Final answer:
In a permanent wave set, the greatest tension and risk of breakage occur when rods are placed tightly and near the scalp, as this increases tensile stress on the hair.
Step-by-step explanation:
The rod placement in a permanent wave set that will create the greatest tension, and thereby increasing the chance of breakage, is when the rods are placed very tightly and near the scalp. Tension is related to how tightly the hair is wound around the rod. The tighter the hair is wound, the more tension there is on the hair strand, which can increase the chance of hair breakage. This is similar to the concept in physics where the tensile stress on a rod is not uniform, being greatest at the top of the rod and least at the bottom, particularly when the weight of the rod itself is considered. Furthermore, materials have different breaking stresses, which is the value of stress at which they will fracture. Different materials, like aluminum and steel, can have very different breaking stresses, which influences their ability to withstand tension without breaking.
The placement of rods in a permanent wave set can affect the tension experienced by the hair. When a rod is placed at the top of the hair strand, closest to the scalp, it creates the greatest tension. This is because the weight of the rod pulls the hair downwards, causing more stress on the strand. On the other hand, when a rod is placed at the bottom of the hair strand, near the ends, it creates the least tension.