Electric Forces and Fields

Chapter 17

ELECTRIC CHARGESection 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

18.4 Charging by Contact and by Induction

Charging by contact.

Transfer of Electric Charge:Contact – (friction and Conduction)

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

18.4 Charging by Contact and by Induction

Charging by induction.

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 FORCESection 2

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

CLASS ASSIGNMENT

DUE BY THE END OF CLASS

will not accept late work

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

The Electric Field

Section 6

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.

18.6 The Electric Field

Electric fields from different sources add as vectors.

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.

Electric Field Lines – sketch this!!

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!!

18.7 Electric Field Lines

Electric Field Lines – sketch this!!

Now sketch two negative charges!!

Electric Field Strength

Electric field strength depends on charge and distance.

CLASS ASSIGNMENT

DUE BY THE END OF CLASS

Will not accept late work