The Treaty of Versailles in the mind of the Germans stabbed them in the back, especially the guilt clause that made them take blame for the First World War. Hitler capaitalized on these sentiments and used them to rally the German people.
Primary sources are the first hand evidence left behind by participants or observers at the time of events. Primary sources originate in the time period that historians are studying. They vary a great deal.
Electricity cannot be mined from the ground like coal. So it is called a secondary source of energy, meaning that it is derived from primary sources, including coal, natural gas, nuclear fission reactions, sunlight, wind, and hydropower. Most direct uses of primary energy are limited to generating heat and motion.