Distinguish between a change in quantity demanded and a change in demand of a commodity.
Or
Distinguish between a movement along a demand curve and shift in the demand curve with the help of diagrams.
Change in quantity demanded (or movement along the demand curve) is associated with a change in the demand curve by a rise/fall in the price of the commodity. It is expressed in the form of an expansion of demand or contraction in demand. When the demand of a good rises due to a fall in the price of the good alone, it is termed as expansion of demand. When the demand of a good falls due to rise in its price, it is called as contraction in demand. Graphically, it means movement along the demand curve. At price OP, the demand is OQ. When price falls to OP2, demand rises to OQ2. In this case the consumer moves from A to B downwards but remains on the same demand curve. When price rises to OP1, demand falls to OQ1. Once again the consumer moves along the same demand curve from A to C.
Change in demand (or shift in demand curve) is associated with the change in demand for a commodity caused by factors other than the price of a commodity such as price of related goods, income of the consumer etc. It is expressed in the form of an increase or decrease in demand. When at the given price, the demand of a good increases, it is called increase in demand. When at the given price, the demand decreases, it is called decrease in demand. Graphically, it means, shift of the demand curve. At price OP, the demand is OQ. When there is an increase in demand at a given price, the demand curve shifts to the right. If there is a decrease in demand at the given price, the demand curve shifts to the left. Thus, change in quantity demanded is due to a fall/rise in price while the change in demand is due to other factors than price.
Under change in quantity demanded, there is a movement along the same demand curve, whereas under change in demand, there is a shift of the demand curve.