Change In Demand (Assuming No Change In Supply)

There is a rightward shift in the demand curve due to increase in demand creating a situation of excess demand at the original equilibrium E. Transmission mechanism operates leading to a rise in equilibrium prices from OP to OP1 and equilibrium quantity to OQ1. Market attains equilibrium at point E1.
Decrease in demand leads to leftward shift in demand curve causing excess supply at the initial equilibrium E2. Transmission mechanism leads to a fall in price from to OP2 and fall in equilibrium quantity from OQ to OQ2.