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Energy

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Title: Slide 1 Author: Jon Pepper Last modified by: George Mason Created Date: 11/12/2006 11:13:52 PM Document presentation format: On-screen Show Other titles – PowerPoint PPT presentation

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Title: Energy


1
Energy
2
Defining Energy
  • Energy
  • the ability to do work or cause change
  • We will address what work is later
  • Energy can come in many forms, many of which you
    are probably already familiar with

3
Forms of Energy
  • Electrical Energy the energy of charged
    attraction
  • example would be the energy we get from a
    household circuit
  • Magnetic Energy the energy of polar attraction
  • a magnet picks up other magnetic substances
    using magnetic energy

4
Forms of Energy (continued)
  • Mechanical Energy the energy of machines doing
    work
  • a turning motor is an example of mechanical
    energy
  • Heat (Thermal) Energy the energy of burning
    calories, usually noticed through changes in
    temperature
  • Example would be warming a cup of water until the
    water undergoes a phase change.

5
Forms of Energy (continued)
  • Light (Radiant) Energy the energy from
    radiating waves
  • UV rays from the sun give off light energy
  • Chemical Energy the energy of breaking bonds in
    chemical reactions
  • an example is a battery that turns a chemical
    reaction inside of itself into energy

6
Forms of Energy (continued)
  • Nuclear Energy the energy of atomic reactions
    in the nucleus of atoms
  • example is radioactive decay, like at nuclear
    power plants
  • Gravitational Energy the energy from using the
    pull of gravity
  • water from a waterfall has gravitational energy

7
Classifying Energy
Despite the many different forms of energy, all
energy can be classified as either kinetic
energy or potential energy Kinetic Energy the
energy of motion Potential Energy the energy of
position The different forms of energy can be
kinetic or potential depending on what is going
on in the system an electric circuit can have
potential energy if the switch is open, and no
current is flowing. Once, the switch is closed,
and current flows, the electric current has
kinetic energy
8
Measuring Energy
Energy is measured in the unit of Joules (J) 1
Joule 1 Newton x meter 1 J 1 N m 1 kg m/s2
x m 1 kg m2/s2 When calculating energy, because
we use Joules, mass must always be in
kilograms!!! (kg)
9
Potential Energy
Potential energy exists whenever an object which
has mass has a position within a force field
(that force could be gravity, electric, magnetic,
etc. . .) Potential energy is stored energy, it
is not being released -- in this class, we will
focus on what is known as gravitational potential
energy, which is the stored energy due to height
above the Earth We can calculate potential energy
using the following formula PE mass x gravity
x height mgh
m mass of the object (kg)
g acceleration due to gravity (9.8 m/s2)
h height of object above the earth (m) PE
is measured in Joules (kg m/s2 m)
10
Kinetic Energy
The stored energy of potential energy is released
when it is converted to kinetic energy. Kinetic
energy exists whenever an object which has mass
is in motion with some velocity. It is the
energy of motion. We can calculate kinetic energy
using the following formula KE (1/2) mass x
velocity2 (1/2)mv2 m mass of the object
(kg) v velocity of the object (m/s) You must
remember to square the velocity before solving
the rest of the formula v2 is in (m/s)2
m2/s2 KE is in Joules (J) kg m2/s2
11
Practice Problems
  • A 100-kg boulder is sitting on top of a hill that
    is 50 meters above the ground. What is the
    potential energy of the boulder?
  • A 85-kg jogger is running at a velocity of 3 m/s.
    What is the kinetic energy of the jogger?

m 100 kg h 50 m g 9.8 m/s2 PE ?
PE m x g x h PE 100 kg x 9.8 m/s2 x 50 m PE
49000 kg m2/s2
PE 49000 J
Click For the Answer
m 85 kg v 3 m/s KE ?
KE (1/2) x m x v x v PE .5 x 85 kg x 3 m/s x
3 m/s PE 382.5 kg m2/s2
PE 382.5 J
Click For the Answer
12
Challenging Practice Problems
  • What is the velocity of a 2500-kg car that is
    traveling with a kinetic energy of 2,250,000 J
    (2250 kJ)?
  • 4. A 3,000-kg car is stopped at a red light.
    The light turns green and the car accelerates for
    5 seconds until its kinetic energy reaches
    1,200,000 J, when it smashes into a guard rail.
    How much force did the car hit the guard rail
    with?

m 2500 kg KE 2250000 J v ?
KE (1/2) x m x v2 v sqrt 2KE/m v sqrt (2 x
2250000 J/2500 kg v 42.43 m/s
v 42.43 m/s
Click For the Answer
CHALLENGE DO THIS ONE ON YOUR OWN AND BRING IT
IN
13
Conservation of Energy
Much like momentum, energy is a conserved
quantity The principle of conservation of energy
says that in a closed system, all energy is
conserved. Think about an object in free
fall -- as the object is at rest at a height,
h, all of its energy is potential. When the
object falls and accelerates due to Earths
gravity, potential energy is converted to kinetic
energy. Velocity increases, so KE increases and
height decreases, so PE decreases. Right before
the object hits the ground, its height 0, so
its PE is zero. All of its energy has been
converted to kinetic energy.
14
Energy Conversions
We can track the energy conversions between
different types of energy. Example Questions An
object is raised off of the ground and
stopped. Kinetic Potential The raised object
falls to the ground Potential
Kinetic Not all conversions have to be between
kinetic and potential energy A firefly (lightning
bug) uses a chemical reaction in its hind
quarters to glow at night Chemical
Radiant (Light)
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