Download Session 26 - Iowa State University

Session 26: EM Waves
Supplemental Instruction
Iowa State University
Leader:
Course:
Instructor:
Date:
Garrett Schieber
Phys 222
Dr. Al-Saqer
11/10/14
Warm up:
1) The electric and magnetic fields in a traveling plane electromagnetic wave in vacuum at a
⃗ , respectively. The
particular point in space and at a particular instant of time are 𝐸⃗ and 𝐵
direction of propagation of the wave is in the direction of ____ .
a) 𝐸⃗
⃗
b) 𝐵
⃗ × 𝐸⃗
c) 𝐵
⃗ × ⃗𝑩
⃗
d) ⃗𝑬
⃗
e) 𝐸⃗ ∙ 𝐵
2) An electromagnetic plane wave in vacuum is moving in the + y direction. At time t = 0 and at
position (x,y,z) = (0,0,0), the electric field is pointing in the + z direction. At that time and
position, the magnetic field in the wave is pointed in the ____ direction.
a) +x
b) +y
c) +z
d) –x
e) –z
Group Activity
3) The electric field of a sinusoidal electromagnetic wave obeys the equation: (Book 32.8)
a) What is the amplitude of the electric field of this wave?
b) What is the amplitude of the magnetic field of this wave?
c) What is the frequency of the wave?
d) What is the wavelength of the wave?
e) What is the period of the wave?
f) Is this light visible to humans?
g) What is the speed of the wave?
Supplemental Instruction
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4) The microwaves in a certain microwave oven have a wavelength of 12.2 cm. (Book 32.35)
a) How wide must this oven be so that it will contain five antinodal planes of the electric
field along its width in the standing wave pattern?
b) What is the frequency of these microwaves?
c) Suppose a manufacturing error occurred and the oven was made 6.0 cm longer than
specified in part (a). In this case, what would have to be the frequency of the microwaves
for there still to be five antinodal planes of the electric field along the width of the oven?
5) We can reasonably model a 90-W incandescent light bulb as a sphere 6.1 cm in diameter.
Typically, only about 5% of the energy goes to visible light; the rest goes largely to nonvisible infrared radiation. (Book 32.15)
a) What is the visible light intensity at the surface of the bulb?
b) What is the amplitude of the electric field at this surface, for a sinusoidal wave with this
intensity?
c) What is the amplitude of the magnetic field at this surface, for a sinusoidal wave with this
intensity?