PHY 102 – Atoms to Galaxies

Our early human ancestors most

certainly looked at the night sky,

and wondered.

PHY 102 – Atoms to Galaxies

Chapter 8: Light & Electromagnetism

Waves

WaveNot a material object, but a moving pattern: bumps on the surface of water, deformations of music strings, variations in air pressure, oscillations of electromagnetic fields, etc.

Two types of waves:

1. Transversal: Waves with propagating direction perpendicular to the oscillation direction.

2. Longitudinal: Waves with propagating direction parallel to the oscillation direction.

Two principles:

1. Speed: Waves move at a constant speed that is determined by the medium where they travel, rather than the waves themselves.

2. Superposition: If two or more waves arrive simultaneously at the same place, the resulting effect is simply the sum of the effects of the waves.

Speed of soundin various media:

Moving bumps and periodic waves (http://members.aol.com/nicholashl/waves/waves.htm)

wavelength (m): length of the wavefrequency f (/s): number of oscillations over time

faudible 20 to 20,000 Hz (humans)

faudible < 20 Hz

faudible > 20,000 Hz

wavelength (m)frequency f (1/s) faudible 20 to 20000 Hz

wave speed v = f

vsound = 340 m/s

vlight = 300 000 000 m/s = 300 000 km/s

= 186 450 mi/s

If Earth-Sun distance is 92 million miles

(150 million km), how long for the light from the Sun to reach the Earth?

17th century physics: planets

Many 17th century scientists did not believe in “speed of light.”

Galileo … 1670's, the Danish astronomer Ole Roemer discovered that Io didn't always appear where it was supposed to be.

c = 300 000 km/s 1888, H. Hertz generated

EM waves in his lab.

Christian Huyghens

In 1673 reported synchronization between two pendulum clocks hanging on the same wall.

Superposition: If two or more waves arrive simultaneously at the same place, the resulting effect is simply the sum of the effects of the waves.

Interference: Result of different waves traveling through the same medium interacting with one another.

Interference

When periodic waves arrive at the same place from two synchronized sources, or from the same source but traversing two different paths, they produce an interference pattern.

Interference

Difraction:

Process by which waves spread out as a result of passing a narrow aperture, or across an edge, typically accompanied by interference between the wave forms produced beyond the aperture or edge.

Difraction: Thomas Young, 1803.

Light: Particle or Wave? From the mid-1660s on Newton conducted a series of experiments on the composition of light, and established the modern study of optics. He adopted the corpuscular theory of light according to which light is made of tiny particles emitted in all directions by a source.

The theory explained well reflection: a reflecting force would push the light particles away from the surface.

Reflection

Light: Particle or Wave?

Newton discovered that white light is composed of the same system of colors that can be seen in the rainbow.

Refraction

Light: Particle or Wave?

Newton’s corpuscular theory of light had a few difficulties, such as explaining refraction.

Light: Particle or Wave?

From the diffraction experiment with light there is good evidence that light is a wave.

Light

George McCoy

So the concept of light as a wave goes beyond the visible spectrum.

Question: If light is a wave, what medium is light

traveling through on its way from the

Sun to Earth? This and other questions were being asked in the 1800’s. The answer is intrinsically related to electricity and magnetism.

Light as a wave:

Light is an electromagnetic wave traveling through an

electromagnetic field.

Electricity

Form of energy resulting from the existence of charged particles, such as electrons or proton.

(Thesaurus Dictionary)

From experiments we know that the charge of the proton (+e) exactly equals the charge of the electron (-e), where e = 1.6 x 10-19 coulomb.

Electricity

Law of conservation of electric charge: During any process, the net electric charge of an isolated system remains constant (is conserved).

Fundamental characteristic of electric charges:

Like charges repel and unlike charges attract each other.

Electricity

Coulomb’s Law:The magnitude of the electrostatic force exerted by one point charge q1 on another point charge q2 is directly proportional to the magnitude of the charges and inversely proportional to the square of the distance r between them:

F = k q1 q2 / r 2

where k = 9 x 109 N m2/C2.

Magnetism

Two nearby bar magnets either attract or repel each other.

The ends of a bar magnet are called north and south magnetic poles.

Each bar magnet has two poles. “Monopoles” are yet to be found.

Magnetism

Fundamental characteristic of magnetic poles:

Like poles repel and unlike poles attract each other.

This attraction (or repelling) force is a new type called magnetic force.

Magnetism

Experiments show that electrically charged objects that are moving exert and feel an additional force beyond the electric force that exists when they are at rest. This additional force is the magnetic force.

All magnetic forces are caused by charges in motion.

The Electric Atom

The planetary model of the atom depicts the atom as almost entirely empty, divisible and made of many parts.

Nucleus (constituted of neutrons and positively charged protons) surrounded by tiny negatively charged electrons.

The Electric Atom

Order of magnitude: the overall size of an atom is about 10-10 m.

The nucleus is about 10,000 times smaller than the atom.

A scaled-up model of the atom with a nucleus the size of a soccer ball would have the electrons as dust specks several kilometers away.

The Electric Atom

Order of magnitude: the overall size of an atom is about 10-10 m.

The nucleus is about 10,000 times smaller than the atom.

A scaled-up model of the atom with a nucleus the size of a soccer ball would have the electrons as dust specks several kilometers away.

Faraday’s Law

When a wire loop is placed in the vicinity of a magnet and when the loop or the magnet is moved, an electric current is created within the loop for a long as the motion continues,

or,

A changing magnetic field creates an electric field.

The principle of electric power generation

Chapter 9: Electromagnetic Radiation

Electromagnetic radiation

James C. Maxwell, Scottish, in the 1860s developed a theory that unified electricity and magnetism.

Electromagnetic radiation

Every vibrating charged object creates a disturbance (wave) in its own electromagnetic field. This disturbance spreads outward through the field at light-speed, 300,000 km/s . Light is just such an electromagnetic wave.

Electromagnetic radiation

Heinrich Hertz, German, in the 1890s demonstrated experimentally that electromagnetic waves can travel in space and induce oscillations at a distance from where they were generated.

Electromagnetic radiation

Guglielmo Marconi, Italian, in the late 1890s developed wireless telegraphy which became the basis for the radio and television revolution.

Ether

The Newtonian clockwork model had no room for new phenomena like light traveling in ‘empty’ space ether Hypothetical medium for transmitting light and heat (radiation), filling all unoccupied space.

• all attempts to demonstrate its existence, most notably the Michelson-Morley experiment of 1887, produced negative results

Field

The special theory of relativity, proposed by Albert Einstein in 1905 eliminated the need for a light-transmitting medium.

• Field: a region of space characterized by a physical property, such as gravitational or electromagnetic force or fluid pressure, having a determinable value at every point in the region.

Solar Radiation and Earth

Solar Radiation and Earth

Solar Radiation and Earth

Ozone DepletionOzone depletion: 1928, GMC, inert chlorofluorocarbons (CFCs)Where are CFCs going? Crutzen, Molina, Rowland (1974)

u-v radiaton Chlorine (Cl)

Cl + O3 ClO + O2

ClO + ClO + sunlight Cl + Cl + O2

A single Cl atom destroys about 100,000 ozone molecules!

Ozone Depletion: Susan Solomon, 1986

Global Warming

Hypothetical Earth with normal atmosphere except for no greenhouse gases.

Global Warming

Realistic Earth with normal atmosphere including trace amounts of greenhouse gases.

Global Warming

Global Warming

Atmospheric concentration of carbon dioxide between 1000 and 2003.

Global Warming

Global Warming

Global Warming

Carbon dioxide concentration

and

temperature

Global Warming

Global Warming Consequences

1. Climate zones shift about 500 km away from Equator

2. 1.25 million species extinction by 20503. Accelerate Greenland and Antarctic ice melting4. Increase rate of sea level rise5. Mixed agricultural effects6. Thrive of mosquitoes and other disease vectors7. Northward spread of tropical diseases

(malaria, dengue)

Global Warming: What to do?

1997, Kyoto: Industrialized nations agreed to reduce their greenhouse gases emission 5% below 1990 levels by the year 2012.

1. As of 2001, emissions were 10% above 1990 levels

2. Actually, 60%-80% reduction is needed3. The U.S. with 4.5% of world’s population

emits 23% of world’s carbon, did not sign the Kyoto Protocol.

Global Warming: Precautionary Principle

Scientific uncertainty should not be a reason to postpone measures to prevent harm.

Chapter 10: Special Theory of RelativityIn the late 1800s it was believed that the era of new discoveries in fundamental physics was likely ended. The future would be to improve the accuracy of known results. But …

In 1900 the German physicist Max Planck introduced a revolutionary idea, the quantum of energy. The new idea was hardly noticed, initially.

Special Theory of Relativity

In 1905 a different but also revolutionary idea was introduced by a patent clerk in Switzerland, Albert Einstein.

Galilean Relativity

Relative motionReference frame

vball/train = 20 m/s

vtrain/ground = 70 m/s

vball/ground = ?

Galilean Relativity

Relative motionReference frame c = 300,000 km/s

vlight/ship = c

vship/ground = 0.25 c

vlight/ground = ?

The Principle of Relativity

Every non-accelerated observer observes the same laws of nature,

or

No experiment performed within a sealed room moving at an unchanging velocity can tell you whether you are standing still or moving.

The Principle of the Constancy of Lightspeed

The sped of light (any electromagnetic radiation) in empty space is the same for all nonaccelerated observers, regardless of the motion of the light source or the observer.

1964, fast subatomic irradiating particle

experimental evidence.

Michelson-Morley experiment, 1887, ether

Time is relative

If the speed (distance over time) of light is constant, then perhaps we need to revisit the concepts of distance and time.

Let’s measure time with a light clock from two distinct reference frames.

Mort and Velma use identical clocks and measure different time intervals for the same event: the relativity of time.

Mort and Velma use identical clocks and measure different time intervals for the same event: the relativity of time.

€

t

tM = time interval measured by Mort

tM = time interval measured by Velma

v = speed of ship (relative speed)

c = lightspeed

€

tM =tV1

1−v2

c2

Duration of one clock tick (1 second in the clock’s reference frame) on a clock moving relative to the reference frame.

Muon ’s lifetime has been shown to be different depending on their speed.

Fast muons live longer.

Time Travel: Back to the Future?

Velma (assumed same age as Mort) travels away at 0.75c relative to Mort and comes back after 60 years on Mort’s clock. How old will she be upon her return?

Relativity of Space and Mass

Time and space are tangled up with each other.

Length contraction

Space is different for different observers.

Mort and Velma use identical meter sticks and measure different lengths for the same object: the relativity of space.

LM = lenght measured by Mort

LM = length measured by Velma

v = speed of ship (relative speed)

c = lightspeed

€

LM = LV 1−v 2

c 2