Electric Forces and Fields Chapter 17. ELECTRIC CHARGE Section 1.
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Transcript of Electric Forces and Fields Chapter 17. ELECTRIC CHARGE Section 1.
Chemistry Review Atoms are small particles that make up matter Atoms are neutral when they have equal numbers of
______________and _____________ Atoms can lose electrons, because electrons are
located in the _________________ Protons are in the _______________and do not
leave the atom When atoms gain electrons, they become
_____________ charged ions When atoms lose electrons, they become
______________ charged ions
Electric forces and charges
The electric force is one of four fundamental forces.
The electric force is a field force. The electric force is the result of the
interaction between electric charges.
Properties of Electric Charge
There are two kinds of electric charges.
Protons have a positive charge; electrons negative.
Opposite charges attract; Like charges repel.
Properties of Electric Charge
Electric charge is conserved.
When a neutral atom gives up an electron, it becomes a positive ion.
The electron is the smallest unit of charge that can be isolated AKA elementary charge
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Insulators and Conductors
A conductor is a material that allows charges to flow easily throughout the material. Most metals are
conductors.
An insulators is a material that does not allow charges to flow easily throughout the material. Glass, rubber, silk and
plastic are examples of insulators.
Transfer of Electric Charge:Contact – (friction and Conduction)
Conductors and insulators can be charged by contact. Example- Rubbing rubber rod with fur, glass rod
with silk. = charging by friction
Transfer of Electric Charge:Induction
Conductors can be charged by induction. A charged object is brought
near the conductor. The conductor is then
grounded, allowing the excess charge to flow away.
The grounding is then broken.
The conductor remains charged, even when the charged object is removed.
© 2013 Mark Lesmeister/Pearland ISD
Transfer of Electric Charge:Polarization
A surface charge can be induced on an insulator by polarization. Ex. Walking across a
carpet on a dry day.
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© 2013 Mark Lesmeister/Pearland ISD
LAW OF CONSERVATION OF ELECTRIC CHARGE
During any process, the net electric charge of an isolated system remains constant (is conserved).
Electric Force When two charged objects are brought close
together, they may experience forces of attraction or repulsion.
The closer the two objects are to each other, the stronger the force between them.
The greater the mass (number) of the charges the stronger the force between them
Magnitude and direction of the electric force depends on:
The amount of charge carried by each object. Charge is measured in Coulombs (C).
An electron (negative elementary charge) has a charge (q) of -1.60 x 10-19 C. A proton (positive elementary charge) has a charge (q) of 1.60 x 10-19 C.
Coulomb’s Law
• The force between 2 charged objects can be calculated using coulomb’s Law
• Kc = 8.99 X 109 N.m2/C2
221
r
qqkF Celectric
2(distance)
2) 1)(charge (charge constant Coulomb force Electric
Fundamental forces of nature Note the similarities and differences between gravitational force and electrical force - only attractive, can’t be shielded
- both attractive and repulsive,
can be shielded
These are two of the four fundamental forces of nature
Example 1 What is the magnitude of the electric force between a proton and
an electron in a hydrogen atom? The two particles are separated by a distance of 5.3 x 10-11 m on average. G: q1 = -1.60 x 10-19 C, q2 = 1.60 x 10-19 C, r = 5.3 x 10-11 m, Kc = 8.99 x 109 N.m2/C2
U: Felectric = ?
E:
S:
S: 8.2 x 10-8 N An attractive force!!
221
r
qqkF Celectric
211
1919229
103.5
106.1106.1/CN.m1099.8
m
CCFelectric
In the Bohr model of the hydrogen atom, the electron is in orbit about the nuclear proton at a radius of 5.29x10-11m. Determine the speed of the electron, assuming the orbit to be circular.
G: ,
U: E: S:
Example 2: A Model of the Hydrogen Atom
sm1018.2
kg109.11
m1029.5N1022.8 631-
118
mFrv
Which of the following statements about charges is true?
A. Charges are created by gaining electrons.
B. Charges are destroyed by losing electrons.
C. Charges are separated through a transfer of electrons.
D. Charges cannot be created, destroyed, or separated.
Section CheckSECTION20.1SECTION CHECK
What will happen if a positively charged rod is suspended freely in air and another positively charged rod is brought near the suspended rod?
A. The more you bring the rod closer to the suspended rod, the more it will attract.
B. The more you bring the rod closer to the suspended rod, the less it will attract.
C. The more you bring the rod closer to the suspended rod, the more it will repel.
D. The more you bring the rod closer to the suspended rod, the less it will repel.
SECTION20.2
Section CheckSECTION CHECK
Electric Field
The electric field is a field force per unit charge.
An electric field exists in a region if a stationary charged object (test charge – usually positive) experiences an electric force because of its charge.
Electric Field Vector
The electric field at any point is a vector. The direction of the electric field is the direction in
which a small positive charge would move if placed there.
The magnitude of the electric field (E) is the force per unit charge at that point:
The electric field is measured in N/C.
charge. test theof magnitude theis where q
q
FE electric
18.6 The Electric Field
It is the surrounding charges that create the electric field at a given point.
Electric Field Lines
Electric field lines are lines of force providing a map of the electric field in the space surrounding electric charges.
Electric field lines represent the strength and direction of an electric field at any point around a charged object.
Electric field lines do not really exist, however, they offer a useful means of analyzing electric fields.
Electric Field Lines
Electric field lines run in the direction of the field at each point.
Lines begin on positive charges (or infinity) and end at negative charges (or infinity).
Lines do not cross.
The magnitude of the field is shown by the number of lines in a given area.
18.7 Electric Field Lines
The number of lines leaving a positive charge or entering a negative charge is proportional to the magnitude of the charge.
Electric Field Lines – sketch this!!