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“Modern” Physics - Relativity
Young’s slit experiments showed conclusively that light had wave-like characteristics.
The next question was “What kind of wave is light?”
Until late 1800’s, light was thought to be a “matter” (or mechanical) wave that
traveled through the ether.
Michelson-Morley experiment disproved the existence of an ether. (They also
“proved” Einstein’s Special Theory of Relativity.)
Maxwell showed how light wasn’t a matter wave at all but rather a wave of
electromagnetic fields.
Electric and Magnetic Fields
An electric field exists wherever an electric charge experiences an electric force.
- Direction of force depends on whether charge is positive or negative
(A gravitational field exists wherever an object with mass experiences a gravitational force.)
A magnetic field exists wherever a magnet experiences a magnetic force.
- A compass aligns itself in the direction of the field
Fields are represented by drawings/constructions of field lines which represent the
direction and strength of the field.
Maxwell was interested in how quickly changes in the fields were felt at distances
away from the change.
- If you wiggle a charge (or a magnet) on one side of the room, how long before
charges (or compasses/magnets) on other side of the room “feel” this wiggle?
Magnetism
A magnetic field is the consequence of a moving electric charge.
A stationary charge has an electric field, but a moving charge has both an electric field
AND a magnetic field!
Charges in atoms are always moving at least a little bit.
- In non-magnetic materials, the charges move in random directions and thus the
magnetic fields created are random and cancel each other out.
- In magnetic materials, the charges move in similar directions, creating
magnetic
fields that are thus aligned and add together to create a large magnetic field.
- In materials that are easy to magnetize, the presence of an external magnetic
field aligns the charges and thus the fields inside the material, turning it into a
magnet.
Electric Current and Magnetism
Since moving charges create magnetic fields, and since an electric current is a steady
motion of electric charges (electrons!), electric currents always produce magnetic
fields!
Ampere figured this out. (including the equations that explain it.)
A current in a wire produces a magnetic field that encircles the wire. (See a
diagram…)
A current in a coil of wire produces a magnetic field that is identical to a bar magnet.
The current and the magnetic field are perpendicular (at right angles) to each other.
A coil of wire is called a solenoid. Each loop of the solenoid produces a magnetic
field, but, since the loops are aligned, the magnetic fields are aligned, and a strong
magnetic field is created. This is an electromagnet.
Other Applications
- electric meters: galvanometer, ammeter, voltmeter
- switches, relays
- microphones, speakers, buzzers, bells
- electric motors
- recording magnetic media: cassette tapes, floppy disks, video tape
- at a tiny level, computer memory!
Magnetism and Electric Current - Electromagnetic Induction
- Magnetic fields exert forces on moving charges
- A moving (changing) magnetic fields exert forces on stationary charges
- If those charges are in a wire, the magnetic field can
- cause a current in that wire, or, cause the wire to move, or, both!
The magnetic field and the current are perpendicular (at right angles) to each other.
Causing the wire to move creates a motor!
Causing a current in the wire is electromagnetic induction.
- This is the principle behind any electric generator.
- This is how we generate all the electricity we use. (dams, nuclear,
windmills…)
- KE turns or moves either
a wire coil in a magnetic field, or,
a magnetic field around a wire coil.
- Either effect creates current electricity!
Faraday figured this out. (including the equations that explain it.)
Maxwell’s “Breakthrough” – Maxwell’s Equations
Maxwell realized that these inter-related electric and magnetic forces were the result
of inter-related electric and magnetic fields.
These fields can exist whether or not there are any charges (electrons or protons) to
feel the fields.
Once created by some moving charge:
- the created magnetic field creates a new electric field
- the newly created electric field creates a new magnetic field
- this newly created magnetic field creates yet another new electric field
- etc!!! throughout space! Even empty space!
The result is an electromagnetic wave – a wave of electric and magnetic fields.
- Each field creates the other.
- The fields are mutually perpendicular to each other.
- The fields can move through a vacuum (empty space.)
Maxwell was able to show (mathematically) that these e.m. waves moved at 3x10 8
m/s.
The speed of light had already been measured to (also) be 3x108 m/s!
Hertz was able to experimentally demonstrate/prove Maxwell’s equations for radio
waves (though radio wasn’t a word or an invention yet!)
Physicists came to understand that there was a large range – a spectrum – of possible
electromagnetic waves.
This spectrum includes visible light!
The Electromagnetic Spectrum
Frequency of e.m. waves ranges from 1 Hz to 1024 Hz.
Wavelength of e.m. waves ranges from 108 m to 10-16 Hz.
Energy depends on frequency. High frequency -> high energy.
From lowest energy to highest:
power, AM, shortwave, TV, FM, radar, microwave, infrared, visible,
ultraviolet, x-ray, gamma ray
The Electromagnetic Spectrum (continued)
We communicate long distances via radio waves.
The universe (outer space!) radiates radio waves. (Thus, radio telescopes.)
Some microwave frequency matches the vibrational frequency of water! (We can cook
with these microwaves!)
Infrared frequencies match thermal vibration frequencies -> WARMTH!
UV A induces vitamin D formation. Can treat some skin disorders.
UV B reacts with the melanin in the skin causing tanning. Or burning.
UV C is high enough energy to cause mutations in cells (cancer.)
UV causes fluorescence. (e.g., fluorescent lighting.) Blacklights!
Glass is opaque to uv (and infrared also actually.)
X-rays are high enough energy to penetrate our soft tissues but not our bones.
These are also high enough energy to cause mutations.
X-rays and gamma rays are “ionizing” radiation.
Gamma rays come from radioactivity, nuclear fission and fusion.
Gamma rays can be used to kill cancer cells.
The small range of frequencies that effect the rods and cones of our eyes, we call
visible light.
ROYGBIV
ALL of these, no matter the frequency (or wavelength, or energy) travel at the same
speed – the speed of light. 3x108 m/s or 186,000 mph.
Reasons we care:
- Obvious….
- Less obvious but equally important
- Ozone (O3)
- created via…
- a trace gas (0.3 ppm)
- Ozone is toxic to us.
- Ozone is protective to us in the stratosphere.
- Myth: There is an ozone layer.
- Fact: The ozone is distributed through the stratosphere
- Absorbs UVB and UVC
- CFC’s (chlorofluorocarbons) are known to destroy ozone
- CFC concentration vs ozone depletion
- South Pole sees the biggest effect
- Montreal Protocol (Ozone Treaty)
- Despite detractors, this is real science - measurable, testable
and, ultimately, worth a Nobel Prize in Chemistry
- Global Warming
- Earth’s atmosphere is transparent to visible light but is
opaque in infrared.
- This infrared is absorbed by the atmosphere, warming the air
and planet.
- This is the greenhouse effect.
- H2O, CO2, methane (CH4) are good i.r. absorbers, thus are called
greenhouse gases.
- The greenhouse effect makes life as we know it possible.
- A greenhouse effect out of balance will change life as we know it.
- 1800, CO2 concentration 280 ppm; today, >370 ppm (>30% increase)
- Increased CO2 concentration from:
- Resulting warming of Earth = about 1oF (0.6oC)
=> Global Warming
- Kyoto Treaty
- Greater economic impact
- Greater environmental impact
- U.S. is in? or out?