Supply

The quantity of a good or service that producers are willing and able to sell at each possible price, over a given time period. Both willingness (profitable enough) and ability (resources available) are required.

Because of the profit motive: higher prices mean higher revenue per unit and more profit, so firms want to produce more. Existing firms increase output and new firms may enter the market when price rises.

A graph showing how much of a good producers are willing to sell at every possible price. Price (P) on the Y-axis, quantity supplied (Q) on the X-axis. It slopes upward from left to right.

As the price of a good rises, the quantity supplied rises — and as the price falls, the quantity supplied falls — ceteris paribus. There is a positive (direct) relationship between price and quantity supplied.

Price (P) on Y-axis, Quantity (Q) on X-axis. Curve slopes upward from left to right. Label the curve "S" (or "S₁" if showing a shift later). Add a title (e.g., "Market for wheat"). Label everything for full marks.

Profit — the difference between total revenue and total costs. Higher prices generally mean higher profit per unit, giving firms a stronger incentive to produce and sell more.

As firms produce more, they face rising costs — less efficient workers, overtime pay, scarcer raw materials. Each additional unit costs more to produce, so firms need a higher price to justify each extra unit. This explains the upward slope.

Demand is the buyer's side (how much consumers want to buy at each price). Supply is the producer's side (how much firms want to sell at each price). Together, they determine the equilibrium price and quantity in a market.

As price rises, quantity supplied rises; as price falls, quantity supplied falls. Price and quantity move in the same direction. High price = top-right (high Q). Low price = bottom-left (low Q).

When the market price rises, profits increase. This makes the market attractive to firms that previously found it unprofitable. They enter the market, increasing total market supply. Firms with higher costs can now also cover those costs.

Demand slopes DOWN (↘) — higher price, less bought. Supply slopes UP (↗) — higher price, more sold. The "S" in Supply looks like a curve going upward. They slope in opposite directions because buyers and sellers respond to price differently.

Willing: it is profitable enough for the firm to bother producing. Able: the firm has the necessary resources, labour, technology, and capacity to actually produce the good at that price.

Demand slopes down because of the income and substitution effects (lower price → consumers buy more). Supply slopes up because of the profit motive (higher price → producers sell more). Both respond to price, but in opposite directions.

Bottom-left: low price, low quantity supplied (few firms willing to sell at a low price). Top-right: high price, high quantity supplied (more firms willing to sell and existing firms produce more).

Because the quantity firms want to sell changes at different prices and over different time frames. Stating "supply is 1,000 units" is incomplete without specifying at what price and per what period (day, week, year).

Rising costs (higher wages, raw material prices, energy, rent) shift supply left — firms produce less at every price because it is more expensive. Falling costs shift supply right — firms can produce more profitably.

More firms entering the market increases total market supply (shifts right) because total output across all producers rises. Firms leaving the market reduces supply (shifts left).

A change in the good's own price causes a movement along the existing supply curve. A change in any non-price factor (costs, technology, government policy, etc.) shifts the entire curve to a new position.

Costs of production (wages, raw materials, energy, rent), technology and innovation, government policy (taxes, subsidies, regulations), number of firms in the market, expectations about future prices, and weather/natural conditions.

Better technology allows firms to produce the same quantity at lower cost, or more quantity at the same cost. Technology improvements almost always shift supply right. Examples: automation, better machinery, digital efficiency tools.

Good weather → bumper harvest → supply shifts right. Bad weather (drought, floods, disease) → poor harvest → supply shifts left. Weather is a major supply factor for agriculture and natural resources.

Indirect taxes (on tobacco, alcohol, etc.) raise production costs → supply shifts left. Subsidies (for renewable energy, farming, etc.) reduce production costs → supply shifts right. Both are government policy tools.

If firms expect the price to rise in the future, they may hold back supply today (shift left now) to sell later at the higher price. This is common in commodity markets like oil, where producers can store output.

An increase in supply — at every price, producers now supply more. The whole curve moves right (S₁ → S₂). Usually caused by lower costs, better technology, subsidies, or more firms entering the market.

A decrease in supply — at every price, producers now supply less. The whole curve moves left (S₁ → S₃). Usually caused by higher costs, new taxes, stricter regulations, or firms exiting the market.

A drought reduces the wheat harvest — supply of wheat shifts left, raising the price of wheat. This is a supply-side change: the cost of producing has not changed, but the physical ability to produce has been reduced by the weather.

Regulations (e.g., pollution standards, safety requirements) raise costs for firms → supply shifts left. Deregulation removes cost burdens and restrictions → supply shifts right. Regulation is a trade-off between protection and efficiency.

When fuel or electricity costs increase, production becomes more expensive across many industries. If oil prices double, transport and manufacturing costs rise, shifting supply left in industries from food to electronics — less is produced at every price.

Anything that makes production cheaper shifts supply right (firms can produce more at every price). Anything that makes production more expensive shifts supply left (firms produce less at every price).

Because saying "supply decreased" is incomplete. You must identify the cause: "rising fuel costs increased production costs, shifting supply left." Naming the specific determinant and explaining the mechanism earns full marks.

A change in any non-price factor: costs of production, technology, government policy (taxes, subsidies, regulations), number of firms, expectations, or weather. The entire curve moves to a new position.

A change in the price of the good itself. When price rises, you move up and right along the curve (Qs rises). When price falls, you move down and left (Qs falls). The curve stays in the same position.

Demand-side factors: income, tastes, related goods, population, expectations → shift demand. Supply-side factors: costs, technology, government policy, weather, number of firms → shift supply. Identify the cause, then shift the correct curve.

A "change in supply" (or "increase/decrease in supply"). This means the whole curve has shifted to a new position — at every price, the quantity supplied is now different.

A "change in quantity supplied" (not a "change in supply"). "Change in supply" means the whole curve shifted. This terminology distinction matters for IB exam marks — examiners check for it.

The demand curve shifted right. When demand increases (shifts right), there is upward pressure on both price and quantity at the new equilibrium. P↑ Q↑ = demand shifted right.

The supply curve shifted left. When supply decreases (shifts left), less is available so price rises but quantity falls. P↑ Q↓ = supply shifted left. Price and quantity moving in opposite directions signals a supply shift.

Supply: price rises → move up and RIGHT (Qs rises). Demand: price rises → move up and LEFT (Qd falls). They move in opposite horizontal directions because buyers and sellers respond to price changes differently.

Right shift: a new technological advance (e.g., automation) lowers production costs, increasing supply. Left shift: a new government regulation (e.g., stricter environmental rules) raises production costs, decreasing supply.

Draw the original supply curve S₁. Then draw S₂ to the right (increase) or left (decrease). Add an arrow showing the direction. Label both curves clearly and mark the new equilibrium if applicable.

There is a movement UP and to the RIGHT along the existing supply curve. Quantity supplied rises because the higher price makes production more profitable. The curve itself does not shift.

Price and quantity move in the SAME direction → demand shifted (P↑Q↑ = D right, P↓Q↓ = D left). Price and quantity in OPPOSITE directions → supply shifted (P↓Q↑ = S right, P↑Q↓ = S left).

It increases. A rightward shift means at every given price, producers now supply a larger quantity. The entire price-quantity relationship has changed — the same price now corresponds to more output.

Because only the good's own price changed — all other factors (costs, technology, government policy) remained the same. The curve represents the relationship at all prices; a price change selects a different point on the same curve.

Step 1: Is the cause a demand or supply factor? Step 2: Does it shift the curve right or left? Step 3: Draw, label axes, both curves (original and shifted), mark old and new equilibrium, and explain the outcome. This keeps your answer accurate under pressure.