Physics 1230
Homework 4
Due Thursday. June 9, 2016
More about waves and the behavior of electromagnetic waves (light!)
Waves
The figure on the right shows a wave on a
string propagating to the right. There is a dot
painted on the string at point 1. The right end
of the string (indicated by the dotted lines)
extends off of the page to the right.
A hand steadily wiggles the left end of the
string up and down. The figure below shows snapshots of the wave on the string at three
instants in time (t1, t2, t3) as the wave travels to the right. The dot painted on the string is
indicated at point 1. The pictures below show two possible physical situations. Three
students argue about which picture is physically correct.
Student A: “The bottom picture is correct,
because the wave is traveling to the right.
Since the wave and the string look exactly
the same, the dot must move with the
wave to the right.”
Student B: “The top picture is correct,
because the wave is a disturbance of the
string, not the string itself. As the wave
travels to the right, the string moves up
and down. Therefore, the dot moves up
and down with the string.”
Student C: “Neither picture is correct. The
string’s position is at y=0 at point 1, which
means the dot just sits there.”
1) Which student do you agree with? Explain. (4 points)
Physics 1230
HW4.1
Summer 2016
Physics 1230
Homework 4
Due Thursday. June 9, 2016
2.
Charge and force. [6 pts] Look at the PHET simulation about balloons and static
electricity. http://phet.colorado.edu/en/simulation/balloons-and-static-electricity
A) Remove the wall on the right in the simulation (yellow button). Use just one balloon.
Rub the balloon against the sweater and pull the balloon away and let go. What happens?
(Just a few word answer.)
B) Choose one: Opposite charges:
A) Attract
B) Repel
C) Depends on the situation.
C) Now add in the wall and two balloons and look at some other situations. Do charges
ever “disappear” in the simulation (in mode “show all charges”) or do they just rearrange
as the objects touch each other?
Physics 1230
HW4.2
Summer 2016
Physics 1230
Homework 4
Due Thursday. June 9, 2016
3.
Making electromagnetic waves. [8 pts] Consider the shaking of a charged stick
to create an electromagnetic wave. Don’t worry about the FACT that you are not going
to be able to shake it fast enough to create light. Just assume you are shaking it up and
down quickly at some fixed frequency and with some fixed amplitude. You are creating
an electromagnetic wave at the frequency of visible light. Answer each of the following
questions:
A) What do you need to do when shaking the stick to make you the light you are
creating "redder"?
B) What do you need to do when shaking the stick to make the light you are
creating "bluer"?
C) What happens to the color of the light if you shake the stick horizontally,
leaving everything else the same?
D) What happens to the polarization of the light if you shake the stick
horizontally, leaving everything else the same?
4. [2 pts] If a light source is emitting electric fields that are oscillating in random
directions, the light is said to be:
A) Polarized
B) Diffused
C) Unpolarized
D) Scattered
Physics 1230
HW4.3
Summer 2016
Physics 1230
Homework 4
Due Thursday. June 9, 2016
5. [6 pts] Which of these phones is emitting
linearly polarized light? (Linearly polarized
light is a term that refers to light that is for
example either horizontally or vertically
polarized.)
A) Nexus 4
B) Nexus 5
Which of these images of the ocean is being
observed through polarizing sunglasses?
A) Left image
B) Right image
6. [4 pts] Review HW2 and HW3, and the solution sets. If you did have some mistakes,
list two problems you did incorrectly and what you did that was incorrect on each of
those problems. If you didn’t have any mistakes or only had one mistake, then please tell
us about the one or two things that are most confusing to you and that you might have
questions about.
Physics 1230
HW4.4
Summer 2016