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

ConceptTestIn 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.

ConceptTestIn 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.

higher energy

lower energy

The Electromagnetic Spectrum

Spectroscopy: The quantitative analysis of spectra

The spectroscopic analysis of an object’s spectrum can reveal the object’s:

CompositionTemperatureVelocity

Spectroscopy: The quantitative analysis of spectra

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”)

Rules for Thermal Emission by Opaque Objects

5000 K

3000 K

4000 K

Wavelength

Ene

rgy

emitt

ed p

er s

quar

e m

eter

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?

At “room temperature”, or

“body-temperature”, objects emit their

peak radiation in the infrared.

The surface of the Earth emits

radiation in the infrared.

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

A

B

To

tal E

ne

rgy

outp

ut p

er

seco

nd

Wavelength

Which of the two stars (A or B) emits light that has a peak emission with the longer wavelength?

(red) Star A

(blue) Star B

(green) The stars’ peak emissions are at the same wavelength

(yellow) None of the above

VIBGYOR

visiblerange

A

B

Wavelength

Which of the two stars (A or B) emits light that has a peak emission with the longer wavelength?

(red) Star A

(blue) Star B

(green) The stars’ peak emissions are at the same wavelength

(yellow) None of the above

VIBGYOR

visiblerange

To

tal E

ne

rgy

outp

ut p

er

seco

nd

A

B

Wavelength

Which of the two stars (A or B) would appear red?

(red) Star A

(blue) Star B

(green) Neither would appear red

(yellow) There is insufficient information to determine the star’s color

VIBGYOR

visiblerange

To

tal E

ne

rgy

outp

ut p

er

seco

nd

A

B

Wavelength

Which of the two stars (A or B) would appear red?

(red) Star A

(blue) Star B

(green) Neither would appear red

(yellow) There is insufficient information to determine the star’s color

VIBGYOR

visiblerange

To

tal E

ne

rgy

outp

ut p

er

seco

nd

The figure shows the spectra of two stars. Which star is hotter?

(red) A

(blue) C

(yellow) neither

A

C

WavelengthVIBGYOR

visiblerange

To

tal E

ne

rgy

outp

ut p

er

seco

nd

The figure shows the spectra of two stars. Which star is hotter?

(red) A

(blue) C

(yellow) neither

A

CEn

erg

y o

utp

ut p

er

seco

nd

WavelengthVIBGYOR

visiblerange

To

tal E

ne

rgy

outp

ut p

er

seco

nd

A

CEn

erg

y o

utp

ut p

er

seco

nd

Wavelength

Which of the following is possible to infer about stars A and C based upon the information provided in the graph?

(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

VIBGYOR

visiblerange

To

tal E

ne

rgy

outp

ut p

er

seco

nd

A

CEn

erg

y o

utp

ut p

er

seco

nd

Wavelength

Which of the following is possible to infer about stars A and C based upon the information provided in the graph?

(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

VIBGYOR

visiblerange

To

tal E

ne

rgy

outp

ut p

er

seco

nd

“Matter” and Light

Atomnucleuselectron

e-

(proton,neutrons)

p+n

● 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.

Incorrect view

better view

Electrons do not “orbit” the nucleus; they are “smeared out” in a cloud which give the atom its size.

Atomic Number Element1 Hydrogen (H)2 Helium (He)3 Lithium (Li)4 Beryllium (Be)5 Boron (B)6 Carbon (C)7 Nitrogen (N)8 Oxygen (O)

The number of protons in the nucleus, i.e., the “atomic number”, determines the element

Relative abundances of elements in the universe

Every element has multiple isotopes

same number of protons (same element)

different numbers of neutrons