Download Period 20 Solutions: Radiant Energy from the Sun

12/22/12
Period 20 Solutions: Radiant Energy from the Sun
20.1 Radiant Energy and the Electromagnetic Spectrum
1)
Vibrating electric charges
a) Your instructor will demonstrate the effect on a radio produced by vibrating
charge. What happens to the radio broadcast when the radio antenna receives
electromagnetic radiation from the spark of the Wimshurst machine?
You hear interference (static), which indicates that energy from the
spark has reached the radio.
b) How did energy from the separated charges on the Wimshurst machine reach
the radio?
There is no direct electrical connection between the Wimshurst
machine and the radio. Energy produced by the spark (the source)
traveled in electromagnetic waves across the room to the radio (the
receiver).
c) Moving electric charge produces an electric current. What type of energy does
vibrating electric charge produce?
Radiant energy (also called electromagnetic radiation)
2)
Examples of electromagnetic radiation Your instructor will demonstrate the
different types of radiant energy (electromagnetic radiation)
a) Radio waves Which devices operate using radio waves?
Radio, walkie-talkies, remote control cars
b) Microwaves Which devices use microwaves?
Microwave oven, cellular phones, garage door opener,
c) Infrared radiation Which devices illustrate infrared radiation?
TV remote, glow coil, infrared camera, radiometer
d) Visible light
1)
Your instructor will demonstrate a large bulb. Can a bulb that does
not appear to be lit radiate energy? If so, what type of radiant
energy?
Yes. The bulb emits infrared radiation.
2)
Observe the bulb through a diffraction grating.
a) What are the major colors of the light do you observe? List the
colors of visible light from longest to shortest wavelength.
A diffraction grating separates the light into a continuous
spectrum of color: red, orange, yellow, green, blue,
indigo, and violet.
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b) Observe the color wheel. What color do you see when the
wheel spins? Why?
When the wheel spins rapidly, the separate colors appear to
combine and you observe white light
e) Ultraviolet light Which devices illustrate ultraviolet radiation?
The “black light” bulbs emit ultraviolet radiation as well as
visible violet visible light.
f) X–rays What are some uses of X – ray radiation?
X–rays can penetrate soft body tissue but not bone, allowing
a picture of the bone structure. X–rays can be used to
diagnose illnesses and treat diseases such as cancer.
g) Gamma rays What materials emit gamma rays?
Some unstable nuclei, such as Cobalt-60, emit gamma rays.
h)
Group Discussion Question: How are radio waves, microwaves, infrared radiation,
visible light, ultraviolet light, X-rays, and gamma rays similar? How are they
different?
They are similar in that they are all forms of radiant energy
(electromagnetic radiation) produced by vibrating electric charges.
They differ in wavelength and energy. Radio waves have the longest
wavelength and gamma rays have the shortest wavelength.
20.2 Radiant Energy Produced in Stars
3) Energy from nuclear fusion reactions
a) In the core of stars, hydrogen nuclei (protons) can fuse into deuterium nuclei. Is
this nuclear reaction endothermic or exothermic? exothermic
b) In the process of producing deuterium, one proton becomes a neutron. What
change happens to the quark trio that makes up a proton to change it into a
neutron?
One of the up quarks of the proton becomes a down quark.
c) What is the source of this energy: the binding energy of the deuterium
nucleus
d)
In the core of star, temperatures of 15,000,000 K provide the activation energy
necessary for fusion. Why does the process of forming a deuterium nucleus
from two protons require such a large amount of activation energy?
The two positive protons repel one another. Energy is required to push the
protons close enough together so that they are subject to the strong force.
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e) Group discussion question: Major challenges to controlling nuclear fusion
reactions on Earth are providing the activation energy and containing the
reactants. What force provides the activation energy and contains the reactants
in fusion reactions in stars?
The gravitational force pulls a star’s matter to the center (core) of the
star and exerts great pressure on the matter at the core. The pressure
from the weight of the star’s matter heats the material in the star’s
core to temperatures to temperatures high enough to activate nuclear
reactions – temperatures of at least 15,000,000 K.
4) Exothermic reactions in stars
a) What is the proton-proton fusion chain?
In stars smaller than 1.2 times the mass of the Sun, two hydrogen
nuclei fuse to form a nucleus of deuterium. Deuterium fuses with
another hydrogen to form tritium, an isotope of helium. Two tritium
fuse to form stable helium plus two hydrogen nuclei.
b) The proton-proton chain begins with two hydrogen nuclei (protons). What is the
end product of the chain?
A helium nucleus and two protons
c) How much energy is released by this fusion reaction?
1.44 + 5.49 + 12.86 = 19.79 MeV
5) Energy released from stars
a) How does the energy from fusion in the star’s core reach the star’s surface?
Energy is radiated through the star’s radiative zone and is then transferred
primarily by convection through the star’s convective zone to its surface.
b) What holds the gas of a star into a spherical shape?
The outward radiation pressure of the hot gas is balanced by the
inward force of gravity.
c) Group Discussion Question: What do you think happens to clumps of matter
that do not reach an internal temperature high enough to begin fusion
reactions?
Less massive gas clouds are bound together by a weaker force of
gravity. If this gravitational force does not result in enough thermal
energy in the core of the clump to begin nuclear fusion reactions, the
clump may become a planet.
20.3
How Is Radiant Energy from the Sun Transmitted to Earth?
Your instructor will discuss the properties of waves.
6) Transferring energy with waves
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a) One end of a stretched spring is vibrated to send sine waves along it. What
can be done to increase the frequency of the waves?
Vibrating the spring faster increases the frequency of the waves
(how often one of the wave crests passes a given point on the
table).
b) What does increasing the frequency of the waves do to the wavelength?
The wavelength becomes shorter.
c) Is it possible to transfer energy without a transfer of matter? Yes
d) Group Discussion Question: List examples of transfer of energy without a
transfer of matter.
Radio and TV broadcasts, energy from the Sun
7) Wave Speed and Frequency
Your instructor will discuss wave periods and frequencies. Use this information
to find the speed of the wave illustrated in the diagrams below.
a) Find the wavelength (in meters) of the wave in the diagram. 6 cm = 0.06 m
Displacement
2
4
6
8
Midpo
10
12
int
14
16
18
20
Distance
(in cm)
b) The diagram below shows the displacement of a wave over time, at a fixed
point along the path of the wave. Find the period of the wave in the
diagram. _1.5 seconds_
Displacement
0.5
1,0
1.5
2.0
Midpo
2.5
3.0
3.5
int
4.0
4.5
5.0
Time
( in sec)
c) Calculate the frequency of the wave (in cycles/second, or Hertz).
frequency = 1/period = 1/1.5 sec = 0.67 cycles/sec = 0.67 Hz
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d) Calculate the speed of this wave.
S = f L = 0.67 Hz x 0.06 m = 0.04 m/s = 4 x 10 – 2 m/s
e) Based on the speed you calculated, could these diagrams represent a wave
of electromagnetic radiation? Why or why not?
This could not be a wave of electromagnetic radiation because it is
traveling too slowly. All waves of electromagnetic radiation travel
at the speed of 3 x 108 m/s (in a vacuum). When traveling in other
media, such as air or water, the speed of electromagnetic waves is
only slightly slower.
f) Find the wavelength of a wave of electromagnetic radiation that has a
frequency of 6 x 1014 Hz.
S = f L, or L = S = 3 x 108 m/s = 0.5 x 10 – 6 m = 5 x 10 – 7 m
f
6 x 1014 1/s
g) Your instructor will demonstrate a vacuum jar that contains a buzzer and a
light bulb.
Describe the differences you observe between sound waves and waves of
electromagnetic radiation.
Sound waves cannot move through a vacuum, as in the bell jar, but
waves of electromagnetic radiation can.
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