Năng lượng là khái niệm quan trọng trong vật lý, dù không cơ bản như khối lượng. Lịch sử của khái niệm này bắt đầu từ cơ học cổ điển, đến điện từ học, rồi trải qua cuộc cách mạng về việc đồng nhất với khái niệm nhiệt lượng trong nhiệt động lực học và đến giờ là quan niệm hiện đại của thuyết tương đối và thuyết lượng tử. | 30 energy a beginner s guide rotation deflects them into the prevailing westerlies that bring plenty of precipitation to America and Europe s western coasts. The fastest near-ground winds are the product of the intensive summer heating that generates cyclonic low-pressure flows ranging from innocuous localized thunderstorms to massive hurricanes. Even big thunderstorms with a power of ten to a few hundred gigawatts do not usually produce winds that strike objects with vertical power densities of more than 15 kW m2 below the threshold for structural damage. North American hurricanes originate off Africa first move westward and then veer clockwise frequently making landfall along the northern Gulf of Mexico Florida and the East Coast. Their Asian counterparts typhoons originate above very warm Pacific waters near the Mariannas move westward and repeatedly affect large parts of Southeast Asia coastal China the Korean peninsula and Japan. Hurricanes or cyclones can have speeds up to 90 m s more than 300 km h and they strike vertical surfaces with power densities of up to 1 MW m2 forces easily resisted by modern steel and concrete structures but not by woodenframed houses. While some hurricanes can endure for weeks and affect sequentially large areas along a path extending for several thousand kilometers tornadoes are more restricted. The average path of an American tornado is only about 125 m wide and often sharply delineated a barely damaged house can stand across the street from a completely destroyed structure and less than 10 km long and they last less than three minutes. In contrast tornadoes in the most violent and relatively rare category can generate winds in excess of 100 m s and can strike vertical surfaces with more power than a typical hurricane. But because of its unique qualities it is water not air which is the Earth s most important carrier as well as largest reservoir of heat. WATER S UNIQUE PROPERTIES Water s high specific heat capacity J gC is
