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Energy
Energy is a property that enables something to do work
Energy has various forms (kinetic, potential, rest energy)
Kinetic energy is 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.
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Potential Energy
Potential energy is a capacity of doing some work
Potential energy is the energy of position
Gravitational Potential Energy
W = Fd = mgh = PE (potential energy)
PE of a 1000-kg car at the top of a 50-m
PE is relative!
multilevel parking lot is:
mgh= (1000 kg)(9.8 m/s2)(50 m)=490 kJ
Rest Energy
Mass and Energy are related to each other and can be
converted into each other.
The rest energy of a body is the energy equivalent of its
mass.
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/s2 x 9000 m ~
105 J
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.
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
Chapter 5
Light and Atomic Structure
• Light and its properties
• Atomic structure
• Interaction between Light and Matter
• Spectrum
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 of
colors)
Light travels with a speed of c = 300,000 km/s
Rainbow
Interaction of Light and Matter
• Emission
• Absorption
• Transmission (passing through)
• Reflection (scattering)
Properties of Light
Light behaves as both a particle and a wave
Light particles are called photons, which can be counted
individually.
Light is also an electromagnetic wave
The wavelength is the distance between adjacent peaks of
the electric or magnetic field
1 nm (nanometer) = 10–9 m
1μm (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
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.
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
Electromagnetic spectrum
Electromagnetic spectrum
Frequency units – Hertz
1 Hz = 1 c1
Light and Matter
The amount of light is called intensity
Studying spectra of celestial bodies one can
learn a wealth of information about them
Atomic Structure
92 chemical elements have been identified in the Universe.
Nearly 20 more have been created artificially.
Each chemical element is made from a different type of
atom.
Atoms are made from particles called protons, neutrons,
and electrons.
Protons and neutrons form the nucleus in the center of the
atom.
Electrons surround the nucleus.
Atomic Structure
Positively charged protons are hold together by the strong
force, 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.
Absorption and Emission in Gases
Since electrons in atoms can have only specific
energies, 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 a photon of light
The photon has exactly the same energy that
the electron has lost
Types of Spectra
Emission line spectrum consists of photons
emitted as each electron falls back to lower levels
Absorption line spectrum appears when photons
are absorbed, causing electrons to jump up in energy
Each element or molecule produces its own distinct
set of spectral lines
Emission by Hydrogen
Hydrogen lines in the visible
Examples of Spectra
Thermal Radiation
``Complex’’ objects - planets, stars, people produce thermal radiation
Its spectrum depends only on the object’s
temperature
Hotter objects emit more total radiation per unit
surface area
The radiated energy is proportional to the fourth
power of the temperature
Hotter objects emit photons with a higher average
energy
Temperature and Color
Temperature and Intensity
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
The Doppler Shift
Radial motion of a distant object can be determined
due 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 are
bluer, the Doppler shift of this object is called a
blueshift
The Doppler shift of a moving away object - redshift
Doppler Effect
Demo Doppler effect
Summary
Spectral information gives us more knowledge
about 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 is
moving toward or away from us