Drawing and interpretation of diagrams that show how changes in output affect costs of production

Microeconomic Decision‑Makers: Firms’ Costs, Revenue and Objectives

1. Understanding Costs

Costs are the prices firms pay to produce goods. They come in two main types: Fixed Costs (FC) and Variable Costs (VC).

Fixed costs stay the same no matter how much you produce. Think of a bakery’s oven rental – you pay the same amount whether you bake 10 or 100 cupcakes. Variable costs change with output: flour, sugar, and the baker’s wages.

The total cost of production is therefore:

$TC = FC + VC$

From TC we can derive two important averages:

• $AC = \frac{TC}{Q}$ – the cost per unit.
• $MC = \frac{\Delta TC}{\Delta Q}$ – the cost of producing one more unit.

2. Visualising Cost Curves

Imagine a graph where the horizontal axis is output (Q) and the vertical axis is cost. The shapes of the curves tell a story:

• U‑shaped AC curve – At first, as you produce more, the average cost falls because fixed costs are spread over more units. After a point, it rises because of diminishing returns.
• MC curve – Starts below the AC curve, cuts it at the lowest point, and then rises.

📈 Analogy: Think of a pizza shop. The first few pizzas are cheap because you’re using the oven efficiently. But when you start making too many, you need extra ovens or overtime, so each pizza costs more.

3. Revenue Basics

Revenue is the money a firm receives from selling its product.

$TR = P \times Q$ where $P$ is the price per unit.

Average revenue (AR) is simply the price: $AR = \frac{TR}{Q} = P$.

Marginal revenue (MR) is the extra revenue from selling one more unit: $MR = \frac{\Delta TR}{\Delta Q}$.

4. Profit Maximisation

Profit ($\pi$) is the difference between total revenue and total cost:

$\pi = TR - TC$

Firms maximise profit where MR equals MC:

$MR = MC$

If MR > MC, producing an extra unit adds more revenue than cost, so profit increases. If MR < MC, the opposite is true.

5. Example: A Small Café

Let’s look at a simple numerical example. The café has:

• Fixed cost (FC) = £500 per month (rent, equipment).
• Variable cost per cup of coffee = £0.50.
• Price per cup (P) = £2.00.

We can calculate costs and revenue for different output levels (Q = 100, 200, 300 cups).

Notice how the café is still losing money at 300 cups. To break even, the café needs to raise the price or reduce costs.

6. Drawing the Cost Curves (Text Version)

Below is a textual sketch of the key curves for the café. Imagine the x‑axis as cups of coffee and the y‑axis as £.

   £
   |
   |          MC
   |          /\
   |         /  \
   |        /    \
   |-------/------\-------- Q
   |      /        \
   |     /          \
   |    /            \
   |   /              \
   |  /                \
   | /                  \
   |/                    \
   +------------------------------ 

The MC curve starts below the AC curve, meets it at the lowest point of AC, and then rises. The AC curve is U‑shaped.

7. Key Take‑aways

1. Fixed costs do not change with output; variable costs do.
2. Average cost (AC) is total cost divided by quantity.
3. Marginal cost (MC) is the cost of producing one more unit.
4. Profit is maximised where MR = MC.
5. Understanding the shapes of cost curves helps firms decide how much to produce.

💡 Remember: Think of cost curves like a roller‑coaster: the first dip (low AC) is exciting, but if you go too far, the ride gets steep (high MC).