Suppose a bat uses sound echoes to locate its insect prey, 3.00 m away. Calculate the echo time at a temperatures of 35.0ºC.

where are the vectors ?

the answer is the first electromagnet (a). this is because it has more coils around the ferromagnetic material. increasing the number of coils increases the surface area in which the metal interacts with the current in the coil. this enables more atoms in the metal to become aligned hence becoming more magnetized.