Transcript fusion_3

Forms of Energy
Freshman Seminar
Energy
• Energy  The ability & capacity to do work
– Energy can take many different forms
– Energy can be quantified
• Law of Conservation of energy
“In any change from one form of energy to other forms,
the total quantity of energy remains constant”
Energy
Energy =The ability & capacity to do work
Measured by the capability of doing work:
potential energy
or
the conversion of this capability to motion:
kinetic energy
Examples of Potential Energy:
Stretching a rubber band..
-Stores energy
Water at the top of a waterfall..
-Stores energy
Yo–Yo in held in your hand..
-Stores energy because of position
Drawing a Bow…
-Stores energy because of position
Calculating Potential Energy…
• Energy due to position or
stored energy.
Example: PE= (mass) (gravity) (height)
Due to gravity
In this case potential energy is calculated by:
The object’s mass, multiplied by the earth’s gravitational pull (9.8
m/sec sq), multiplied by the distance the object can fall.
Potential Energy Converted
to Kinetic Energy…
When stored energy begins to move,
the object now transfers from
potential energy into kinetic energy.
Standing still
Running
Examples of Kinetic Energy…
• Shooting a rubber
band.
• Water falling over
the fall.
• A Yo-Yo in motion.
• Releasing the arrow
from the bow.
Definition of Kinetic Energy…
The energy of motion.
Measured by:
2
KE= ½ (Mass) (Velocity)
Kinetic energy is calculated by one half of the object’s
mass, multiplied by the object’s speed- squared.
Mechanical Energy
Mechanical energy E is the sum of the potential
and kinetic energies of an object.
E=U+K
The total mechanical energy in any isolated
system of objects remains constant if the
objects interact only through conservative
forces:
E = constant
Ef = Ei  Uf + Kf = Ui+ Ki
Conservation of Mechanical Energy
If friction and wind resistance are ignored, a bobsled run
illustrates how kinetic and potential energy can be interconverted,
while the total mechanical energy remains constant.
Heat and temperature
Kinetic molecular theory
Measures of heat
Metric units
English system
• calorie (cal) - energy
needed to raise
temperature of 1 g of
water 1 degree Celsius
• kilocalorie (kcal,
Calorie, Cal) - energy
needed to raise
temperature of 1 kg of
water 1 degree Celsius
• British thermal unit
(BTU) - energy needed
to raise the temperature
of 1 lb of water 1
degree Fahrenheit
Mechanical
equivalence
• 4.184 J = 1 cal
Heat
• A form of energy transfer
between two objects
• External energy - total
potential and kinetic
energy of an every-day
sized object
• Internal energy - total
kinetic energy of the
molecules in that object
• External can be
transferred to internal,
resulting in a temperature
increase
Heat versus temperature
Temperature
• A measure of hotness or
coldness of an object
• Based on average molecular
kinetic energy
– Recall difference in KE and temp.
between solids, liquids & gases
Heat
• Based on total internal energy
of molecules
• Doubling amount at same
temperature doubles heat
Specific heat
Variables involved in heating
• Temperature change
• Mass
• Type of material
– Different materials
require different amounts
of heat to produce the
same temperature
change
– Measure = specific heat
Summarized in one equation
Energy, heat, and molecular
theory
Two responses of matter
to heat
1. Temperature increase
within a given phase
–
–
Heat goes mostly into
internal kinetic energy
Specific heat
2. Phase change at
constant temperature
–
–
Related to changes in
internal potential energy
Latent heat
Heat flow
Three mechanisms for heat transfer due to a
temperature difference
1. Conduction
2. Convection
3. Radiation
Natural flow is always from higher temperature
regions to cooler ones
Conduction
• Heat flowing through
matter
• Mechanism
– Hotter atoms collide
with cooler ones,
transferring some of
their energy
– Direct physical contact
required; cannot occur
in a vacuum
• Poor conductors =
insulators (Styrofoam,
wool, air…)
Sample conductivities
Material
Relative conductivity
Silver
0.97
Iron
0.11
Water
1.3x10-3
Styrofoam
1.0x10-4
Air
6.0x10-5
Vacuum
0
Convection
• Energy transfer
through the bulk
motion of hot
material
• Examples
– Space heater
– Gas furnace (forced)
• Natural convection
mechanism - “hot air
rises”
Radiation
• Radiant energy - energy associated with
electromagnetic waves
• Can operate through a vacuum
• All objects emit and absorb radiation
• Temperature determines
– Emission rate
– Intensity of emitted light
– Type of radiation given off
• Temperature determined by balance between rates
of emission and absorption
– Example: Global warming
Thermodynamics
• The study of heat and
its relationship to
mechanical and other
forms of energy
• Thermodynamic
analysis includes
– System
– Surroundings (everything
else)
– Internal energy (the total
internal potential and
kinetic energy of the
object in question)
• Energy conversion
– Friction - converts
mechanical energy into
heat
– Heat engines - devices
converting heat into
mechanical energy
– Other applications: heat
pumps, refrigerators,
organisms, hurricanes,
stars, black holes, …,
virtually any system with
energy inputs and
outputs
The first law of
thermodynamics
• Conservation of energy
• Components
– Internal energy
– Heat
– Work
• Stated in terms of
changes in internal
energy
• Application: heat
engines
Electric current
• A flow of charge is called an
electric current
I  Q / t
• It is measured in ampere
(A=C/s)
• Need free charge to have
electric current. Use
conductors.
Note: net charge =0
+
+
+
+
+
+
+
Skiing  electric circuit
High PE
High PE
Low PE
Low PE
Skiers
Charges
go from points with high PE to low PE
To complete the circuit need a device that brings
you back
to high PE:
Ski lift
Battery
Ohm’s law
• Electric current is proportional to voltage.
I V
V  IR
• Coefficient in this dependence
is called resistance R
• Resistance is measured in
Ohm (W = V/A)
I
R
V
Resistance and Temperature
• When electrons move through the conductor
they collide with atoms:
– Temperature of the conductor increases because
of the current (through collisions)
– Electrical energy is transformed into thermal
energy
– Resistors dissipate energy
– Power – energy per unit of time- (in W=J/s)
dissipated by a resistor
PI R
2
Electric power
• Electric energy can be
converted into other
kinds of energy:
–
–
–
–
Thermal ( toaster)
Light (bulbs)
Mechanical (washer)
Chemical
• Electric power (energy
per unit of time):
P  IV
What Produces Voltage?
A Battery
9V
Lab Power Supply
Solar Cell
1.5 V
Electric Power Plant
13,500 V
A few
Volts
Nerve Cell
A few millivolts
when activated by
a synapse
Simple Electromagnetic Energy
Moving electric charges.
Examples:
• Power lines carry
electricity
• Electric motors are
driven by electromagnetic
energy
• Light is this form of
energy (X-rays, radio
waves, laser light etc.)
Or Radiation Energy?
• Electromagnetic
radiation -- photons
(visible light, infrared,
ultraviolet, x-rays,
and radio waves).
• Light is an
electromagnetic wave
Chemical Energy
• Energy that exists in the
bonds that hold atoms
together.
• When bonds are broken,
chemical energy is released.
Examples:
• Digesting food…bonds are
broken to release energy for
your body to store and use.
• Sports… your body uses energy
stored in your muscles
obtained from food.
• Fire–a chemical change.
Sodium metal reacts with water.
Nuclear Energy
• When the nucleus of an atom splits,
nuclear energy is released.
• Nuclear energy is the most concentrated
form of energy.
• Fission/fusion