policy to the constraint named in the question: here it is the zero lower bound, and that is precisely what makes fiscal policy the answer.

• Match a policy to an objective: fiscal and monetary policy act on demand; supply-side policy raises long-run capacity.
• Explain how a change in interest rates works through AD, and fiscal policy through $G$ and $T$.
• Evaluate policy with time lags, side effects and conflicts between objectives.
• Supply-side measures (education, tax incentives) are long-run and slow to work.

international trade 国际贸易 lets countries buy and sell with each other. Trade lets each country specialise 专业化 in what it makes best, so the world makes more in total.

Absolute and comparative advantage

• a country has an absolute advantage 绝对优势 in a good if it can make more of it with the same resources than another country.
• a country has a comparative advantage 比较优势 in a good if it can make it at a lower opportunity cost 机会成本 than another country.

Comparative advantage is the key idea: a country should make the goods it gives up least to produce, and trade for the rest. Look at this example, where each country uses the same resources:

Country A is better at both (absolute advantage in both). But think about opportunity cost:

• in A, making 1 cloth means giving up 2 wine.
• in B, making 1 cloth means giving up 1 wine.

So B gives up less to make cloth — B has the comparative advantage in cloth. A has it in wine. If A makes only wine and B makes only cloth, and they trade, both can end up with more than before.

Terms of trade

The terms of trade 贸易条件 measure the price of a country's exports compared with the price of its imports. It is worked out as an index:

If export prices rise faster than import prices, the terms of trade "improve" — the country can buy more imports for the same exports.

Worked example. Over a year, a country's export price index rises from $100$ to $110$, while its import price index rises from $100$ to $105$. Find the new terms of trade.

The figure is above $100$, so the terms of trade have improved: each unit of exports now buys about $4.8\%$ more imports than before.

Limits of the theory

The gains are real, but the simple theory assumes no transport costs, no trade barriers, and that costs do not change with output. In the real world these do not fully hold, and specialising in one good is risky if its price falls.

protectionism 贸易保护主义 means a government protecting home producers from foreign competition. The main tools:

• a tariff 关税 — a tax on imports, which raises their price.
• a quota 配额 — a legal limit on the quantity of a good that may be imported.
• a subsidy 补贴 to home producers, so they can sell more cheaply than foreign rivals.
• an embargo 禁运 — a complete ban on trade with a country or in a good.
• administrative barriers 行政壁垒 — extra paperwork and tough standards that make importing hard.

For and against

Arguments for protection include protecting an infant industry 幼稚产业 (a young industry that needs time to grow), saving jobs, and stopping dumping 倾销 (foreign firms selling below cost to destroy local rivals).

Arguments against are strong: protection raises prices for consumers, protects weak firms so they stay inefficient, and can lead to retaliation 报复 — other countries hitting back with their own barriers.

Effects on stakeholders

A tariff affects each stakeholder 利益相关者 differently: home producers gain, the government gains tariff revenue, but consumers lose (higher prices, less choice) and foreign producers lose sales.

The balance of payments 国际收支 is a record of all money flows between one country and the rest of the world. Its most-studied part is the current account 经常账户, which has four parts:

• trade in goods 货物贸易 — exports and imports of physical goods.
• trade in services 服务贸易 — exports and imports of services (tourism, banking).
• primary income 初次收入 — income from work and investment abroad (wages, profit, interest).
• secondary income 二次收入 — transfers with nothing given back, like foreign aid.

A current account deficit 经常账户赤字 means a country buys more from abroad than it sells. A current account surplus 经常账户盈余 is the opposite. A long deficit can mean rising debt to other countries; a large surplus can mean the country saves a lot but consumes little.

An exchange rate 汇率 is the price of one currency in terms of another. There are three main systems.

• a floating exchange rate 浮动汇率 is set freely by the demand for and supply of the currency. If more people want a currency (to buy its exports), its value rises.
• a fixed exchange rate 固定汇率 is held at a set value by the central bank, which buys and sells the currency to keep it there.
• a managed exchange rate 管理浮动汇率 mostly floats, but the central bank steps in now and then to steady it.

Appreciation and depreciation

Under floating rates:

• appreciation 升值 is a rise in the currency's value. Exports become dearer for foreigners, and imports become cheaper.
• depreciation 贬值 is a fall in the currency's value. Exports become cheaper, and imports become dearer.

A depreciation can therefore raise export sales and cut imports, helping the current account — but it can also raise inflation, because imports cost more.

Worked example. Suppose the exchange rate is $\text{\pounds} 1 = \$1.40$.

A UK exporter's $\text{\pounds} 200$ machine costs an American buyer $\text{\pounds} 200 \times 1.40 = \$280$. If the pound then appreciates to $\text{\pounds} 1 = \$1.60$, the same machine costs $\text{\pounds} 200 \times 1.60 = \$320$ — dearer for the American, so fewer are sold. Going the other way, a $\$700$ invoice from a US supplier costs a UK firm $\$700 \div 1.40 = \text{\pounds} 500$.

A government can use three kinds of policy to cut a deficit:

• expenditure-reducing 支出减少 policies lower total demand (higher taxes or interest rates), so people buy fewer imports. But they also slow growth and raise unemployment.
• expenditure-switching 支出转换 policies move spending away from imports towards home goods (a tariff, or a lower exchange rate). But they can cause retaliation or inflation.
• supply-side policies raise the competitiveness 竞争力 of home firms (better skills and technology), so exports rise. These work best in the long run but are slow.

• Explain comparative advantage through opportunity cost (specialise where you give up least) and state its assumptions and limits.
• Evaluate protectionism (tariffs, quotas): it can protect jobs but raises prices and risks retaliation.
• Read the balance of payments (the current account covers trade, income and transfers).
• A depreciation makes exports cheaper and imports dearer, helping the current account only if demand is elastic enough (Marshall-Lerner).

utility 效用 is the satisfaction a consumer gets from a good.

• total utility 总效用 is the satisfaction from all the units consumed.
• marginal utility 边际效用 is the extra satisfaction from one more unit.

The law of diminishing marginal utility 边际效用递减 says each extra unit gives less extra satisfaction. Total utility still rises, but more and more slowly.

The equi-marginal principle

A consumer with a fixed budget gets the most total utility when the last dollar spent on each good gives the same extra utility. This is the equi-marginal principle 等边际原则:

Because marginal utility falls as you buy more, you only buy extra units when the price falls. This is why the demand curve 需求曲线 slopes downward.

Limitations

The approach is hard to use in practice: utility cannot really be measured in numbers, and people do not calculate like this. It also ignores habit and advertising.

An indifference curve 无差异曲线 shows all the combinations of two goods that give a consumer the same total utility. It slopes downward (more of one good means less of the other) and bows in towards the origin.

A budget line 预算线 shows all the combinations the consumer can just afford with their income, at the current prices.

consumer equilibrium 消费者均衡 is the best the consumer can do: the point where the budget line just touches the highest possible indifference curve.

Income and substitution effects

When the price of a good falls, the consumer buys more for two reasons:

• the substitution effect 替代效应 — the good is now cheaper than others, so the consumer switches towards it.
• the income effect 收入效应 — the consumer's money now buys more, so real income has risen.

These effects work differently for different goods:

• for a normal good 正常品, both effects raise the quantity bought.
• for an inferior good 低档品, the income effect works the other way (higher real income lowers demand), but it is usually weaker, so demand still rises when price falls.
• a Giffen good 吉芬品 is a rare, very inferior good where the income effect is so strong that demand actually falls when the price falls. Its demand curve slopes upward.

Economists judge how well resources are used.

• productive efficiency 生产效率 — making goods at the lowest possible cost (using the fewest resources).
• allocative efficiency 配置效率 — making the goods people most want, where price equals marginal cost.
• Pareto optimality 帕累托最优 — a point where no one can be made better off without making someone else worse off.
• dynamic efficiency 动态效率 — improving over time through new technology and better products.

market failure 市场失灵 happens when the free market does not reach an efficient result. The main causes are monopoly power, externalities 外部性, public goods 公共物品, information failure 信息失灵, and factor immobility 要素不流动性 (factors cannot move quickly to where they are needed).

Some costs and benefits fall on people outside the deal. We separate three levels:

• a private cost 私人成本 or private benefit 私人收益 falls on the buyer or seller.
• an external cost 外部成本 or external benefit 外部收益 falls on a third party.
• the social cost 社会成本 is private cost plus external cost; the social benefit 社会收益 is private benefit plus external benefit.

Marginal analysis

We study externalities with marginal analysis 边际分析, using these curves:

• marginal private cost 边际私人成本 (MPC) and marginal social cost 边际社会成本 (MSC).
• marginal private benefit 边际私人收益 (MPB) and marginal social benefit 边际社会收益 (MSB).

The best outcome for society is where MSC = MSB. But the market produces where MPC = MPB. The gap between these creates lost welfare.

• a negative externality 负外部性 (like pollution) means MSC is above MPC. The market makes too much.
• a positive externality 正外部性 (like education) means MSB is above MPB. The market makes too little.

The lost welfare from making too much or too little is the deadweight welfare loss 无谓损失 — the triangle on the diagram between the social and private curves.

Costs

In the short run 短期, at least one factor is fixed; in the long run 长期, all factors can change.

Worked example. Making $10$ units costs £500 in total; making $11$ units costs £540. Find the average cost at $10$ units and the marginal cost of the 11th unit.

Here $MC$ ($\text{\pounds} 40$) is below $AC$ ($\text{\pounds} 50$), so making the 11th unit pulls the average cost down — whenever marginal cost is below average cost, average cost falls.

In the short run, output is limited by the law of diminishing returns 边际报酬递减: as you add more of a variable factor (workers) to a fixed factor (machines), each extra worker eventually adds less output. This makes marginal cost rise.

In the long run, all factors vary, so we look at returns to scale 规模报酬. As a firm grows, it can gain economies of scale 规模经济 (lower average cost from buying in bulk, using big machines, and specialising). If it grows too big, it may suffer diseconomies of scale 规模不经济 (poor communication and control), and average cost rises.

Revenue

Worked example. A firm sells $10$ units at £50 each. To sell an 11th unit it must lower the price to £48 on every unit. Find the total revenue at each output and the marginal revenue of the 11th unit.

The marginal revenue ($\text{\pounds} 28$) is well below the £48 price, because the lower price applies to all $11$ units, not just the extra one.

Profit

Profit is total revenue minus total cost. Economists include the opportunity cost of the owner's resources in cost, so:

• normal profit 正常利润 is just enough profit to keep the firm in the industry. It counts as a cost.
• supernormal profit 超常利润 (also called abnormal profit) is profit above normal.
• subnormal profit 次正常利润 is profit below normal — the firm may leave in the long run.

A firm makes the most profit at the output where $MR = MC$.

Worked example. At its best output a firm makes $100$ units, sells each for £12 (its average revenue), and has an average cost of £9. Find the supernormal profit.

Because average revenue (£12) is above average cost (£9), the firm earns supernormal profit. If instead $AR = AC$, it would make only normal profit; if $AR < AC$, it would make a loss.

A market structure 市场结构 describes a market by the number of firms, the barriers to entry, and the type of product.