ECON 1B03 Lecture Notes - Lecture 4: Demand Curve, Negative Number

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Point Elasticity
- The other formula we use is the formula for point elasticity
- It measures the impact of a marginal change in price on quantity demanded
- The formula for point elasticity is:
- This is actually easy to compute, since dQ/dP is just the slope of a linear demand curve
when demand is in the form Q= f(P)
Example
- Demand is given by the equation Qd = 200-3P
- What is the point elasticity of demand when price is $15
- Slope = -3
- When P = 15, Qd = 155
- Ep = (-3) (15/155) = -0.29
In the last three examples, we have an elasticity coefficient that is a negative sign
- But, remember the law of demand: as P↑, Qd↓. The coefficient will always be a negative
number.
- Sice we’re sart ecooists ad kow this, when we calculate price elasticity, we
drop the negative sign, (we know it will always be negative)
Some Generalities About Demand Elasticities and their Determinants
1. Goods that are necessities tend to have inelastic demand
- insulin would be perfectly inelastic (if it keeps you alive)
- Textbooks
2. Goods that are luxuries tend to have elastic demand
- Plasa T.V.’s
3. Goods that have close substitutes tend to have elastic demand
- Coke and pepsi
4. Goods tend to have more elastic demand over longer time horizon
- You can find substitutes in the long ru here you a’t i the short ru
5. How you define the market makes a difference
- Food- inelastic
- Vegetables- more elastic
- Broccoli- even more elastic
- The more narrowly defined the market, the more elastic the demand for that good
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ECON 1B03 Full Course Notes
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