Resistance Opposition to Current Flow
Do you ever stand in the shower with the water temperature adjusted so that it is just right, and then somebody flushes a nearby toilet? Often, when this happens the water in the shower suddenly gets very hot. The reason this happens is as follows: The water (both hot and cold) comes to you from a reservoir or a pressurized tank. As it flows to your shower, it travels through pipes that, depending on their sizes, restrict the flow of water. After all, you do not want all of the water to arrive at once in a flood. Also, when you adjust the water temperature, you further restrict the flow of water by partially closing valves. If the water is too cold, you close the cold valve a bit to restrict its flow, and you open the hot valve a tad to allow it to flow more freely. Now, when you have it just right and somebody flushes a toilet without warning, the cold water
follows the less-restricted path to the toilet. This is especially serious if the toilet draws a lot of water, as do those of the institutional variety found in college dormitories and gymnasiums. Since you have already restricted the flow by partially closing the cold valve in your shower, the cold water goes mostly to the toilet and you are left with only hot water.