Q.Explain the Law of Demand with the help of a demand schedule and a demand curve. Why does the demand curve slope downward from left to right?

This question combines the statement of the law, its tabular and graphical illustration, and the underlying economic reasons for its characteristic shape.

The Law of Demand, most closely associated with Alfred Marshall, states that other things remaining constant, the quantity demanded of a commodity varies inversely with its price — demand rises as price falls and falls as price rises.

Demand schedule. This inverse relationship can be shown in a table. Consider a hypothetical individual demand schedule for mangoes:

As price falls steadily from Rs 100 to Rs 20 per kg, quantity demanded rises steadily from 2 kg to 10 kg per week.

Demand curve. Plotting each price-quantity pair from the schedule, with price on the Y-axis and quantity demanded on the X-axis, and joining the points gives the demand curve, which slopes downward from left to right.

Why the curve slopes downward. Five reasons, working together, explain the negative slope. First, the law of diminishing marginal utility — successive units give less additional satisfaction, so a buyer purchases more only if price falls to justify the lower marginal utility of the next unit. Second, the income effect — a price fall raises the buyer's real income (purchasing power), part of which is spent on buying more of the now-cheaper good. Third, the substitution effect — a fall in a good's price, with substitute prices unchanged, makes it relatively cheaper, so buyers substitute toward it and away from its substitutes. Fourth, the new consumers (extension of the market) effect — buyers previously priced out of the market can now afford the good as its price falls, adding their demand to the total. Fifth, the different uses effect — a commodity with multiple uses (electricity, water) is extended to progressively less urgent uses as its price falls, raising the total quantity a single buyer demands.

Students often state the law and draw the curve correctly but give only one or two reasons (usually just diminishing marginal utility) for the downward slope, missing the income effect, substitution effect, new-consumers effect and different-uses effect that a full-mark long answer is expected to cover.