The primary particle that carries charge (and therefore can be lost or gained) in an atom is a/an:
Proto
n
Neutro
n
Electr
on
0% 0%0%
1. Proton2. Neutron3. Electron
When a positively charged object comes close to an negatively charged object, the negative object will:
Be attracte
d
Be repelle
d
Do noth
ing
0% 0%0%
1. Be attracted2. Be repelled3. Do nothing
When a positively charged object comes close to an neutral object, the neutral object will:
Be attracte
d
Be repelle
d
Do noth
ing
0% 0%0%
1. Be attracted2. Be repelled3. Do nothing
Anytime an object at rest starts to move, what must be present to cause it to move?
Diff
erence
in electr
ic c..
.
Electr
icity
A force
Matter
0% 0%0%0%
1. Difference in electric charge
2. Electricity3. A force4. Matter
A Bit of History
Ancient Greeks Observed electric and magnetic
phenomena as early as 700 BC Found that amber, when rubbed, became
“electrified” and attracted pieces of straw or feathers
Properties of Electric Charges
Two types of charges exist They are called positive and negative Arbitrarily named by Benjamin Franklin
Like charges repel and unlike charges attract one another
More Properties of Charge
Positive charge – protons Negative charge – electrons
Gaining or losing electrons is how an object becomes charged; protons remain with the nucleus
Conservation of Charge
Electric charge is always conserved Charge is not created, only
exchanged Objects become charged because
negative charge is transferred from one object to another
More review:
A force is …? Anything that produces acceleration
or a change in motion. Contact vs. Field forces?
Contact: require matter to be in contact (ex. friction)
Field: matter not required (ex. gravitational, electrical, magnetic)
Properties of Charge, final
Charge is quantized All charge is a multiple of a
fundamental unit of charge, symbolized by e
Electrons have a charge of –e Protons have a charge of +e The SI unit of charge is the Coulomb
(C) 1 e = 1.6 x 10-19 C
Conductors
Conductors are materials in which the electric charges move freely Copper, aluminum and silver are good
conductors
Insulators
Insulators are materials in which electric charges do not move freely Glass and rubber are examples of
insulators When insulators are charged by
rubbing, only the rubbed area becomes charged
There is no tendency for the charge to move into other regions of the material as opposed to conductors
Charging by Friction
The act of rubbing creates friction which removes or adds electrons to the objects involved in the friction.
Charging by Conduction A charged object (the
rod) is placed in contact with another object (the sphere)
Some electrons on the rod can move to the sphere
When the rod is removed, the sphere is left with a charge
The object being charged is always left with a charge having the same sign as the object doing the charging
Charging by Induction
A negatively charged rubber rod is brought near an uncharged sphere
The charges in the sphere are redistributed Some of the electrons
in the sphere are repelled from the electrons in the rod
Charging by Induction, final
The wire to ground is removed, the sphere is left with an excess of induced positive charge
The positive charge on the sphere is evenly distributed due to the repulsion between the positive charges
Charging by induction requires no contact with the object inducing the charge
A charged rod is brought close to a neutral electroscope. When touched by the rod, the leaves both become positive and repel. The rod must have been …
Positive
ly cha..
.
Negatively
cha...
Not e
nough in
f...
0% 0%0%
1. Positively charged
2. Negatively charged
3. Not enough information to tell
A neutral electroscope is charged by induction. When touched by the rod, the leaves both become negative and repel. The rod must have been …
Positive
ly cha..
.
Negatively
cha...
Not e
nough in
f...
0% 0%0%
1. Positively charged
2. Negatively charged
3. Not enough information to tell
A postively charged rod is brought close to a neutral electroscope to charge it by induction. The top of the electroscope must be:
Positive
ly cha..
.
Negatively
cha...
Not e
nough in
f...
0% 0%0%
1. Positively charged
2. Negatively charged
3. Not enough information to tell
Coulomb’s Law
Mathematically,
kc is called the Coulomb Constant kc = 8.99 x 109 N m2/C2
221
r
qqkF celec
Coulomb’s Law
Typical charges can be in the µC range Remember, Coulombs must be used in the
equation
Remember that force is a vector quantity
It is attractive if the charges are of opposite signs and repulsive if the charges have the same signs (you must note if it is attractive or repulsive after the magnitude – 3 N attractive)
221
r
qqkF celec
How is the magnitude of the charges proportional to the electric force between them?
Dire
ctly
Inverse
ly
Exponentially
0% 0%0%
1. Directly 2. Inversely3. Exponentially
How is the square of the distance between two charges proportional to the electric force between them?
Dire
ctly
Inverse
ly
Exponentially
0% 0%0%
1. Directly 2. Inversely3. Exponentially
Coulomb’s Law
It is attractive if the charges are of opposite signs and repulsive if the charges have the same signs (you must note if it is
221
r
qqkF celec
Things that make you go, “Hmmmm…”
A) The electric force is significantly stronger than the gravitational force. However, although we are attracted to Earth by gravity, we do not usually feel the effects of the electric force.
Explain why.
B) An ordinary nickel contains about 1024 electrons, all repelling one another.
Why don’t these electrons fly off the nickel?
C) When the distance between two negatively charged balloons is doubled, by what factor does the repulsive force between them change?
Electrical Force Compared to Gravitational Force
Both are inverse square laws The mathematical form of both
laws is the same Electrical forces can be either
attractive or repulsive Gravitational forces are always
attractive
If all other variables are held constant, is the electric force or the gravitational force greater for two oppositely charged objects?
Grav
itational ..
.
Electr
ic Fo
rce Sa
me
Unable to
tell
0% 0%0%0%
1. Gravitational Force
2. Electric Force3. Same4. Unable to tell
Compare Gravitational and Electric Force
Calculate the gravitational force as well as the electric force for an electron and a proton which are located 1 cm from each other.
Electrical Field
An electric field is said to exist in the region of space around a charged object When another charged object enters
this electric field, the field exerts a force on the second charged object
Electric Fields The concept of a field is used to describe any
quantity that has a value for all points in space. You can think of the field as the way forces are
transmitted between objects. Charge creates an electric field that creates
forces on other charges.
Van de GraaffGenerator
An electrostatic generator designed and built by Robert J. Van de Graaff in 1929
Charge is transferred to the dome by means of a rotating belt
Eventually an electrostatic discharge takes place
Electrical Field
An electric field is said to exist in the region of space around a charged object When another charged object enters
this electric field, the field exerts a force on the second charged object
Electric Field, cont.
A charged particle, with charge Q, produces an electric field in the region of space around it
A small test charge, qo, placed in the field, will experience a force
Electric Field Lines
Electric field is a vector. There are electric field lines that
help visualize this field and were introduced by Michael Faraday
Electric Field Lines
Electric field lines are drawn to visualize electric field strength and direction.
Introduced by Michael Faraday
Electric Field Line Rules
Electric Field Lines are drawn pointing in the way that a positive point charge would move when placed by the charged object.
Field lines can never cross
Electric fields exist even in theabsence of a point charge.
Electric Field Line Rules
The direction of the electric field is in the same direction as the electric force on the point charge.
The relative strength of the electric field is proportional to the number of field lines in a given location.
Electric field lines accumulate as sharp points more than rounded objects.
Electric Potential Energy of a Charge Wants to move when it
has high PE Point b
PE = max KE = min
Point a PE = min KE = max
Electric Field Intensity Equation
E = F/q
F is the force in Newtons acting on the test charge q in coulombs.
Combined with Coulomb’s law,
E = kQ/r2
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