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.