Very Basic Electromagnetism ASTR101 Mini-Lecture Mike Chu Montgomery College

The 4 fundamental forces of nature Gravity Electricity and Magnetism (Electromagnetism) Strong Nuclear Force Weak Nuclear Force Each force is uniquely responsible for certain interesting phenomena that we know. Q: Name a common EM phenomenon that we know or encounter every day!

Basic Static EM Static case Static charge: Electric Field (E-field) Static current (flow of charges): Magnetic Field (B-Field) A physics undergraduate can calculate E-field, B-field, the motion of a point charge in an E-field, etc. The charge carriers are electrons for negative charge and protons for positive charge. Others are simply composite of them. We will not worry about the subatomic component of protons and neutrons, known as quarks that carry -2/3 and 1/3 charge. CHEMSTRY is the quantum mechanical outcome of EM interactions of electrons of atoms/molecules

Dynamic EM Things are different when charges or currents accelerates (non-constant motion) through space Dynamic case – Electromagnetic Radiation All light that we see is the result of emission/radiation due to EM interactions (but keep in mind our eyes see only in the visible range) Light is the manifestation of EM Light is EM wave (traveling through space) In fact “visible light” is only a small part of the entire EM spectrum. “photon” is the language of modern physics and quantum mechancis Lights (photons) that Astronomers observe

An EM Wave – Classical View An EM wave travels through space, and does not (cannot) stop. It has both E-field and B-field components oscillating in unison as it propagates. Its propagation is described by (as in basic waves) frequency Wavelength velocity (speed) v = f l

A Photon – A Modern View What modern physics and quantum mechanics add to the further understanding is the concept of a “photon” that enabled a straight-forward energy relation Energy of a photon = planck’s constant * frequency Photons are real, and have been measured singly. The Higher the frequency the more energetic the photon. Ex: a gamma ray photon is more energetic than a red-light photon.

The Electromagnetic Spectrum Our human eyes see only the visible spectrum of the entire EM spectrum, which spans from Gamma to Long-Wave Radio. The visible spectrum is at its full glory when you see a rainbow after the rain or near a moist region. The water molecules disperse different frequencies into different directions, spreading out the original white light into the rainbow spectrum. But other frequencies are there too, just that we cannot see them. Despite having different numerical values (f and l), they are all EM waves obeying the same physical laws following the same equations.

Visible Light/Spectrum Also called “optical” (Rainbow Spectrum) Frequency: 430 – 770 Tera-hertz (Thz) Wavelength: 390 to 700 nm (0.00005 cm) This is the spectral range where the very long history of astronomical observations are made (until this past century when modern technology enabled other spectral range) Now: imaging, photometry, spectroscopy and polarimetry

Radio Waves Frequency: 3 kilo-hertz to 300 Giga-hertz (Ghz) Wavelength: 100 kilometer (km) to 1 millimeter Typical AM radio stations ~ 1000 khz  wavelength is 300 meters