Transcript energy conversion

S ECTION 15–2:
E NERGY C ONVERSION
AND
C ONSERVATION
O BJECTIVES

Describe conversions of energy from one form to
another.

State and apply the law of conservation of energy.

Analyze how energy is conserved in conversions
between kinetic energy and potential energy and solve
equations that equate initial energy to final energy.

Describe the relationship between energy and mass and
calculate how much energy is equivalent to a given
mass.
E NERGY C ONVERSION

Energy can be converted
from one form to another.

The process of changing
energy from one form to
another is energy
conversion.

These conversions happen
very frequently. Light bulbs
convert electrical energy into
thermal energy and
electromagnetic energy.
C ONSERVATION
OF
E NERGY

When energy changes from one form to another, the
total energy remains unchanged even though many
energy conversions may occur.

The law of conservation of energy states that energy
cannot be created or destroyed.

According to the law of conservation of energy, energy
can be converted from one form to another, but you will
always finish with what you started with in a closed
system.
E NERGY C ONVERSIONS

One of the most common energy
conversions is between potential
energy and kinetic energy.

The gravitational potential energy of
an object is converted to the kinetic
energy of motion as the object falls.

Sea gulls use energy conversions to
eat. They can’t break open the shells
of oysters, so they pick them up, fly
to a high altitude, and drop the
oyster over the rocks. The potential
energy of the oyster at its height is
gradually converted to all kinetic
energy as it hits the rocks.
E NERGY C ONVERSION IN P ENDULUMS

Christiaan Huygens, a Dutch scientist, was
the first person to use a pendulum in a
clock.

The time it takes a pendulum to swing
back and forth once is precisely related to
its length.

At the highest point of its swing, the
pendulum has nothing but gravitational
potential energy. As it reaches the lowest
part of its swing, the gravitational
potential energy converts to kinetic
energy. As it swings back upward, the
kinetic energy is converted to
gravitational potential energy again.
E NERGY C ONVERSION C ALCULATIONS

When friction is small enough to be ignored, and no
mechanical energy is added to a system, then the
system’s mechanical energy does not change.

Total mechanical energy is equal to the total kinetic
energy (KE) plus the total potential energy (PE).
Mechanical Energy = KE + PE

The conservation of mechanical energy says that
mechanical energy remains constant during any
process.
(KE + PE)beginning = (KE + PE)end
S AMPLE P ROBLEM – C ONSERVATION
OF M ECHANICAL E NERGY

At a construction site, a 1.50-kg brick is dropped from
rest and hits the ground at a speed of 26.0 m/s.
Assuming air resistance can be ignored, calculate the
gravitational potential energy of the brick before it was
dropped.

Given:
Mass: 1.50 kg
Speed: 26.0 m/s
Beginning KE: 0 J
Ending PE: 0 J
S AMPLE P ROBLEM – C ONSERVATION
OF M ECHANICAL E NERGY

At a construction site, a 1.50-kg brick is dropped from
rest and hits the ground at a speed of 26.0 m/s.
Assuming air resistance can be ignored, calculate the
gravitational potential energy of the brick before it was
dropped.

Solve:
(KE + PE)beginning = (KE + PE)end
(0 J + PE)beginning = (KE + 0 J)end
(PE)beginning = (KE)end
Ending KE: ½mv2 = ½(1.50 kg)(26.0 m/s)2
= 507 J
Beginning PE: 507 J
E NERGY

Albert Einstein developed his
special theory of relativity in 1905.
This theory included the nowfamous E = mc2.


E = energy, m = mass, c = speed of
light
Einstein’s equation says that energy
and mass are equivalent and can be
converted into each other. In other
words, energy is released as matter
is destroyed, and matter can be
created from energy.
AND
M ASS
V OCABULARY

Energy conversion