Download Electric Force

Electric Force
EQ: How can an object be charged and what affect does that charge have upon other objects around it?
LO: We will identify the basic properties of an electric charge and learn how to apply Coulomb’s Law
CT: I will differentiate between conductors and insulators and distinguish between the different methods of
charging a material.
Electric Force pg. 62
EQ: How can an object be
charged and what affect does that
charge have upon other objects
around it?
• Electric charges at rest (static
electricity)
• Electric Force, which is a force between
objects with charge, just as the gravitational
force is a force between objects with mass.
The Basics-Atomic structure
Summary of Subatomic
Particles
Proton
Neutron
Electron
*In
nucleus
*Tightly
Bound
*Positive
Charge
*Massive
*In
nucleus
*Tightly
Bound
*No
Charge
*Massive
*Outside
nucleus
*Weakly
Bound
*Negative
Charge
*Not very
massive
The Basics- 2 types of charges
A charge is present if there is an imbalance of protons
and electrons
+
+
+
-
Charged versus Uncharged Particles
Positively Charged
Negatively Charged
Uncharged
Possesses more protons
than electrons
Possesses more
electrons than protons
Equal numbers of
protons and electrons
electrons are migrants - constantly on the move and always
ready to try out a new atomic environment or persuade other
electrons to try out their atomic environment.
The Basics-checkpoint
____ are the charged parts of an
atom
• a. Only electrons
• b. Only protons
• c. Neutrons only
• d. Electrons and neutrons
• e. Electrons and protons
• f. Protons and neutrons
The Basics-the fundamental law
Like charges repel; unlike charges attract.
Neg
Pos
Neg
Pos
Neg Pos
The Basics-checkpoint
Electrical forces ____.
a. can cause objects to only attract each other
b. can cause objects to only repel each other
c. can cause objects to attract or repel each other
d. have no effect on objects
The Basics-checkpoint
Upon entering the room, you observe two balloons
suspended from the ceiling. You notice that instead of
hanging straight down vertically, the balloons seems to be
repelling each other. You can conclusively say ...
a. both balloons have a negative charge.
b. both balloons have a positive charge.
c. one balloon is charge positively and the other negatively.
d. both balloons are charged with the same type of charge.
The Basics-conductors and insulators
The movement of charge is limited by the substance
the charge is trying to pass through. There are
generally 2 types of substances.
Conductors: Allow charge to
move readily through it
(metals such as copper, gold,
aluminum)
Insulators: Restrict the
movement of the charge
(plastic, rubber, glass)
Charged
conductor
Charged
insulator
The Basics-conductors and insulators
• Semiconductors: Are materials characterized by
electrical properties that are somewhere between
those of insulators and conductors. A foundation of
modern electronics
• Superconductors: Are materials that will conduct
electricity without any resistance when cooled below
their critical temperature. Benefit-energy would not
be lost. Downfall- cooling constraints.
The Basics-conservation of charge
Electrons are never created nor
destroyed, but are simply transferred
from one material to another
•No case of the creation or
destruction of net electric
charge has ever been
found
The Basics-checkpoint
A conductor differs from an insulator in that a conductor
________.
a. has an excess of protons
b. has an excess of electrons
c. can become charged and an insulator cannot
d. has faster moving molecules
e. does not have any neutrons to get in the way of electron
flow
f. when charge is placed on a conductor, it redistributes to the
outer surface.
The Basics-polarization
Polarization is the separation of charge
In a conductor, “free” electrons can move around the
surface of the material, leaving one side positive and the
other side negative.
In an insulator, the electrons “realign” themselves
within the atom (or molecule), leaving one side of the
atom positive and the other side of the atom negative.
Polarization is not necessarily a charge imbalance!
The Basics-Closing Task
• Title pg. 63 “Electric Field Hockey
• Answer the questions on the handout on pg. 63.
Do not write on the handout
1. Friction – friction by rubbing
2. Induction –method used to charge an object
without actually touching the object to any
other charged object then grounding the
conductor
3. Conduction - involves the contact of a charged
object to a neutral object
Charging Methods-contact
When a rubber rod is rubbed against fur, electrons
are removed from the fur and deposited on the rod.
Electrons
move from
fur to the
rubber rod.
negative
- positive
- ++++
The rod is said to be negatively charged because of an
excess of electrons. The fur is said to be positively
charged because of a deficiency of electrons.
Charging Methods-contact
glass
silk
positive
+ +
negative
+ +
- - - -
Which material gives
up the electrons?
Triboelectric effect
Charging Methods-induction
Induction uses the influence of one charged object to
“coerce” charge flow.
Step 1. A charged rod is brought
near an isolated conductor. The
influence of the charge object
polarizes the conductor but does
not yet charge it.
Step 2. The conductor is
grounded to the Earth,
allowing charge to flow out
between it and the Earth.
Charging Methods-induction
Step 3. The ground is removed
while the charge rod is still
nearby the conductor.
Step 4. The rod is removed
and the conductor is now
charge (opposite of rod).
An object charged by induction has the opposite sign
of the influencing body.
Notice that the original charged object does not lose charge.
https://www.youtube.com/watch?v=-JsVZwc1dOo
Charging Methods-induction
What is grounding?
*Grounding means making objects neutral or uncharged
*The ground serves as an infinite source to supply electrons to object or receive electrons
from an object until the object has an equal amount of protons and electrons to be
considered neutral.
*Grounding with an inductor present allows the conducting
object to obtain a charge rather than neutralizing it
Writing prompt
In terms of electron movement, explain
what is happening in Diagrams B and C
above. Finally, state
the charge acquired by the left and the
right can as a result of this process.
Charging Methods-conduction
When a charged conductor makes contact with a
neutral conductor there is a transfer of charge.
CHARGING NEGATIVELY
Electrons are transferred from
the rod to the ball, leaving them
both negatively charged.
CHARGING POSITIVELY
Electrons are transferred from
the ball to the rod, leaving
them both positively charged.
Remember, only electrons are free to move in solids.
Notice that the original charged object loses some charge.
Charging Methods-conduction
Writing prompt:
Now consider the conducting charging of the
person using a positively charged sphere. In
terms of electron movement explain what is
happening for the person’s hair to have static
CHARLES COULOMB
(1736-1806)
MICHAEL FARADAY
(1791-1867)
The Quantity of Charge
The quantity of charge (q) can be defined in
terms of the number of electrons
The Coulomb (C) is the unit
The Coulomb: 1 C = 6.25 x 1018 electrons
1/e
Which means that the charge on a single electron is:
1 electron: e- = -1.6 x 10-19 C
Units of Charge
The coulomb (selected for use with electric
currents) is actually a very large unit for static
electricity. Thus, we often encounter a need to
use the metric prefixes.
1 mC = 1 x 10-6 C
1 nC = 1 x 10-9 C
1 pC = 1 x 10-12 C
The Electrostatic Force-Coulomb’s (C) Law
The force of attraction or repulsion between two
point charges is directly proportional to the product
of the two charges and inversely proportional to the
square of the distance between them.
k - The constant of proportionality =
8.99 x 109 Nm2/C2.
COULOMB’S LAW OF
ELECTROSTATIC FORCE
constant
kq1q2
Fe  2
r
electrostatic
force
(N)
Charges
(C)
Distance
(m)
A negative force is attractive,
and a positive force is repulsive.
The sign (+ or –) is different from
a vector direction (left or right)
The Electrostatic Force-Coulomb’s (C) Law
Draw and label
givens on figure:
-5 C
q
-
F
r
+3 C
+
q’
2 mm
Example A –5 C charge is placed 2 mm
from a +3 C charge. Find the force
between the two charges.
Note: Signs are used ONLY to determine force direction.
The Electrostatic Force-Coulomb’s (C) Law
EXAMPLE 1 - Find the force between these two charges
9.0  10 5  10


9
Fe
6

C 8  10 6 C
0.04 m 2

Fe  225 N
The negative signs means force of attraction,
but does not indicate left or right direction
EXAMPLE 2 - Find the net force on the left charge
9.0  10 5  10


9
Fe
Fe  360 N
6

C 5  10 6 C
0.025 m 2

(force of repulsion)
Fnet  Fleft  Fright
Fnet  360 N  225 N  135 N, to the left
The Electrostatic Force-Coulomb’s (C) Law
EXAMPLE 3 - Find the net force on the upper left charge
Fe,x  225 N, right
Fe,y  360 N, up
Fe  Fe,x 2  Fe,y 2  225 2  360 2  425 N
425 N,
58˚
 Fe,y 
1  360 
  tan 

tan

  58.0Þ

225
 Fe,x 
1
Electric Field Strength
Field Theory Visualizes Force At A Distance
DEFINITION OF
GRAVITATIONAL
FIELD
DEFINITION OF
ELECTRIC
FIELD
force
g field 
mass
g
E field 
force
charge
Fe
E
q0
Fg
m
q0 is a small, positive test charge
Electric field is a vector quantity
E field points toward negative charges
E field points away from positive charges
SI unit of electric field
click for
applet
newton
N

coulomb C
Electric Field Lines
Single Point Charges
POSITIVE CHARGE
NEGATIVE CHARGE
Density of field
lines indicates
electric field
strength
Inverse square
law obeyed
click for applet
Definition of E Field for single point charge
Fe kq0 q / r 2
E

q0
q0
electric
field
constant
charge
kq
E 2
r
distance
Electric Field Lines
Electric fields for multiple point charges
POSITIVE AND NEGATIVE POINT CHARGES
TWO POSITIVE POINT CHARGES
OPPOSITE MAGNETIC POLES
ALIKE MAGNETIC POLES
Summary of Formulas:
Like Charges Repel; Unlike Charges Attract.
kqq '
F 2
r
1 mC = 1 x 10-6 C
1 electron: e- = -1.6 x 10-19 C
1 nC = 1 x 10-9 C
1 pC = 1 x 10-12 C
Bell Ringer
•Draw an atom and label the
protons, neutrons and
electrons
•Which of these particles is
the easiest to remove?