Transcript Energy - CCPhysics.US
Energy Energy requires Work Work is the product of Force times Distance – W = Fd Not the same as torque, which is a force at a distance that causes rotation. Energy is measured in Joules, a Newton meter. Also ergs, foot-pounds, and calories. Work can be transformed into various kinds of energy. Force times distance: This means no work is done if the direction of the force is perpendicular to the direction of displacement. Therefore, twirling a mass on a string requires no work! (the length of the string doesn’t change in the direction of the centripetal force.) Mechanical Energy Kinetic Energy: the energy of motion. KE depends on the mass of the moving object, but more importantly its velocity squared. Potential Energy: the energy of position. Stored energy: in heights, springs, etc. Conservation of Energy Energy cannot be created nor destroyed, only changed in form (KE or PE, chemical to heat, etc.) In all transformations, some is lost to low level heat and is irretrievable. Sources of Energy Most of the energy on Earth can be traced back to the Sun. Solar (obviously), wind (differential heating), hydroelectric (Sun evaporates water, winds move vapor to high altitudes) “Fossil” fuels: plants and animals get their energy from Sunlight. Nuclear Fission is the major exception. The Sun’s Source: Nuclear Fusion Not like a nuclear plant Changes Hydrogen into Helium Some mass becomes energy E = mc2 So most energy on Earth is some kind of nuclear energy! Efficiency The ratio of work (energy) out of a system compared with the work put into a system. Expressed as a percent, never > 100% Cars are about 20%, solar cells about 10%. Don’t think of it as “how well the energy is used”; efficiency is a number. Power The rate of energy usage. Energy relates to power as distance relates to speed. Power is measured in Watts, Joules/sec. – 750W = 1 hp One gallon of gas in a lawnmower has the same energy as if it was in a Corvette; the Corvette just uses it up faster.