The law of supply follows the next unit

A higher output price makes additional high-cost units worthwhile.

A supply schedule lists quantities sellers plan to offer at possible prices. Holding technology, input prices, expectations, and other determinants constant, a higher price usually raises quantity supplied. The response is movement along a fixed supply curve.

Supply requires willingness and ability to sell during a stated period. Inventory that an owner refuses to offer at any relevant price is not part of current quantity supplied. Actual sales are also not the supply curve. Sales occur where supply meets demand. A farmer may be willing to sell 1,000 crates at $30, yet only 800 are purchased if demand at that price is 800.

The law of supply is a ceteris paribus relationship between a good’s own price and quantity supplied. A higher price raises the marginal revenue from an additional unit and makes units with higher opportunity cost worthwhile. A lower price removes some of those units from the profitable plan. The curve summarizes the resulting quantities at several possible prices.

The upward slope need not mean each existing unit becomes physically harder to make. Industry supply can rise because higher-cost firms enter production or because firms use overtime, less-suitable equipment, or more expensive inputs. The common principle is rising opportunity cost for added output.

In the short run, a competitive firm’s supply comes from the rising portion of marginal cost above average variable cost. Price does not change the firm’s technology. It changes how far along that cost schedule the firm chooses to operate. At a market level, additional firms or capacity can contribute to the upward slope over the relevant horizon.

Own price changes the chosen quantity

A bakery’s marginal costs for successive batches are $18, $24, $31, and $39. At a market price of $32 per batch, producing the first three batches covers each batch’s marginal cost. The fourth does not. If price rises to $41, the fourth becomes worthwhile. The higher price causes a movement to a larger quantity supplied on the existing marginal-cost relationship.

Do not call the higher price a supply increase. At each price, the production conditions are unchanged, so the curve stays fixed. Quantity supplied rises because the selected point changes. By contrast, cheaper flour lowers the marginal cost of batches and causes the bakery to offer more at every possible output price, shifting supply right.

The law of supply can have specialized exceptions, such as a backward-bending individual labor-supply curve at high wages, but an ordinary product-market question uses the upward-sloping relationship unless the stem states otherwise. Apply the model the question builds rather than importing an exception from another market.

A reliable final sentence names both what stayed fixed and what changed: “With production conditions unchanged, the higher product price raises quantity supplied along the existing supply curve.” That wording rules out a technology shift, seller entry, and an input-cost change. It also keeps supply distinct from the quantity actually sold, which demand helps determine.

Quantity supplied is therefore a plan at a price, not a guaranteed sale.

Actual sales require a buyer, so the market intersection-not supply alone-determines the quantity exchanged.