electric charge and field basics fundamental charges are carried by electrons (negative) and protons...

Electric Charge and Field Basics

Fundamental charges are carried by electrons (negative) and protons (positive).

Charge on the electron is – 1.6 x 10-19 C

Charge on the proton is + 1.6 x 10-19 C

Like sign charges repel, opposite sign charges attract

Ability to exert electric forces on other charges is represented by the electric field of the charge

Direction of the electric field is that same as the direction of the electric force that would be exerted on a positive charge when placed in the electric field

+ -Away from positive

Toward negative

EMWaves

Electric Charge and Field Basics

Fundamental charges are carried by electrons (negative) and protons (positive).

Charge on the electron is – 1.6 x 10-19 C

Charge on the proton is + 1.6 x 10-19 C

e = 1.6 x 10-19 CCalled the elemental charge

Current and Magnetic Field Basics

A moving charge is called a current

Moving charges have the ability to exert a second type of force on other moving charges.

This force is called a magnetic force.

Ability to exert magnetic forces on other charges is represented by the magnetic field of the current

Direction of the magnetic field is determined by a Right Hand Rule (will not be used in this course)

Electromagnetic Wave Basics

If a charge is made to jiggle (or more appropriately accelerate) it will produce a varying electric field. If the charge oscillates with a regular period, it will produce an oscillating electric field, with all the characteristics of a wave.

v

Charge

Faraday’s Law: A changing magnetic field will create an electric field. The direction of the electric field will be perpendicular to the direction of the magnetic field.

Maxwell’s Proposal: A changing electric field will create a magnetic field. The direction of the magnetic field will be perpendicular to the direction of the electric field.


James C. Maxwell (1831 – 1879): In 1864 presented a theory which combined both of these effects into one entity called the electromagnetic wave.

Since one field was “created” by the other, there was no need for a medium in Maxwell’s version of electromagnetic theory.

In order to conserve energy, Maxwell calculated that ALL electromagnetic waves travel at the same wave speed, 3 x 108 m/sec.

c = 3 x 108 m/sec


Special Property of EM waves:

* The can be created in and move though a vacuum

* No medium is required for an EM wave to exist.

* EM waves are the only wave phenomena that can exist without a medium


Electromagnetic wave phenomena

* Gamma Rays

* X-Rays

* Radio Waves

* Microwaves

* Ultraviolet (UV) waves

* Infrared (IR) waves

* Light


10000 10-8.00011 10-12

Wavelength in meters


The Electromagnetic Spectrum

Radio

Microwave

IR

UV

X-Ray

10000 10-8.00011 10-12




The Visible Window



The range of wavelengths detectible by the average human eye

Shortest detectible wavelength ≈ 400 mn

Longest detectible wavelength ≈ 700 mn


Transparency of the Earth’s Atmosphere

10000 10-8.00011 10-12




Very transparent Partially transparent

10000 10-8.00011 10-12




Very transparent Partially transparent

Radio

Microwave

IR

UV

X-Ray



Implications:

Earth based astronomy is based upon the receipt and analysis of electromagnetic waves. Astronomical observations can only be carried out at wavelengths that can penetrate through the atmosphere to telescopes.

Astrophysics and Cosmology

The Wavelengths are the same in all directions only if the atoms in the gas are not moving.

Doppler Shift

, f , f


The Wavelengths get “squished” in the direction of motion, and “stretched” in the opposite direction.

Doppler Shift

Longer , lower f

Shorter , higher f


In which direction is the bird moving?

Doppler Shift

Astrophysics and CosmologyDoppler Shift

Longer , lower f

Shorter , higher f

In the visible spectrum, the color blue is associated with the shorter wavelength end of the visible window. The squishing of the wave to a shorter wavelength is called a “blue shift”


Longer , lower f

Shorter , higher f

In the visible spectrum, the color red is associated with the longer wavelength end of the visible window. The stretching of the wave to a longer wavelength is called a “red shift”


Longer , lower f

Shorter , higher f

“Red Shift” “Blue Shift”


Longer , lower f

Shorter , higher f

In general

Apparent Wavelength

True Wavelength Apparent Frequency

True Frequency Velocity of Source

Wave Speed= = 1 +

Note: If the source and detector are moving apart, the Velocity of the Source is POSITIVE. If the source and detector are toward one another, the Velocity

of the Source is NEGATIVE.

“Red Shift” “Blue Shift”

electric charge and field basics fundamental charges are carried by electrons (negative) and protons...

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