Chapter Seven: Energy
7.1 Energy and Systems
7.2 Energy Transformations
7.3 Conservation of Energy
7.3 Conservation of Energy
 The idea that energy tranforms from
one form into another without a
change in the total amount is called
the law of conservation of energy.
 The law of energy conservation says
the total energy before the change
equals the total energy after it.
(Energy can never be created or
destroyed.)
Energy Resources
Many people are concerned about
“running out” of energy. What they
worry about is running out of certain
resources of energy that are easy to
use, such as fossil fuels like oil and
coal.
7.3 Conservation of Energy
 When you throw a ball
in the air, the energy
transforms from
kinetic to potential and
then back to kinetic.
Solving Problems
A 2 kg car moving with a speed of 2
m/sec starts up a hill.
How high does the car roll before it
stops?
Solving Problems
1. Looking for:
 …height of hill
2. Given
 … mass = 2 kg, v = 2 m/s
3. Relationships:
 Energy transformed from EK to EP
 EK = ½ mv2
 EP =mgh
Solving Problems
1. Solution
 Find beginning EK
 EK = ½ (2 kg) (2 m/s)2 = 4 Joules
 Assume energy before = energy after
 EK = E P
 EP =mgh
4 J = mgh
 h = (4 Nm)/(2 kg)(9.8 N/kg) = .2 m
7.3 Conservation of Energy
 Many people are concerned about
“running out” of energy.
 What they worry about is running out of
certain forms of energy that are easy to
use, such as fossil fuels like oil and gas.
7.3 Conservation of Energy
 It took millions of years to
accumulate these fuels
because they are derived
from decaying, ancient
plants that obtained their
energy from the Sun when
they were alive.
 Because it took a long time
for these plants to grow,
decay, and become oil and
gas, fossil fuels are a
limited resource.
7.3 Conservation of Energy
 Regular
(incandescent) light
bulbs convert only
10% of electrical
energy to light.
 That means 90% of
the energy is
released as wasted
heat.
7.3 Conservation of Energy
 Other forms of
energy, such as
thermal energy,
flowing water, wind,
and solar energy are
not as limited.
2013 National Electricity
Energy Sources
Natural gas
27%
Oil and other
liquids
1%
Coal
40%
Renewables
13%
Nuclear
19%
Where Does Electricity Come
From in Our Area?
1.30%
0.09%
 Fossil Fuel(Coal): 42%
 Nuclear: 40%
 Fossil Fuel (Gas): 13%
 Fossil Fuel (Oil): 1.1
42.20%
40.30%
 Hydroelectric: <1%
 Other Renewables: 1.3%
13.10%
1.10%