4.3.1. Macroeconomic Equilibrium with Price Level Fixed

As already explained, the consumers demand can be expressed by the equation C=C+cY.

This equation represents the consumption function, where C is the total consumption expenditure, c is the marginal propensity to consume, and Y is the income. It shows how consumption changes as income changes. The equation reflects that as income increases, so does consumption, but the rate at which consumption increases is defined by the marginal propensity to consume (c).

Think of it like a growing tree: as the tree (income) grows taller, its branches (consumption) also expand. The branches don’t grow as fast as the trunk—this is like the marginal propensity to consume, which determines how much faster consumption grows relative to income.

Recall the 'intercept form of the linear equation', Y = a + bX, where 'a' is the intercept on the Y-axis and 'b' is the slope of the line.

In this linear equation, 'Y' represents the dependent variable (like total consumption), and 'X' represents the independent variable (like income). The intercept 'a' indicates the value of Y when X is zero, and the slope 'b' quantifies how much Y changes for each unit increase in X. Understanding this form helps us visualize relationships in economics through graphs.

Consider a ramp: the steepness of the ramp represents the slope (b), while the point where the ramp touches the ground is the intercept (a). The steeper the ramp, the quicker you can ascend (increase in consumption with income).

Using the same logic, the consumption function can be shown as follows: C = C + cY.

This representation helps visualize the consumption function: the intercept (C) shows autonomous consumption, which occurs even at zero income, while the slope (cY) indicates the portion of additional income that will be spent on consumption. The graph thus depicts how consumption levels are influenced by changes in income.

Imagine you're planning a party. No matter what, you spend a minimum amount (autonomous consumption), but if more friends come (increase in income), you'll spend more on snacks proportionally (marginal propensity to consume).

In a two sector model, there are two sources of final demand, the first is consumption and the second is investment.

The investment function, represented as I = I, indicates a constant level of investment in this model; it remains unchanged regardless of income fluctuations. This simplification allows us to focus on how consumption and investment together influence aggregate demand.

Think of it like a business that has a steady budget for new equipment every year (investment), regardless of how much revenue it makes. This ensures that the business maintains its ability to operate efficiently and meet demand.

The Aggregate Demand function shows the total demand (made up of consumption + investment) at each level of income.

Graphically, the aggregate demand function can be derived by adding the consumption and investment functions. This function displays how overall economic demand changes with varying income levels, showing a consistent relationship between income and total demand in the economy.

Imagine a community potluck where the total amount of food (aggregate demand) depends on how many people show up (income). If more people come, the total food amount increases, showcasing how consumption and investment drive overall demand.

In the first stage of macroeconomic theory, we are taking the price level as fixed. Here, aggregate supply or the GDP is assumed to smoothly move up or down since they are unused resources of all types available.

This approach assumes that in the short run, the economy can adjust its output without changing the prices due to excess capacity. Therefore, all available resources can contribute to fulfilling aggregate demand, illustrated by a 45-degree line where output equals demand.

Picture a factory with plenty of machines and workers on standby. They can ramp up production without needing to increase costs, as they haven't been fully utilized yet. This situation allows for a direct relationship where every increase in demand can be met with an equal increase in supply.

Equilibrium is shown graphically by putting ex ante aggregate demand and supply together in a diagram.

Graphically, equilibrium occurs at the point where the aggregate demand curve intersects with the aggregate supply line. This point represents the output level where planned consumption and investment match what is available for production. At this level, there is no excess supply or demand in the economy.

Imagine a seesaw perfectly balanced: on one side is the amount of goods being produced (supply), and on the other is the demand for those goods. When they balance out, neither side has to adjust—everything is in equilibrium.

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