Transcript Lecture7

Vesta, the second
largest object in
the asteroid belt,
was recently
imaged for the first
time by the robotic
Dawn satellite that
arrived last month.
Homework #3
Will be posted soon
The first exam has been shifted to Monday,
October 3.
Don’t confuse
mass and weight
Weight is the
force due to
gravity that acts
on a given mass
Same mass
different weight
ConceptTest
The gravitational potential energy of an interstellar
cloud of gas that is slowly shrinking in size …
(yellow) stays the same
(red) slowing changes into other forms of energy
(green) slowly increases
(blue) interstellar clouds do not have gravitational
potential energy
ConceptTest
The gravitational potential energy of an interstellar
cloud of gas that is slowly shrinking in size …
(yellow) stays the same
(red) slowing changes into other forms of energy
(green) slowly increases
(blue) interstellar clouds do not have gravitational
potential energy
ConceptTest
In the not too distant future scientists will develop an
engine that produces more energy than it uses. This
statement is …
(yellow) likely to be true
(red) false
(green) difficult to determine if this will happen or not.
(blue) this has already happened, but the government is
suppressing this discovery.
ConceptTest
In the not too distant future scientists will develop an
engine that produces more energy than it uses. This
statement is …
(yellow) likely to be true
(red) false
(green) difficult to determine if this will happen or not.
(blue) this has already happened, but the government is
suppressing this discovery.
Radiative energy:
energy carried by
electromagnetic
waves (light).
Properties of Waves
WAVELENGTH (: Distance
between adjacent crests
FREQUENCY (f): number of crests that pass through a point
each second. It is measured in units of hertz (Hz), which are
the number of cycles per second.
AMPLITUDE: A measure of the strength of the wave.
SPEED (s): how fast the wave pattern moves.
For any wave:
s=f
Light as a Wave
• The speed of light is a constant: s = c !!!
• Therefore, for light:
f=c
• The higher f is, the smaller  is, and vice versa.
• In the visible part of the spectrum, our eyes recognize
f (or ) as color!
Light as a Particle
 Light can also be treated as photons – packets of energy.
 The energy carried by each photon depends on its frequency
(color)
 Energy:
E = hf = hc/  [“h” is called Planck’s Constant]
Shorter wavelengths:
more energy per photon.
The Electromagnetic Spectrum
lower
energy
higher
energy
Spectroscopy: The quantitative analysis of spectra
Spectroscopy: The quantitative analysis of spectra
The spectroscopic analysis of an object’s spectrum
can reveal the object’s:
Composition
Temperature
Velocity
Four Ways in Which
Light can Interact with Matter
1.
emission – matter releases energy as light
2.
absorption – matter takes energy from light
3.
transmission – matter allows light to pass through it
4.
reflection – matter reflects light
The type of interaction is determined
by characteristics of the “matter” and
the wavelength of light.
Different
wavelengths
of light
interact
differently
with the
atmosphere
Three types of spectra
 Continuous spectra
 Absorption spectra
 Emission line spectra
An example: stars have many colors
 Continuous spectra
are usually related to the
temperature of an object that is
emitting radiation.
 Absorption & emission
line spectra are related to the
composition of the material
absorbing or emitting radiation.
Thermal Emission
A hot, dense glowing object (solid or gas)
emits a continuous spectrum.
1. Hotter objects emit
more total radiation
per unit surface
area.
2. Hotter objects have
peak emissions at
shorter wavelengths
(they will appear
“bluer”)
Energy emitted per square meter
Rules for Thermal Emission by Opaque Objects
5000 K
4000 K
3000 K
Wavelength
The sun emits its
peak radiation in
the yellow portion
of the visible
spectrum.
The human eye has its peak
sensitivity at the same wavelength.
Coincidence?
infrared
At “room
temperature”, or
“body-temperature”,
objects emit their
peak radiation in the
infrared.
The surface of the
Earth emits
radiation in the
infrared.
visible
Extremely hot objects will emit
most of their radiation in the
ultraviolet, x-ray or even the
gamma ray portion of the spectrum
Intermission time
Which of the two stars (A or B) emits light that has a
peak emission with the longer wavelength?
(blue) Star B
(green) The stars’
peak emissions are at
the same wavelength
(yellow) None of the
above
Total Energy output per second
(red) Star A
visible
range
A
B
VIBGYOR
Wavelength
Which of the two stars (A or B) emits light that has a
peak emission with the longer wavelength?
(blue) Star B
(green) The stars’
peak emissions are at
the same wavelength
(yellow) None of the
above
Total Energy output per second
(red) Star A
visible
range
A
B
VIBGYOR
Wavelength
Which of the two stars (A or B) would appear red?
(blue) Star B
(green) Neither would
appear red
(yellow) There is
insufficient
information to
determine the star’s
color
Total Energy output per second
(red) Star A
visible
range
A
B
VIBGYOR
Wavelength
Which of the two stars (A or B) would appear red?
(blue) Star B
(green) Neither would
appear red
(yellow) There is
insufficient
information to
determine the star’s
color
Total Energy output per second
(red) Star A
visible
range
A
B
VIBGYOR
Wavelength
(red) A
(blue) C
(yellow) neither
Total Energy output per second
The figure shows the spectra of two stars.
Which star is hotter?
visible
range
A
C
VIBGYOR
Wavelength
(red) A
(blue) C
(yellow) neither
per second
output
Energy
Total
per second
output
Energy
The figure shows the spectra of two stars.
Which star is hotter?
visible
range
A
C
VIBGYOR
Wavelength
(red) Star A is
smaller than star C
(blue) Star A is
larger than star C
(green) The stars
are the same size
(yellow) It is not
possible to infer
any of these
relationships
per second
output
Energy
Total
per second
output
Energy
Which of the following is possible to infer about stars A
and C based upon the information provided in the graph?
visible
range
A
C
VIBGYOR
Wavelength
(red) Star A is
smaller than star C
(blue) Star A is
larger than star C
(green) The stars
are the same size
(yellow) It is not
possible to infer
any of these
relationships
per second
output
Energy
Total
per second
output
Energy
Which of the following is possible to infer about stars A
and C based upon the information provided in the graph?
visible
range
A
C
VIBGYOR
Wavelength
“Matter” and Light
nucleus
Atom
electron
(proton,neutrons)
p+
n
●
●
●
e-
10,000,000 atoms can fit across a period in your textbook.
The nucleus is nearly 100,000 times smaller than the entire atom (if
atom filled the classroom auditorium, the nucleus would be barely
visible at its center).
Although it is the smallest part of the atom, most of the atom’s mass
is contained in the nucleus.
Electrons do not “orbit” the nucleus; they are “smeared
out” in a cloud which give the atom its size.
Incorrect
view
better
view
The number of protons in the nucleus, i.e., the
“atomic number”, determines the element
Atomic Number
1
2
3
4
5
6
7
8
Element
Hydrogen (H)
Helium (He)
Lithium (Li)
Beryllium (Be)
Boron (B)
Carbon (C)
Nitrogen (N)
Oxygen (O)
Relative abundances of
elements in the universe
Every element has multiple isotopes
 same number of protons (same element)
 different numbers of neutrons