The price of Petroleum will increase as the demand for petroleum rises because at this point there is a positive relationship between the two. Thus, the rise in demand will always lead to a rise in price (in the case of normal goods). However, the law of demand says as the price of goods rises if its demand falls. Conversely, fall in the price of commodity will leads to a rise in demand because people will be more willing to buy goods at a lower rate. But when there is a change in demand and this change in demand leads to change in price then there is a positive relationship between demand and price.
