Electric Charge and Field Basics Fundamental charges are carried by electrons (negative) and protons (positive). Charge on the electron is – 1.6 x C Charge on the proton is x 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 C Charge on the proton is x C e = 1.6 x C Called 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. Electromagnetic Wave Basics

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 10 8 m/sec. c = 3 x 10 8 m/sec Electromagnetic Wave Basics

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 Basics

Electromagnetic wave phenomena * Gamma Rays * X-Rays * Radio Waves * Microwaves * Ultraviolet (UV) waves * Infrared (IR) waves * Light Electromagnetic Wave Basics

Wavelength in meters Electromagnetic Wave Basics The Electromagnetic Spectrum Radio Microwave IR UV X-Ray

Wavelength in meters Electromagnetic Wave Basics The Electromagnetic Spectrum The Visible Window

Electromagnetic Wave Basics The Electromagnetic Spectrum The range of wavelengths detectible by the average human eye Shortest detectible wavelength ≈ 400 mn Longest detectible wavelength ≈ 700 mn

Electromagnetic Wave Basics Transparency of the Earth’s Atmosphere

Wavelength in meters Electromagnetic Wave Basics Transparency of the Earth’s Atmosphere Very transparentPartially transparent

Wavelength in meters Electromagnetic Wave Basics Transparency of the Earth’s Atmosphere Very transparentPartially transparent Radio Microwave IR UV X-Ray

Wavelength in meters Electromagnetic Wave Basics Transparency of the Earth’s Atmosphere Very transparentPartially transparent Radio Microwave IR UV X-Ray

Electromagnetic Wave Basics Transparency of the Earth’s Atmosphere 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

Astrophysics and Cosmology The Wavelengths get “squished” in the direction of motion, and “stretched” in the opposite direction. Doppler Shift Longer, lower f Shorter, higher f

Astrophysics and Cosmology In which direction is the bird moving? Doppler Shift

Astrophysics and Cosmology Doppler 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”

Astrophysics and Cosmology Doppler 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”

Astrophysics and Cosmology Doppler Shift Longer, lower f Shorter, higher f “Red Shift”“Blue Shift”

Astrophysics and Cosmology Doppler Shift Longer, lower f Shorter, higher f In general Apparent Wavelength True WavelengthApparent Frequency True FrequencyVelocity 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”