Transcript electrostatics problems

Electrostatics – Practice Problems
Problem 1 – Lunar Athletes & Apparent Weight
Problem 2 – Electric Field Safety Net
Problem 3 – Long Distance Crater Jumping
Problem 4 – Athlete’s Field
Click on this icon to return to the this slide.
Copyright - Adam Randall
+
+
+
+
+
+
P
r
o
b
l
e
m
1
q = 0.7 C
m = 78 kg
_
_
_
_
_
magnitude of charge on
athlete’s suit
athlete’s mass
_
In the future, many Olympic athletes will train on the moon. The moon’s gravitational
“force” field is nearly 1/6th of the Earth’s. Special training facilities will be built to
maintain variable strength / uniform electric force fields inside different training rooms.
Athletes will wear electrically charged body suits designed to evenly distribute charge
and the resulting electric force. The result will be a unique training environment able to
create apparent weights ranging from 1/6th mg to 2mg.
A. If the athlete above wanted an apparent weight equal to their weight on the Earth,
would their suit need an excess or deficiency electrons.
B. Predict the strength of the electric force field needed to give the athlete an
apparent weight on the moon equal to their weight on the Earth.
Copyright - Adam Randall
Coulomb’s Lunar Safety Net
A 72 kg Lunar athlete falls from a 15 meter high
platform wearing a body suit charged with 0.35 C. As a
built in safety measure a photogate is placed 5 meters
above the floor and measures the speed of the falling
athlete.
Instantly a uniform electric field is created to gently
slow them down and decelerate them to rest on the
surface of the floor.
A. Predict the charge polarity of the athlete’s
suit in order for electric safety net to work.
B. Predict the strength of the electric force
field needed in order to save the athlete.
+
+
+
Photogate
5m
_
_
_
Copyright - Adam Randall
P
r
o
b
l
e
m
3
Consider a 60 kg athlete trying to jump a 1250 meter wide lunar
crater shown below. Their initial velocity is 4.5 m/s at 45 degrees.
If the electric field points up and has a strength of 25000 N/C,
what in the minimum net charge the athlete must have on their
body suit to jump the crater?
Copyright - Adam Randall
H
i
n
t
3
Hints
• Decompose the initial velocity vector into components.
• Determine the time in flight from one side of the crater to the other.
• Determine the vertical acceleration using ΔY= 0 meters
• Use Newton’s 2nd Law to find the charge on jumper.
Vo
Vox
Voy
Copyright - Adam Randall
Proble
m4
Imagine four electrically charged athletes standing on the vertices of a
square 2 meters on a side. The two athletes on the top of the square carry
-0.25 Coulombs of charge. The two athletes on the bottom of the square
carry +0.25 Coulombs of charge.
A. Determine the electric field (vector) at the exact
center of the square.
2 meters
B. Determine the electric force (vector) on a 0.1
gram water drop with 10,000 extra electrons on
it, at the center of the square.
2 meters
C. Could you use the standard equations of
kinematics to predict the motion of the water
drop through space and time?
Copyright - Adam Randall