Phy 133- Assignment 10:
A.
B.
C. 1. The magnitude of the Poynting vector is the wave's intensity, and its
direction is the wave's direction of propagation.
2.
D. 1. No. It's not a wave because nothing is "waving." These steady,
unchanging fields are like the steady, unchanging surface of a smooth pond.
There's no water wave on the pond, there's no electromagnetic wave in these
fields.
2.
E. There are other ways to do this, but it seems easiest to do part b first:
Picture the beam starting to come out of the laser when you first turn it on.
How much time does it take for a 1.00 m length of beam to come out?
Distance = (speed)(time)
1.0 m = (2.998 x 108 m/s)t
t = 3.336 x 10-9 s
15.0 mW = .015 W means .015 joules per second. So, the energy it puts out in
this time is
(.015 J/s)(3.336 x 10-9 s) = 5.00 x 10-11 J answer, part b.
a) volume of the 1 m length of beam = r2h= (.001 m)2(1.00 m) = 3.14 x 10-6
m3
u = energy/volume = (5.00 x 10-11 J)/( 3.14 x 10-6 m3) = 1.59 x 10-5 J/m3
This is the average energy density, because even in this short time, the E and B
fields in visible light will have vibrated through many cycles.
uave = .5
o
Emax2
Emax = (2 uave / o) = [(2)( 1.59 x 10-5 )/(8.854 x 10-12)] = (3.59 x 10-6) =
1.90 x 103 V/m ans. a
F.