Comprehensive Notes on Purchasing Power and the Law of Demand

Definition: Purchasing power is defined as the measure of how much one unit of a specific currency (e.g., a dollar, a rupee, etc.) can buy at a particular point in time.
Contextual Variation: The value of purchasing power is not fixed; it is tied to specific times and economic conditions.

Defining Demand: Demand is the specific quantity of a product or service that people are both willing and able to purchase at a specific price.
Key Distinction: Demand is distinct from a simple desire to own something. For desire to become demand, it must be complemented by the following two factors:
- Willingness: The consumer's readiness to buy the product.
- Ability/Purchasing Power: The consumer's financial capacity to pay for the product.
Influencing Factors: The quantity demanded is dependent on several variables, including:
- Consumer needs and preferences.
- Seasonal changes (e.g., the approach of the mango season).
- Current trends.
- Consumer income levels.

General Behavior: Observations of consumer behavior, such as during mango season, show that when prices are high, consumers buy smaller quantities. Conversely, as prices fall, consumers prefer to buy larger quantities.
The Principle: The Law of Demand highlights an inverse relationship between the price of a product or service ( $P$ ) and its quantity demanded ( $Q$ ).
Inverse Correlation:
- When the price ( $P$ ) of a product rises, the quantity demanded ( $Q$ ) decreases.
- When the price ( $P$ ) of a product falls, the quantity demanded ( $Q$ ) increases.
Variables: In economic diagrams and formulas, $P$ represents Price and $Q$ represents Quantity.

Definition of Individual Demand: This refers to the quantity of a good or service that a single individual consumer intends to buy at various price points, assuming all other factors remain constant (ceteris paribus).
The Srivalli Example: A consumer named Srivalli illustrates how individual demand changes as prices drop during the mango season:
- At the start of the season (high price), the price was $150$ per kg. Srivalli purchased only $1\,\text{kg}$ .
- As mangoes became more available, the price fell to $100$ . Consequently, she bought $2\,\text{kg}$ .
- When the price dropped further to $50$ per kg, her purchase increased to $3\,\text{kg}$ .

The Demand Schedule: This is a tabular representation of the relationship between price and quantity demanded for an individual. For Srivalli, the schedule is as follows:
- Price of mango per kg ( $150$ ): Quantity demanded is $1\,\text{kg}$ .
- Price of mango per kg ( $100$ ): Quantity demanded is $2\,\text{kg}$ .
- Price of mango per kg ( $50$ ): Quantity demanded is $3\,\text{kg}$ .
The Demand Curve: This is the graphical representation of the demand schedule, typically labeled as Figure 9.2 (Individual demand curve).
- Axes: The vertical axis (y-axis) represents the Price ( $P$ ), and the horizontal axis (x-axis) represents the Quantity demanded ( $Q$ ).
- The Slope: The points derived from the demand schedule are plotted and connected to form a downward-sloping line (often denoted as line $D-D'$ ). This downward slope visually demonstrates the inverse relationship between price and quantity.

Transition to Market Demand: While individual demand focuses on one consumer, market demand looks at the collective behavior of all consumers.
Calculation: Market demand is determined by summing together the individual quantities demanded by every consumer in the market at various price points.