Energy

1
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.
Visit www.worldofteaching.com For 100s of free
powerpoints
2
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
multilevel parking lot is mgh (1000 kg)(9.8
m/s2)(50 m)490 kJ
PE is relative!
3
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 (m1 kg) 1 kg x (3 108)2 (cm/s)2 1017
J PE (m1 kg, h9 km) mgh 1 kg x 9.8 m/s2 x
9000 m 105 J
4
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.
5
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
6
Chapter 5

• Light and Atomic Structure

• Light and its properties
• Atomic structure
• Interaction between Light and Matter
• Spectrum

7
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
8
Rainbow
9
Interaction of Light and Matter

• Emission
• Absorption
• Transmission (passing through)
• Reflection (scattering)

10
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) 109 m 1µm (micron) 106
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
11
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.
12
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
13
Electromagnetic spectrum
14
Electromagnetic spectrum
Frequency units Hertz 1 Hz 1 c?1
15
Light and Matter
The amount of light is called intensity
Studying spectra of celestial bodies one can
learn a wealth of information about them
16
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.
17
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.
18
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
19
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
20
Emission by Hydrogen
21
Hydrogen lines in the visible
22
Examples of Spectra
23
Thermal Radiation
Complex objects - planets, stars, people -
produce thermal radiation
Its spectrum depends only on the
objects 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
24
Temperature and Color
25
Temperature and Intensity
26
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
27
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
28
Doppler Effect
Demo Doppler effect
29
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