Phy2005 Applied Physics II Spring 2017 Announcements: Test 2 Wednesday, March 22 Covers Ch. 22-25 2 practice tests posted on course Tests page Review session in class March 20 in class + March 20 530pm NPB 2205 + March 21 5pm (Anne Tolson) NPB 2165 Reminder: Mechanical Universe Videos (see links page)
Science highlight First up-close and personal view of Pan, one of Saturn’s currently known 63 moons Science magazine article calls it a “space empanada”
Last time R C V = V e-t/RC Charging a capacitor in RC circuit Vc t Vc = V (1 – e-t/RC) V tC=RC: time constant 0.63V e = 2.71828183 tc = RC Vc = V (1 – e-1) = 0.63 V t Discharging R C +Q -Q + V = V e-t/RC 0.37 RC
Electromagnetic WAVES
v = f l y = sin(kx) Wavelength: l = 2p/k Traveling sin(kx) with a velocity v: y = sin{k(x-vt)} t period: T Frequency: f = 1/T v = f l
v = f l Any traveling wave satisfies this equation. Sound Wave Sound is a longitudinal mechanical wave that travels through an elastic medium. The speed of sound in air at 27C is 347 m/s. The middle A note (A4) has 440 Hz frequency. We can use the above equation to calculate the wavelength of the middle A note sound wave. l = v/f = (347 m/s / 440 1/s) = 0.79 m (Note: 1 Hz = 1/s)
From quantities present in his equations Maxwell was able to calculate the speed with which electromagnetism moves Turned out to be c, the speed of light (known from Romer astronomical expts. in 17th century.) Light was known to be a wave (Young experiment 1801) “We can scarcely avoid the inference that light consists of the transverse undulations of the same medium which is the cause of magnetic and electric phenomena” -- Maxwell
Fiat Lux! Velocity of the wave solutions to Maxwell equations Note e0,m0 come from laboratory measurements of electricity and magnetism c=2.9986 x 108 m/s
Ex WUFT broadcast at 89. 1 MHz Ex WUFT broadcast at 89.1 MHz. What is the wavelength of this radio wave? l = c/f = (3 x 108 m/s)/(89.1 x 106 /s) = 3.37 m
Hertz’s Experiment (1887)
B E propagation http://atom.physics.calpoly.edu/Graphics/em_surface.gif
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Q1 Choose a wrong statement on an EM wave (light). Light is a wave. Light travels in vacuum at a speed of 3 x 108 m/s An EM wave has oscillating E and B fields. The direction of E field in a wave is in the direction of propagation.
Q2 (24.11) A laser emits light of frequency 4.74 x 1014 sec-1. What is the wavelength of the light in nm? 1. 1.42 x 1023 nm 2. 1.42 x 104 nm 3. 6.32 x 102 nm 1.58 x 10-3 nm 1.58 x 106 nm
- - E B Transmitter Receiver
1. An EM wave can travel in the absence of medium. 2. In an EM wave, both E and B fields oscillate in a mutually perpendicular direction and perpendicular to the propagating direction. 3. The speed of EM wave (light) in vacuum is c = 3 x 108 m/s 4. EM wave travels slower in other media.
3 x 108 = f l nano = 10-9 http://laxmi.nuc.ucla.edu:8248/M248_99/iphysics/spectrum.gif
Light Intensity All electromagnetic waves such as light transmit energy. Light intensity = power flowing through the area / area (power density) [w/m2] Q. A laser sends out a beam of 3 mW power. The diameter of the beam is 3 mm. What is the intensity of the laser beam in w/m2? Intensity = (0.003 w)/(p x 0.00152 m2) = 424.4 w/m2 My laser pointer produces about 5 mW power!!!
A laser emits a narrow beam of light A laser emits a narrow beam of light. The radius of the beam is 1 mm, and the power is 1.2 x 10-3 W. What is the intensity of the laser beam? Intensity = (1.2 x 10-3 W)/(p x (0.001 m)2) = 380 W/m2
1 lumen (lm) = light power of 1/683 W in yellow (lighting industry) When a light bulb is on, it radiates out broad spectrum of EM wave. The total radiant energy emitted by the lamp per unit time is called radiant flux. Note this is just a special name for total power. Only a small portion of this is in visible range (400 – 700 nm). The portion of radiant energy in visible range is called luminous flux (F). In a incandescent light bulb, only 10% is luminous flux. 1 lumen (lm) = light power of 1/683 W in yellow (lighting industry)
Solid Angle 2 Dimensions = s/R [radian] W = A/R2 [steradian] s R Total angle: q = 2pR/R = 2p (rad) A W = A/R2 [steradian] W R Total solid angle: W = 4pR2/R2 = 4p (sr)