Download ELECTROSTATICS CONCEPTS (physics 2) #1

ELECTROSTATICS CONCEPTS (physics 2)
1) When two different materials are rubbed together, do
the two materials acquire the same type of charge or
different types of charge? Explain how you could
justify your answer with a simple experiment.
They acquire OPPOSITE charges
(TRIBOLELECTRIC charging). If you bring the
objects together, they will attract each other.
2) When a metal ball is charged by induction using a
negatively charged plastic rod, what type of charge
does the metal ball acquire? Explain.
It acquires a POSITIVE charge.
The neutral object polarizes as the rod
approaches. Then it is grounded, and electrons
are pushed off the neutral object onto the ground.
When the grounding path is taken away (before
removing the charged object), the once-neutral
object has lost electrons.
3) If, when charging by induction, you remove the
charged rod from the vicinity of the metal ball before
moving your finger from the ball, what will happen?
Will the ball end up being charged? Explain.
The electrons will redistribute themselves and the
ball will end up being neutral (it will not have lost
nor gained any electrons in the process).
4) Can pith balls be initially attracted to a charged rod
and then later repelled by the same rod, even though
they have not touched any other charged object?
Explain.
If the pith balls contact the rod during the initial
attraction – due to polarization of pith balls – they
may obtain the same charge as the rod due to
CONDUCTION. They will then be repelled.
5) If the distance between two small charged objects is
doubled, will the electrostatic force that one object
exerts on the other be cut in half? Explain.
No; since F 
1
2 and r2 = 2 r1 , then F2 = ¼ F1 .
r
6) If two charges are both doubled in magnitude without
changing the distance between them, will the force
that one charge exerts on the other also be doubled?
Explain.
No; since F  q and both q1 and q2 are doubled,
then F2 = 4 F1 .
7) If a positive charge is moved toward a negative
charge, does the potential energy of the positive
charge increase or decrease? Explain.
It decreases. It is being moved closer to its
“endpoint”. (Same as moving a mass closer to
the ground – it loses PEg.)
8) If a negative charge is moved in the same direction as
the electric field lines in some region of space, does
the potential energy of the negative charge increase
or decrease?
It increases. It is moving closer to a repulsive
area – it is experiencing force acting opposite the
direction it is moving. (The E-field direction is the
direction of the force on a positive charge; a
negative charge experiences the opposite force.)
9) Is electric potential the same thing as electric potential
energy? Explain.
Electric potential is defined as the electric
PEe
potential energy per unit charge.
V
q
Electric potential measures the “energy-giving
ability” of a location in an electric field and does
not depend upon the charge that is placed at that
spot.
10) A positive charge is placed in an electric field. It is
then moved in a direction opposite that of the electric
field direction. Did it move to a region of higher or
lower potential?
Since it would be experiencing a force opposite
the direction it is moving, it would be moving
away from its endpoint. Therefore, it would be
gaining PEe and would be moving to a HIGHER
potential region.
11) What is a capacitor made of? What would you do
with it? How does it work?
Two parallel conductive (metal) plates with an
insulator in between. It is used to store electric
energy by storing charge in the E-field between
the plates.
Electrons are placed onto one plate and taken off
the other. The excess electrons move into the
dielectric and are stored there. When the plates
are connected by a conductive path, the electrons
move from the negative plate to the positive plate,
releasing their electrical energy.
12) During an electrical storm, a parked car becomes
charged negatively by a passing thundercloud.
Describe the location of the excess electrons gained
by the car. (Where on the car would you find them?)
Since the frame of the car is a conductive shell,
the excess charge would only be found on the
outside surface of that shell, with greatest density
at sharp corners and edges. No charge would be
on the inside of the frame shell.
13) The diagram below shows an electric field, illustrated
by E-field lines. Is this an example of a uniform
electric field? How can you tell?
No; the E-field intensity decreases from left to
right. The lines spread out, becoming less dense.
14) Two positive charges, each of magnitude 3.0 x 10-6
C, are located a distance of 10.0 cm from each other.
What is the magnitude of the force exerted upon each
charge?
q1q2
9
F  k 2  9.0 x 10
r
-6
(
3
.
0
x
10
C)(
Nm 2

C2
(0.100
F  8.1 N
15) A uniform electric field is directed upwards and has
a magnitude of 2.5 N/C.
Calculate the force on a -5.0 x 10-3 C charge placed
in this field.
F
E
q
F  Eq  (2.5
N
)(5.0 x 10 C)  0
-3
C

F  0.0125 N, down
16) What is the magnitude and direction of the electric
field at a point 2.00 m away from a +0.0250 C
charge?

q
E  k 2  9.0 x 109
r

E  5.63 x 107
N m 2
N
C
C2

0.0250 C
 5.6
2
(2.00 m)
, away from the ch
17) Suppose that the top plate of a parallel-plate
capacitor has an electric potential of 0.0 V and the
bottom plate has a potential of 450 V. The plates are
2.0 cm apart. What is the magnitude of the electric
field between the plates?
V  Ed
V V2  V1 450 V - 0.0 V
E


 23,0
d
d
0.020 m