well for starters a physical network design well, as the name implies, involves the physical connectivity of each of your hosts on the network. this speaks of the actual physical layout of your network which includes the actual nodes, segments and hosts that appear on the actual layout. on the other hand, a logical network speaks more of data flow in your network. this may be of the different forms of network topology (e.g bus, ring, etc).
the difference of the two lies in the way they are presented. one being the real connection layout (physical) while the other is of how data flows within the network (logical).
think of it as like cars and roads. the roads represent the physical network wherein each road are interconnected and ends with a destination while the cars represent the logical part of the network wherein they move towards a certain destination within the network. although physical and logical networks vary in meaning still they work hand in hand to better define the same single and structured network.