The amplitude of a wave is a measure of the maximum disturbance in the medium during one wave cycle. (the maximum distance from the highest point of the crest to the equilibrium). The wavelength (denoted as λ) is the distance between two sequential crests (or troughs). This generally has the unit of meters. A wavenumber Period Phase velocity:
Light as a Wave (1) Light waves are characterized by a wavelength and a frequency f. f = c/ c = 300,000 km/s = 3*10 8 m/s f and are related through
The Electromagnetic Spectrum Need satellites to observe Wavelength Frequency High flying air planes or satellites
Dual, wave-particle nature of light 1 eV = 1.6x10 -19 J c = 3x10 8 m/s 1 Angstrom = 10 -10 m Speed of light in matter: c m = c/n, where n is refractive index Note: n is a function of
Light as a Wave (2) Wavelengths of light are measured in units of nanometers (nm) or Ångström (Å): 1 nm = 10 -9 m 1 Å = 10 -10 m = 0.1 nm Visible light has wavelengths between 4000 Å and 7000 Å (= 400 – 700 nm).
Light as Particles Light can also appear as particles, called photons (explains, e.g., photoelectric effect). A photon has a specific energy E, proportional to the frequency f: E = h*f h = 6.626x10 -34 J*s is the Planck constant. The energy of a photon does not depend on the intensity of the light!!!
k is a wave number, is a wave length, T is the period Velocity of propagation
Coulomb’s Law Charge Conservation of electric charge Charge is conserved: in any isolated system, the total charge cannot change. If it does change, then the system is not isolated: charge either went somewhere or came in from somewhere is the permittivity of free space
Charge Let’s denote the force that exerts on as and force exerted by on as ; r is the distance between charges. (Newton’s third law works!) Like charges repel; opposites attract
Exercise: If two electrons are placed meters apart, what is the magnitude of the Coulomb force between them? Compare this to the gravitational force between them. Solution: The magnitude of electric force The magnitude of gravitational force (no matter what the separation is) r
Gauss’s Law A conducting sphere, conducting shell, insulating sphere, shell …..
d + + + + + + a - - - (the total field at any point between the plates) Two parallel conducting plates
Capacitors Consider two large metal plates which are parallel to each other and separated by a distance small compared with their width. Area A The field between plates is L
Using Crossed and Fields Velocity selector independent of the mass of the particle!
Ampere’s Law The field produced by an infinite wire
Biot-Savart Law Infinitesimally small element of a current carrying wire produces an infinitesimally small magnetic field (Also called Ampere’s principle) is called permeability of free space
Force exerted on a current carrying wire For a straight, finite wire of length and uniform magnetic field
Faraday’s Law of Induction The induced EMF in a closed loop equals the negative of the time rate of change of magnetic flux through the loop
There can be EMF produced in a number of ways: A time varying magnetic field An area whose size is varying A time varying angle between and Any combination of the above
R From Faraday’s law: a time varying flux through a circuit will induce an EMF in the circuit. If the circuit consists only of a loop of wire with one resistor, with resistance R, a current Which way? Lenz’s Law: if a current is induced by some change, the direction of the current is such that it opposes the change.