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Energy

Energy is a property that enables something to do work

Energy has various forms (kinetic, potential, rest energy)

Kinetic energyis the energy of a moving object

mv2

KE = ---------2

It takes ~1000 N (or ~ 225 lbs) to hit a nail and drive it 5

mm into a surface.

100s of free ppts fromwww.pptpoint.com library

http://bc1.lbl.gov/CBP_pages/educational/WoB/cycle1p04a.htmhttp://www.pptpoint.com/http://www.pptpoint.com/http://bc1.lbl.gov/CBP_pages/educational/WoB/cycle1p04a.htm

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Potential Energy

Potential energy is a capacity of doing some work

Potential energyis the energy of position

Gravitational Potential Energy

W = Fd = m

gh =

PE (potential energy)

PE is relative!PE of a 1000-kg car at the top of a 50-m

multilevel parking lot is:

mg

h= (1000 kg)(9.8 m/s2)(50 m)=490 kJ

http://www.glenbrook.k12.il.us/gbssci/phys/Class/energy/u5l1b.htmlhttp://www.glenbrook.k12.il.us/gbssci/phys/Class/energy/u5l1b.html

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Rest Energy

Mass and Energy are related to each other and can beconverted into each other.

The rest energy of a body is the energy equivalent of itsmass.

E0 = m0c2

E0 (m=1 kg) = 1 kg x (3 108)2 (cm/s)2 ~ 1017 J

PE (m=1 kg, h=9 km) = mgh = 1 kg x 9.8 m/s2x 9000 m ~

105 J

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Energy Transformations

Many mechanical processes involve interchanges

between KE, PE, and work.

Energy exists in some other forms: chemical energy, heat

energy, radiant energy, etc.

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Conservation of Energy

Energy cannot be created or destroyed.It can only be changed from one form to another.

The above statement is called the law of conservation of

energy

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Chapter 5

Light and Atomic Structure

Light and its properties

Atomic structure

Interaction between Light and Matter

Spectrum

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Light in Everyday Life

Light is a form of energy, radiative energy

1 Watt = 1 Joule/sec

Light has color

A prism split light into a spectrum (rainbow ofcolors)

Light travels with a speed ofc = 300,000 km/s

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Rainbow

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Interaction of Light and Matter

Emission

Absorption

Transmission (passing through)

Reflection (scattering)

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Properties of Light

Light behaves as both a particle and a wave

Light particles are called photons, whichcan be counted

individually.

Light is also an electromagnetic wave

The wavelength is the distance between adjacent peaks of

the electric or magnetic field1 nm (nanometer) = 109m 1 m (micron) = 10 6 m

The frequency is the number of peaks that pass by any point

each second, measured in cycles per second or Hertz (Hz).

light demo

http://acept.la.asu.edu/PiN/rdg/color/color.shtmlhttp://acept.la.asu.edu/PiN/rdg/color/color.shtml

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Light is an electromagnetic wave

Light consists of many individual photons.

Each travels at the speed c and can be

characterized by a wavelength and a frequency.

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Many Forms of Light

The spectrum of light is called the electromagnetic

spectrum

Different portions of the spectrum are called:

The visible light - what we see with our eyes

The infrared light - beyond of the red end of rainbow

The ultraviolet light - beyond the blue end

Radio waves - light with the longest wavelengths

X rays - wavelengths shorter than ultraviolet

Gamma rays - the shortest wavelength light

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Electromagnetic spectrum

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Electromagnetic spectrum

Frequency units Hertz1 Hz = 1 c1

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Light and Matter

The amount of light is called intensity

Studying spectra of celestial bodies one can

learn a wealth of information about them

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Atomic Structure

92 chemical elementshave been identified in the Universe.Nearly 20 more have been created artificially.

Each chemical element is made from a different type ofatom.

Atoms are made from particles calledprotons, neutrons, and

electrons.

Protons and neutrons form the nucleus in the center of theatom.

Electrons surround the nucleus.

http://www.webelements.com/http://www.webelements.com/http://www.webelements.com/

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Atomic Structure

Positively charged protons are hold together by the strongforce, which overcomes electrical repulsion.

Negatively charged electrons are attracted to the nucleus.

The number of protons in an atom is called the atomic

number, which is unique for different chemical elements.

The combined number of protons and neutrons in an atom

is called the atomic mass number.

Atoms of the same element with different number of

neutrons are called isotopes.

http://web.jjay.cuny.edu/~acarpi/NSC/3-atoms.htmhttp://web.jjay.cuny.edu/~acarpi/NSC/3-atoms.htm

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Absorption and Emission in Gases

Since electrons in atoms can have only specificenergies, the atoms can absorb or release energy

only in these amounts (quanta)

Electron gets energy, jumps to an excited state,

release the energy, and falls back down

The energy is emitted as aphoton of light

The photon has exactly the same energy that

the electron has lost

http://www.colorado.edu/physics/2000/quantumzone/bohr2.htmlhttp://www.colorado.edu/physics/2000/quantumzone/bohr2.html

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Types of Spectra

Emission line spectrum consists of photonsemitted as each electron falls back to lower levels

Absorption line spectrum appears when photonsare absorbed, causing electrons to jump up in energy

Each element or molecule produces its own distinctset of spectral lines

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Examples of Spectra

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Thermal Radiation

``Complex objects - planets, stars, people -produce thermal radiation

Its spectrum depends only on the objects

temperature

Hotter objects emit more total radiationper unit

surface area

The radiated energy is proportional to the fourthpower of the temperature

Hotter objects emit photons with a higher average

energy

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Temperature and Color

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Temperature and Intensity

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Reflected light

When the light (for example, sunlight) strikes an

object (ground, clouds, people), we see only the

wavelengths of light that are reflected

Different objects (fruits, rocks, atmospheric gases)

reflect and absorb light at different wavelengths

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The Doppler Shift

Radial motion of a distant object can be determineddue to the Doppler effect

The Doppler effect causes shifts in the wavelengths

of light

If an object is moving toward us, its entire spectrum

is shifted to shorter wavelengths

Because shorter wavelengths of the visible light arebluer, the Doppler shift of this object is calleda

blueshift

The Doppler shift of a moving away object - redshift

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Doppler Effect

Demo Doppler effect

http://www.phy.ntnu.edu.tw/java/Doppler/Doppler.htmlhttp://www.phy.ntnu.edu.tw/java/Doppler/Doppler.html

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Summary

Spectral information gives us more knowledgeabout the objects (composition, surface

temperature, moving properties)

Visible light is only a small portion of the

electromagnetic spectrum

The Doppler effect tells us how quickly light ismoving toward or away from us