Energy - CCPhysics.US
Transcript Energy - CCPhysics.US
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
Force times distance:
This means no work is done if the direction
of the force is perpendicular to the direction
Therefore, twirling a mass on a string
requires no work! (the length of the string
doesn’t change in the direction of the
Kinetic Energy: the energy of motion.
KE depends on the mass of the moving
object, but more importantly its velocity
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:
Not like a nuclear plant
Changes Hydrogen into Helium
Some mass becomes energy
E = mc2
So most energy on Earth is some kind of
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.
The rate of energy usage.
Energy relates to power as distance relates
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.