(i) According to Marshall," The excess of price which a person would be willing to pay a thing rather than go without the thing, over that which he actually does pay is the economic measure of this surplus satisfaction".
(ii) Consumer's surplus = potential price - actual price.

Features (or) Characteristics of Human Wants:
(a) Wants are unlimited: Human wants are countless in number and various in kinds. When one want is satisfied another want crops up. Human wants multiply with the growth of civilization and development.
(b) Wants become habits: Wants become habits; for example, when a man starts reading newspaper in the morning, it becomes a habit. Same is the case with drinking tea or chewing pans.
(c) Wants are Satiable: Though we cannot satisfy all our wants, at the same time we can satisfy particular wants at a given time. When one feels hungry, he takes food and that want is satisfied.
(d) Wants are Alternative: There are alternative ways to: satisfy a particular want ego idly, dosa or chappathi.
(e) Wants are Competitive: All our wants are not equally important. So, there is competition among wants. Hence, we have to choose more urgent wants than less urgent wants.
(f) Wants are Complementary: Sometimes, satisfaction of a particular want requires the use of more than one commodity. Example: Car and Petrol, Ink and Pen.
(g) Wants are Recurring: Some wants occur again and again. For example, if we feel hungry, we take food and satisfy our want. But after sometime, we again feel hungry. and want food.

Introduction: A shift in the demand curve occurs with a change in the value of a variable other than its price in the general demand function. Shifts in the Demand Curve: An increase or decrease in demand due to change in conditions of demand is shown by way of shifts in the demand curve.

On the left hand side of the diagram the original demand curve is d₁d₁, the price is OP₁ and the quantity demanded is OQ₁. Due to change in the condition of demand (change in income, taste or change in prices of substitutes or complements) the quantity demanded decreases from OQ₁ to OQ₂. This is shown in the demand curve to the left. The new demand curve is d₁ d₁. This is called decrease in demand.
On the right hand side of the diagram the original price OP₁ and the quantity demanded is OQ₁.
Due to changes in other conditions the quantity purchased has increased to OQ₂. Thus the demand curve shifts to the right d₁d₁. This is called increase in demand.

Utility is defined as the power of commodity or a service to satisfy a human want.
a. Total Utility: Total Utility refers to the sum of utilities of all units of a commodity consumed. For example, if a consumer consumes ten biscuits, then the total utility is the sum of satisfaction of consuming all the ten biscuits.
b. Marginal Utility: Marginal Utility is the addition made to the total utility by consuming one more unit of a commodity. For example, if a consumer consumes 10 biscuits, the marginal utility is the utility derived from the 10th unit. It is nothing but the total utility of 10 biscuits minus the total utility of 9 biscuits.
Thus MU_n = TU_n - TU_n-1
Where
MU_n = Marginal Utility of 'nth' commodity.
TU_n = Total Utility of n units.
TU_n-1 = Total Utility of n-1 units.
c. Relationship between Marginal Utility and Total Utility:

The slope of a linear demand curve is given as \(\frac{\Delta p}{\Delta q}\)
(i) While price elasticity of demand is given as
\(Ed=\frac{\Delta Q}{\Delta P}\times \frac{P}{Q}\)
(ii) Hence, we can write above equation
(iii) (i) as \(Ed=\frac{1}{Slope\ of\ Demand\ Curve}\times \frac{P}{Q}\)
\(Ed=\frac{1}{\frac PQ}\times \frac{P}{Q}\)
(iv) The above equation states that there exist an inverse relationship between slope of a linear demand curve and Price elasticity of demand.