Electromagnetism Unit- Electrostatics Sub-Unit
Regents Physics
4.1.1 Elementary Charge
Atomic Structure Review
electrons
nucleus
excess
electrons
lack of
electrons
_____________
negative charge
two types of nucleons
Only subatomic
particles that are
allowed to move
around WITHIN matter
___________ + charge
___________ 0 charge
Example #1
•
Nucleons are not
allowed to move
around WITHIN matter
An object has three excess electrons
–
What is its “elementary charge”? ____________
–
What is its charge in coulombs?
Definitions
•
charge – a fundamental property of matter
–
•
charge on an object is carried by particles:
–
–
__________________
•
______________________
•
______________________
•
______________________
•
______________________
Objects are normally electrically __________________ (same number of protons and
electrons)
•
An object has 75 protons and 65 electrons.
–
What is its “elementary charge”? ____________
–
What is its charge in coulombs?
__________________
Charged Objects
•
Example #2
measured in __________________ or __________________________
Law of Conservation of Charge
The total amount of charge in a closed system remains constant – charge is not created or
destroyed, it only moves from one object to another
Charge “moves” as a result of ELECTRON movement ONLY!!!
1|Page
Example #3
•
A metal sphere with two excess electrons touches a neutral metal sphere and is then taken
away.
–
What is do you predict the charge on each sphere will be after they make
contact?
Review Questions
4.1.1 – Convert between elementary charge and charge in coulombs. Determine if a
charge is possible. Determine how charge will be distributed in conservation of charge
problems.
-
–
What is the charge on an object with four excess electrons?
What is the total charge on both spheres after they come in contact?
[-4e; 6.4 x 10-19 C]
-
What is the charge on an object that contains 35 electrons and 85 protons?
PRACTICE
1)
A metallic sphere gains 500 electrons. What is the charge on the sphere in coulombs?
[+50; 8.0 x 10-18 C]
-
2)
Which set of particles would produce a charge of +3.2 x 10-18 coulombs?
(1)
(2)
(3)
(4)
How many electrons does it take to generate -3.5 coulombs of charge?
400 electons and 200 protons
40 electrons and 20 protons
200 electrons and 400 protons
20 electrons and 40 protons
[4]
3)
(a) 3.2 x 10-19 C
(b) 6.5 C
(c) -20 e
(d) 1.2 x 102 e
Which of the following charges is allowable in the ‘real world’?
a.
4.8 x 10-16 C
b.
3.5 e
c.
500 e
d.
9.92 x 10-17 C
e.
1.5 x 103 e
f.
1.6 x 10-20 C
Which charges could NOT exist on a real-world object?
(e) 0.5 x 10-19 C
(f) 3.2 x 10-21 C
(g) 3.5 e
(h) 1.6 x 10-19 e
[e, f, g, h]
-
Object A begins with a charge of -3.0 coulombs; object B begins with a charge
of -7.0 coulombs; and object C begins with no charge. Object A is touched to
object B then removed. Object B is then touched to object C and removed.
What is the final charge on each of the three objects?
.
[A = -5.0C;
B = -2.5C;
C = -2.5C]
Electromagnetism Unit- Electrostatics Sub-Unit
Regents Physics
Induction
4.1.2 Transfer of Charge
induction – movement of charge _______________________
Conductive Properties of Materials
Negative Rod
HIGH CONDUCTIVITY
LOW RESISTIVITY
LOW CONDUCTIVITY
HIGH RESISTIVITY
_______________________
_______________________
_______________________
_______________________
Allows electrons to flow freely
Strongly resists flow of electrons
1
2
3
Positive Rod
Charging by Friction
•
Rub two insulators together
1
–
_________________ move from one to the other
–
One object becomes ________ the other _________
RULE #1 – ________________________________________________
Grounding
•
2
3
RULE #3 – ________________________________________________
RULE #4 – ________________________________________________
Since the Earth is very large it can neutralize:
–
negative objects by ____________________ electrons
–
positive objects by _____________________ electrons
RULE #2 – _______________________________________________
3|Page
Conduction
Induction
conduction – movement of charge _______________________
Negative Rod
Negative Rod
1
2
3
2
1
3
Positive Rod
Positive Rod
1
2
3
2
1
3
RULE #5 – ________________________________________________
Charging by Conduction
Negative Rod
Electroscope
1
2
3
4
5
3
4
5
Positive Rod
1
2
Electromagnetism Unit- Electrostatics Sub-Unit
Regents Physics
Charging by Induction
2)
Negative Rod
1
2
3
4
5
A metal sphere with a charge of -3 coulombs is brought into contact with a metal
sphere with a charge of +5 coulombs.
a.
Draw an arrow between the two spheres in the “Before” picture to show
the direction in which electrons will flow when the spheres touch.
b.
Determine the charge on each of the two pith balls after they come into
contact, then are separated.
-3 C
6
+5 C
Before
After
Positive Rod
3)
1
2
3
4
5
6
A metal sphere with a charge of +16 coulombs is brought into contact with a metal
sphere with a charge of +4 coulombs.
a.
Draw an arrow between the two spheres in the “Before” picture to show
the direction in which electrons will flow when the spheres touch.
b.
Determine the charge on each of the two pith balls after they come into
contact, then are separated.
RULE #6 – ________________________________________________
+16 C
+4 C
PRACTICE
1)
A rubber balloon is rubbed on a person’s hair.
a.
Explain how the balloon becomes charged.
b.
Sketch a diagram of the charge distribution when the balloon is stuck to a
wall.
Before
After
5|Page
-
Testing Electroscopes
Object A is negatively charged and is used to charge object B by conduction.
What is the final charge on objects A and B?
[A -; B -]
Object A is negatively charged and is brought near neutral object B without
touching it. What is the final charge on objects A and B?
-
-
+
+
-
-
+
[A -; B=0]
-
Object A is negatively charged and is used to charge object B by induction.
What is the final charge on objects A and B?
[A -; B +]
-
-
+
-
-
+
+
A small object is tested to determine its charge. What test could be used to
prove that the object is positively charged? What test could be used to prove
that the object is not charged?
[repelled from +
attracted to + AND -]
-
Review Questions
4.1.2 – Determine the direction in which electrons flow. Explain the process of conduction,
induction, and charging by induction. Explain the rules for testing an object to determine
its charge. Explain how an uncharged object will react to the presence of a charge one.
Explain how electroscopes become charged and how to test them.
-
[diverge then converge]
-
In which direction will electrons flow in each case?
a.
-3C
-6C
c.
+3C
-6C
b.
-4C
+5C
d.
+8C
+2C
A negatively charged rod is brought near a neutral electroscope then removed,
what is the reaction of the electroscope leaves?
A negatively charged rod is brought near a charged electroscope causing its
leaves to converge. What is the charge on the electroscope?
[+]
-
A negatively charged rod is brought near a charged electroscope causing its
leaves to diverge. What is the charge on the electroscope?
[-]
[; ; ; ]
Electromagnetism Unit- Electrostatics Sub-Unit
Regents Physics
4.1.3 Coulomb’s Law
Electrostatic Force
-
Which of the following shows an electroscope being charged negatively by
conduction?
[C]
-
•
Same charges repel
•
Opposite charges attract
•
Amount of force depends on:
Which of the following electroscopes could be charged positively by induction?
[F]
-
In which of the following are the leaves of the electroscope negatively
charged?
–
______________________________________________________________
–
______________________________________________________________
–
______________________________________________________________
[C; D; E]
– – – – –
+ + + + +
A
Fe
– – – – –
+ + + + +
B
C
D
Equation
r
– – – – –
+ + + + +
[; ; ; ]
E
F
Electrostatic vs. Gravitational Force
•
Consider two electrons that are 1.0 micrometer apart.
–
Gravitationally ______________________
–
Electrostatically _____________________
e-
e-
7|Page
Example #1
•
Determine the electrostatic force between an object with a +2.0 coulomb charge and an
object with a -2.0 coulomb charge if they are separated by 2.0 meters.
Review Questions
4.1.3 – Use equation to determine electrostatic force; charge; and/or distance. Determine
the effects of changing parameters on strength of electrostatic force.
-
Determine the electrostatic force that a +4.0 coulomb charge exerts on a -6.0
coulomb charge when 2.0 meters apart.
[5.4 x 1010 N]
Example #2
•
Determine the electrostatic force between an object with -2.0 micro-coulombs of charge
and an object with -5.0 micro-coulombs of charge if they are separated by 5.0 nanometers.
If two charged objects are attracted to one another by an electrostatic force of
5.0 newtons, what force would they be attracted by if the charge on both of
them were doubled?
[20N]
-
An electrostatic force F acts between two objects with charges +q and +q when
they are a distance R apart. If the distance between the objects is halved, the
electrostatic force would become
(1) F/2
(2) F/4
(3) 2F
(4) 4F
[4]
PRACTICE
1)
When two spheres, each with charge Q are positioned a distance R apart, they are
attracted to one another by a force of magnitude F. What force would they be
attracted by if…
a. …one of the charges is tripled while the other is doubled?
b.
…the separation distance is changed to 3R?
c.
…one of the charges is changed to Q/2?
d.
…one of the charges is doubled and the separation distance is R/2?
4.1.4 Electric Fields
Electric Field Maps
•
Electric fields…
–
exist near __________________________________________
–
mapped using a ______________________ test charge
•
•
•
+
1 – place test charge
2 – draw arrow in direction pushed
3 – this is the field direction
-
Electromagnetism Unit- Electrostatics Sub-Unit
Regents Physics
Example #1
•
+
An object with a charge of -2.0 coulombs positioned in a uniform electric field experiences
a force of -300 newtons. What is the strength of the electric field?
-
Example #2
•
+
+
What is the force experienced by a 0.05 coulomb charge when it is placed in an electric
field with a strength of 3.0 x 103 newtons per coulomb?
Parallel Plates
Electric Field Strength
•
•
Field strength…
Field strength…
–
–
Gets _____________________ with distance
–
__________________ exerted per unit ______________________
Between plates
•
–
E  Fe = _______________________________________
Outside plates = ____________________
Equation
E
+
r
C
A
B
9|Page
An object with a charge of -4 x 10-3 coulombs is placed between a set of oppositely
charged, parallel plates. The object experiences a force of 5 x 10-6 newtons while
between the plates.
3)
Hollow Charged Sphere
•
Field strength…
–
Normal field pattern outside
–
___________________ inside
PRACTICE
1)
An electron is placed within a uniform electric field with strength of 5.0 x 10-8
newtons per coulomb.
a.
Calculate the electrostatic force that is experienced by the electron.
b.
Calculate the acceleration of the electron. [electron mass = 9.11 x 10-31 kg]
a.
Sketch a set of vectors showing the direction in which the electric field lines
point between the plates.
b.
Sketch a vector showing the direction in which the object will travel when
placed between the plates.
+
+
+
+
+
+
+
+
+
+
-Review Questions
+
+
4.1.4– Use equation to determine electric field strength; force; and/or charge. Map or
+
recognize proper electric field maps for point charges or systems of charges. Explain the
+
effect of charged parallel plates on electric field strength, electrostatic force, and
+
acceleration of charges between such plates.
+
A +2.0 coulomb charge experiences a force +of 10 newtons while in an electric
field. What is the strength of the electric field?
[5N/C]
2)
An electron is fired from a point outside two parallel, oppositely charged plates into
the region between the plates. Sketch the path of the electron as it moves between
the plates and comes out the other side. [Hint: which plate will the electron be
deflected toward?]
-
-
-
-
-
-
-
-
-
-
-
What force will a 3.0 coulomb charge experience if place in an electric field
with a strength of 15 newtons per coulomb?
[45N]
-
What is the charge on an object that experiences a force of 20 newtons when it
is placed in an electric field with a strength of 4.0 newtons per coulomb?
+ + + + + + + + + + + + + + + + +
[5C]
Electromagnetism Unit- Electrostatics Sub-Unit
Regents Physics
4.1.5 Electrical Potential (Voltage)
-
Sketch the electric field in the area around each point charge, set of point
charges, and set of plates.
Electrical PE
•
To increase PE
– – – – – – – – – – – – –
– –
–
–
+
–
+
-
+
+
-
+
+ + + + + + + + + + + +
•
-
A
+
Determine the direction of the electric field at the points selected in each
picture.
+
+
B
–
C
-
To decrease PE
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
D
+
+
+
+
+
+
+
+
+
+
+
+
Calculating Potential Difference
–
E
An proton is fired into the space between two parallel, oppositely charged
plates that produce an electric field of strength 2.0 x 10-11 newtons per
coulomb.
o Sketch the path of the electron as it passes between the plates.
o Determine the amount of force exerted on the proton.
o
Determine the acceleration of the proton.
[1.9E-3m/s2]
•
___________________________________________________
•
___________________________________________________
Equation
[A;B ;C↓;D;E=0]
[3.2E-30N]
Amount of potential difference:
Example #1
•
6.0 joules of work are done in pushing an object with +3.0 coulombs of charge toward a
charged plate.
–
What type of charge does the plate have on it?
–
How much potential energy was stored in the electric fields?
–
How much electrical potential was generated?
+ + + + + + + + + + + +
– – – – – – – – – – – – –
– –
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Example #2
•
An object with a 2.0 coulomb charge is accelerated through a potential difference of 10
volts.
– How much kinetic energy does the object gain?
PRACTICE
4)
45 joules of work are done in moving an object with a charge of -3.0 micro- coulombs
toward a negatively charged plate.
a.
Calculate the change in electrical potential (voltage) of the object after it is
moved.
b.
If the object were released, how much kinetic energy would it gain?
Electron-volts
•
Alternate unit for work/energy:
–
Raises 1e to an electrical potential of 1 V
–
___________________________________________________
•
What is the energy needed to raise two electrons to a potential of
1.0 volt?
•
What is the energy needed to raise four electrons to a potential of
2.5 volts?
Example #3
•
An electron travels a distance of 2.0 x 10-3 meter as its electrical potential is raised by 300
volts.
– How much work is done on the electron?
5)
5.0 electron-volts of energy are used to move two electrons through an electric field.
Calculate the change in electrical potential (voltage) for these electrons.
4)
In each case, determine if electrical potential is: increasing, decreasing, or not
changing.
-
+
+
-
– – – – – – – – – – – – –
– –
+
+
– – – – – – – – – – – – –
– –
Electromagnetism Unit- Electrostatics Sub-Unit
Regents Physics
Review Questions
-
4.1.5 – Explain how electrical potential relates to changes in distance between charged
objects. Use equation to determine electrical potential, work/energy, and/or charge.
Convert between joules and electron-volts.
-
In which of the cases below is the electrical potential energy increasing? In
which cases is the electrical potential energy decreasing? In which cases is the
electrical potential energy remaining constant?
+
+
[3eV;
4.8E-19J]
–
+
-
–
+ + + + + + + +
+
+ + + + + + + +
What is the amount of energy needed to move an electron through an
electrical potential of 3.0 volts? Express this energy in both joules and
electron-volts.
-
– – – – – – – –
An object with a 4.0 coulomb charge is accelerated through an electrical
potential of 12.0 volts. What amount of kinetic energy does the object gain?
Express this energy in both joules and electron-volts.
–
[48J;
3E20eV]
– – – – – – – –
[A=inc; B=dec
C=dec; D=dec;
E=inc; F=const]
-
What is the electrical potential generated when 15.0 joules of work are done in
moving a 5.0 coulomb charge through an electric field?
[3V]
-
What is the amount of work needed to increase the electrical potential of a 2.0
coulomb charge by 8.0 volts?
[16J]
-
What is the charge on an object that requires 6.4 x 10-19 joules of work to be
moved through an electrical potential of 2.0 volts?
[3.2E-=19C
or 2e]
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