Econ question

Change in quantity demanded means the movement ALONG the demand curve. Example: Demand curve is negatively-sloped with respect to the origin. If the price of a motor car increases, the quantity demanded on the motor car will falls.However, if the price of a motor car decreases, the quantity demanded on the motor car will rises.

Change in demand means the movement ON THE WHOLE demand curve that is to say, the demand curve will shift leftwards or rightwards. Example: if Hong Kong economy grows so that all the employees in HK have salaries increase, more people will buy motor cars, hence the demand curve of motor cars will move to the right (increase in demand). Conversely, if HK economy is in recession so that less people can afford to buy a car, the demand curve on motor cars will shift to the left (decrease in demand).

change in quantity demanded 表示由需求曲線上一點移去另一點,
主要受內生變數(Endogenous variable)影響。
change in demand表示整條需求曲線左右上下移動,
主要受外生變數(Exogenous variable)影響