Price of demand(PED) is always negative because price of commodity and demand for that commodity has inverse relationship.
We know,
## PED = ” %change in quantity”/” % change in price” ##
PED is negative because of law of demand i.e. increase in price of commodity decreases the quantity demand of that commodity or in other words positive change in price of a commodity has negative change in quantity demanded for that product.
This negative relation causes PED to become negative.