📦 What Is Supply?
Definition: Supply is the quantity of a good or service that producers are willing and able to sell at each possible price, over a given time period.
The basics
Just like demand, supply has two requirements: producers must be willing (it is profitable enough) and able (they have the resources) to produce and sell.
- Willingness to sell — it is profitable enough to be worth producing
- Ability to sell — the firm has the resources, labour, and technology
- At a specific price — supply always relates to a price level
- Over a time period — per day, per week, per year
Why do producers supply?
The main motivation is profit. Higher prices generally mean higher profit per unit, which gives firms a stronger incentive to produce and sell.
Think of supply as the producer's side of the story. Demand asks 'how much will people buy?' and supply asks 'how much will firms sell?' Together, they determine the market outcome.
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📈 The Supply Curve
What it shows: A supply curve is a graph showing how much of a good producers are willing to sell at every possible price. It slopes upward from left to right.
Interactive tool
Use this interactive learning tool in study mode.
How to draw a supply curve
- Price (P) goes on the vertical (Y) axis
- Quantity supplied (Q) goes on the horizontal (X) axis
- The curve slopes UPWARD from left to right
- Label the curve 'S' (or 'S₁' if you will show a shift later)
- Always add a title like 'Market for wheat'
Demand slopes DOWN ↘️, Supply slopes UP ↗️. An easy way to remember: the S in Supply looks like a curve going upward.
Reading the curve
At a high price, the quantity supplied is high (top-right of the curve). At a low price, the quantity supplied is low (bottom-left). This is the positive relationship between price and quantity supplied — they move in the same direction.
On your exam diagram, the supply curve should start from the left (near the origin) and go up to the right. If you draw it the other way around, you have a demand curve!
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🤔 Why Does Supply Slope Upward?
The upward slope comes down to one thing: profit. When the price of a good rises, producing it becomes more profitable.
The profit motive
- Higher prices → higher revenue per unit → more profit → firms want to produce more
- Existing firms increase their output to earn more profit
- New firms may enter the market because it is now profitable enough
- Firms that had high costs can now cover them and start producing
The law of increasing opportunity cost
As firms produce more, they often face rising costs — they need to hire less efficient workers, use less ideal resources, or pay overtime. This means they need a higher price to justify each additional unit of output.
The Law of Supply: As the price of a good rises, the quantity supplied rises — and as the price falls, the quantity supplied falls — ceteris paribus.
Supply slopes up because of the profit incentive: higher price → more profitable → firms supply more. Simple as that. 💰