NEWS: March 12 Cassini flys above Enceladus Plumes

Solar System ExplorationSolar System Exploration

Mission Type Launch Date MilestoneCassini Flagship 15 Oct 1997 Arrival 1 July 04Dawn (Vesta & Ceres) Discovery 27 Sep 2007Deep Impact Discovery 12 Jan 2005 Tempel impact 05Mars Rovers (Spirit & Opportunity)Mars Scout Summer 2003 Land Jan 04Mars Express (ASPERA instru) Mars Scout 2 June 2003 Arrival Dec 03Mars Odyssey Mars Scout 7 April 2001 Arrival Oct 01Mars Reconnaissance Orbiter Mars Scout 12 Aug 2005 Arrival Aug 05Mercury MESSENGER Discovery 3 Aug 2004 Arrival 2011Pluto New Horizons New Frontiers 19 Jan 2006 Arrival 2015Phoenix Discovery 4 Aug 2007 Arrival 25 May 08Stardust Discovery 7 Feb 1999 Sample return 06Voyager Flagship Summer 1977 Neptune 1989

Apollo 16 (1972)Apollo 16 (1972)

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Apollo 16 LandingApollo 16 Landing

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Carbon Dating Carbon Dating

1. Solar neutrons enter Earth’s atmosphere.

2. Neutrons collide with N14 (7p, 7n), creating C14 (6p,8n) [n + N14 p + C14 ]

3. Living bodies continually absorb C14 (e.g. as CO2 in photosynthesis).

4. When the plant or animal dies, it no longer assimilates C14.

5. The C14 decays (half life of 5730 yrs). [C14 N14 + e- + ve ] (n p + e- + ve )

6. The e- emission rate reveals the age.

Measure ages < 70,000 yrs

Carbon Dating first tested Carbon Dating first tested

Radiocarbon dating was discovered by Willard Libby in 1949.

He first demonstrated the accuracy by accurately dating a the wood from an ancient Egyptian royal barge whose age was known from historical documents.

In 1960, Libby received the Nobel Prize in Chemistry for his work

Potassium-Argon (K-Ar) Dating

Potassium-Argon (K-Ar) Dating

40K -> 40Ar + 40Ca

Half life of 1.26 billion years

Because Ar is a gas, it can escape from molten rock.However after the rock solidifies 40Ar produced by potassium decay begins to accumulate in crystal lattices. The ratio of 40Ar to 40K tells us the time elapsed since the rock solidified and was able to trap Ar

Can date the solidification of rocks 100,000 yrs old at 1% accuracy.

Radioactive Dating SystemsRadioactive Dating Systems Parent

Carbon-14

Daughter

Nitrogen-14

Half Life

5,730 yrs*

Potassium-40 Argon-40 1.25 billion yrs

Uranium-238 Lead-206 4.5 billion yrs

Thorium-232 Lead-208 14 billion yrs

Rubidium-87 Strontium-87 48.8 billion yrs

Samarium-147 Neodymium-143 106 billion yrs

Uranium-235 Lead-207 704 billion yrs

*Time that it takes wood to have half the 14C of a living plant.

Swisher et al. 1992, Science

The Electric ForceThe Electric ForceLike Charges Repel, Opposites Attract Coulomb’s Law

The force between two charged objects is proportional to the product of their charges divided by the square of the distance between them.

Charge unit is Coulomb, denoted C.

1 proton has a charge of 1.7x10-19 C

1 electron has a charge of -1.7x10-19 C

K = 9x109 N-m2/C2 is Coulomb’s constant

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Notice the similarityNotice the similarity

Electric Force:

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Dipole and Monopole fieldsDipole and Monopole fields

Monopole FieldDipole Field

A dipole field can be created with multiple electric charges, but monopole magnetic fields do not exist.

Electric Field of a proton Magnetic Field

Changing currents are changing electric fieldsChanging currents are changing electric fields

Electric Field

In metals like copper, electrons are free to move.

+-

electrons

Direction of electron motion

Direction of current is that for a positron

Electric Currents Produce Magnetic Fields

Electric Currents Produce Magnetic Fields

While giving a lecture to introductory physics students, Hans Oersted noticed that a compass needle twitched every time that he turned on an electric circuit. Further experimentation showed that a magnetic field was produced whenever a current flowed.

Shape of the magnetic field for wire arrangements.

Magnetic InductionMagnetic InductionImagine a coil of wire in a magnetic field, as shown to the right. The magnetic flux is proportional to the number of lines that go through the electric circuit. If the plane of the coil of wire is 90° to the B field, then the magnetic flux is large. If the coil were rotated 90 degrees, so that no B field lines intersected the coil, the flux would be zero.

If circuit contains N coils of wire rather than 1, the flux is increased by a factor of N.

Magnetic InductionMagnetic InductionImagine a coil of wire in a magnetic field, as shown to the right. The magnetic flux is proportional to the number of lines that go through the electric circuit. If the plane of the coil of wire is 90° to the B field, then the magnetic flux is large. If the coil were rotated 90 degrees, so that no B field lines intersected the coil, the flux would be zero.

If circuit contains N coils of wire rather than 1, the flux is increased by a factor of N.

A changing magnetic flux induces a current in the wire.

Magnetic InductionMagnetic Induction

The magnetic flux is proportional to the number of lines that go through the electric circuit hoop.

As the magnet is moved closer to the hoop the flux increases and a current results. If circuit contains N coils of wire rather than 1, the flux is increased by a factor of N.

Summary of E&M fundamentals

Summary of E&M fundamentals

• Coulomb’s Law: like charges repel and opposites attract:

• Oersted’s Discovery: A changing electric field creates a magnetic

field.

• Faraday’s law: A changing magnetic flux creates an electric field.

• There are no magnetic monopoles.

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Consider thisConsider thisA changing magnetic field creates an electric field. A changing electric field creates a magnetic field, which then creates another electric field, and so on in leap-frog fashion.

Electricity and magnetism are two aspects of the same force – electromagnetism.

What does this mean?

What are these constantly changing electric and magnetic fields?

The Electromagnetic Spectrum

The Electromagnetic Spectrum

Vibrating electric & magnetic fields regenerate each

other and make up an electromagnetic

wave.

Vibrating electric & magnetic fields regenerate each

other and make up an electromagnetic

wave.

Electromagnetic waves are characterized by their wavelength

Electromagnetic waves are characterized by their wavelength

Waves: definitionsWaves: definitions The peak to peak distance

is the wavelength, . The time that it takes for a

wave to oscillate from peak to peak is the period, T.

The number of waves that pass a point per unit time is called the frequency, v:

= 1/T

The wave’s speed, u, is: u = / T =

The peak to peak distance is the wavelength, .

The time that it takes for a wave to oscillate from peak to peak is the period, T.

The number of waves that pass a point per unit time is called the frequency, v:

= 1/T

The wave’s speed, u, is: u = / T =

Velocity of lightVelocity of light

Light travels at a velocity c=2.99x108 m/s in a vacuum.Light travels at a velocity c=2.99x108 m/s in a vacuum.

Is this a little odd?

Light: both waves and particles

Light: both waves and particles

Light acts like a bunch of particles: It is absorbed by molecules as discrete particle packages.

Light acts likes waves: e.g. it can be refracted and can combine (or superpose).

Yes, very!Yes, very!Imagine standing on the top of a train moving at a velocity vTrain and shining a flashlight towards someone in front of the train on the ground. The velocity of light relative to the train is vLight = c. The velocity of the light received by the person on the ground is c too, not c+v!

vTrain

vBall vBall+vTrain

vTrain

vLight vLight

ConsequencesConsequences

Length contraction: objects in motion are shorten in the direction of motion

Time dilation: Time slows with motion but this is only apparent to someone in a different reference frame because the moving clock slows too. This has been tested by flying atomic clocks on fast planes. The flown clock recorded the passage of less time, compared to the not-flown clock!

Photons: the particles of light

Photons: the particles of light

The energy of a photon depends solely on its frequencyThe energy of a photon depends solely on its frequency

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Planck’s constant: h = 6.626 x 10-34 J s

speed of light

= 2.998 x 108 m/sPhoton frequency

Photon wavelength

Radio Waves Radio Waves

Radio waves have useful properties:

1) Travel far through the atmosphere

2) Easily generated by setting up currents in metal antennas

3) They bounce off the ionosphere, and thus travel long distances around Earth.

AM = Amplitude modulation

FM = Frequency modulation

MicrowavesMicrowaves

Microwave photo of the Amazon

Cosmic microwave background

Microwaves are not obstructed by clouds, haze or any other particle smaller than the microwave wavelengths (~ 1 cm). They can be used to image Earth’s surface through the clouds that cover ½ the disk.

Infrared RadiationInfrared Radiation

We emit IR light. In fact all the objects around us emit IR radiation. Hotter objects emit more IR radiation, which we are warmed by, and which we sense with our skin.

We emit IR light. In fact all the objects around us emit IR radiation. Hotter objects emit more IR radiation, which we are warmed by, and which we sense with our skin.

IR photo of a man

IR photo of the dust in the Solar System

Wavelength: 1 um – 1000 um

Visible LightVisible Light

We see the very part of the electromagnetic that the Sun emits. That is we have detectors that are sensitive the radiation that illuminates our world.

We see the very part of the electromagnetic that the Sun emits. That is we have detectors that are sensitive the radiation that illuminates our world.

White light is made up of light from all different colors

Ultraviolet LightUltraviolet Light

Ultraviolet light has just the right energy to break molecular bonds apart. For this reason it is detrimental to life. The Earth has a natural shield to Solar UV light in the form of an ozone layer (80 km above the surface).

Some birds & bees can see UV as well as visible light.

10% of sunlight is in the UV

X-raysX-rays

X-rays were discovered 1895 by Wilhelm Conrad Roentgen (a German scientist) by accident. A week later he took this x-ray of his wife.

Gamma RaysGamma Rays

Gamma Ray Bursts:

Can release more energy in 10 seconds, then released by the Sun in 10 billion yrs.

Their source is unknown.

Wavelength: 10-12 – 10-13 m

Gamma Rays are the most energetic of all electromagnetic waves. These EM waves are therefore emitted in the hottest events.

SummarySummary Changing electric fields produce changing

magnetic fields, which produce changing electric fields, which propagate as waves.

These waves are called EM waves. EM waves behave as particles & as waves They are characterized by their

wavelength. The energy of light particles depends on

the frequency (E = hv), or equivalently, their wavelength.

Light travels at 2.99x108 m/s in a vacuum.

Changing electric fields produce changing magnetic fields, which produce changing electric fields, which propagate as waves.

These waves are called EM waves. EM waves behave as particles & as waves They are characterized by their

wavelength. The energy of light particles depends on

the frequency (E = hv), or equivalently, their wavelength.

Light travels at 2.99x108 m/s in a vacuum.

An expert is a man who has made all the mistakes, which can

be made in a narrow field.

An expert is a man who has made all the mistakes, which can

be made in a narrow field.

Neils BohrNeils Bohr

A theory which unifies allA theory which unifies allRelativistic Quantum Theory can explain the electromagnetic, weak and strong force, as long as gravity is ignored.

General relativity works when the quantum behavior of nature is ignored.