Chapter 21 1

Chapter 21 Electric Charge and Electric Field

•Electrostatics - interaction between electric charges that are at rest•Electron - negatively charged particle that revolves around the nucleus of an atom with mass approx. equal to 9.1 x 10-31 kg.•Proton - positively charged particle inside the nucleus of an atom with a mass approx. equal to 1.67 x 10-27 kg.•Neutron - uncharged particle inside the nucleus of an atom with a mass approx. equal to 1.67 x 10-27 kg.

•http://www.youtube.com/watch?v=FquL0GG9RGI

Chapter 21 2

Chapter 21 Electric Charge and Electric Field

•Unlike charges attract and like charges repel•Positive Ion - an atom that lost one or more electrons.•Negative Ion - an atom that gained one or more electrons.•Ionization - gaining or losing electrons•Conductor - permits the easy movement of charge•Insulator - does not permit the easy movement of charge•Conservation of Charge Principle - The algebraic sum of all electric charges in a closed system is constant.•Magnitude of charge on an electron or proton (1.6 x 10-19

coulombs) is a natural unit of charge (all observable charge is an integral multiple).

Atom to Nucleus Ratio

Chapter 21 3

6km diameter(10-10 m)

Tennis ball6 cm diameter(10-15 m)

Chapter 21 4

Charge by Induction - give another body a charge of opposite sign without losing its own charge

Chapter 21 Electric Charge and Electric Field

Chapter 21 5

Coulombs Law (Chapter 21, Sec 3)

where F = force in newtonsq = charge in coulombs (disregard sign) r = distance between charges in meters

Force between two point charges (diameter of q much less than r)

+ +r

FF

q1q2

9

0

1094

1xk

120 10854.8 x

where 0 = permittivity of free space

Chapter 21 6

Superposition Principle (Chapter 21, Sec 3)

+q1

+q2

+q0

F

F2

F1

r1

r2

21

011

r

qqkF

22

022

r

qqkF

F = F1 + F2 (Vector Sum)

Chapter 21 7

Electric Field (Chapter 21, Sec 4)Definition of an electric field (E) - The electric force on a charged body is exerted by the electric field created by other charged bodies.

000

limq

FE

q in newtons/coulomb

Figure 21-13

Chapter 21 8

Force on a Charge in an Electric Field

Electric Field (Chapter 21, Sec 4)

F0 = q0E

F0 = – q0E

Chapter 21 9

Electric Field (Chapter 21, Sec 4)

+

q

204

1

r

qE

r

p•

Electric field (E) at point p distance r away from point charge q

+Test charge q0

+

r

q

20

r

qqkF

Coulomb’s law

20

20 4

1

r

q

r

qk

q

FE

(where q0 is much less than q)

(21-6)

Chapter 21 10

Electric Field (Chapter 21, Sec 4)Electric field (E) distance r away from point charge q (vector notation)

Figure 21-15

Chapter 21 11

Electric Field (Chapter 21, Sec 4)Electric field (E) of a point charge q

Figure 21-16

Chapter 21 12

Example 21-8

Figure 21-19

Charge on electronq = – e = – 1.6 x 10-19 coulombs

m

eE

m

qE

m

Fa yy

tvx 0

22

2

1

2

1t

m

eEtay y

20

22

v

xt 2

0

2

2

1

v

x

m

eEy

Electric Field (Chapter 21, Sec 4)

Chapter 21 13

+q1

+q2

E

E2

E1

r1

r2

21

11

r

qkE

22

22

r

qkE

E = E1 + E2 (Vector Sum)

Superposition Principle for an Electric Field

p

•

Electric-Field Calculations (Chapter 21, Sec 5)

Chapter 21 14

Example 21-9

Figure 21- 20

q1 = q2 = 12 nC

Electric-Field Calculations (Chapter 21, Sec 5)

Chapter 21 15

Electric Field of a Line Charge (Example 21-11)

Electric-Field Calculations (Chapter 21, Sec 5)

Chapter 21 16

Electric field distance r from an infinite line charge coul/m

•p

r rE

02

1

coul/m

line length >> r

Electric Field of a Line Charge

Line out of page

Electric-Field Calculations (Chapter 21, Sec 5)

Chapter 21 17

Electric-Field Calculations (Chapter 21, Sec 5)

Chapter 21 18

Electric Field of an Infinite Plane Charge

02

E

= surface charge density in coul/m2

Note that E is is constant (not dependent on thedistance from the charged surface).

(Eq 21-12)

Electric-Field Calculations (Chapter 21, Sec 5)

Chapter 21 19

Electric Field of two oppositely charged infinite plates (Ex 21-13)

Figure 21-24

021 2

EE

021

EEE

Electric-Field Calculations (Chapter 21, Sec 5)

Chapter 21 20

Electric Field Lines (Chapter 21, Sec 6)

Figure 21-26

When you rub a plastic rod with fur, the plastic rod becomes negatively charged and the fur becomes positively charged. As a consequence of rubbing the rod with the fur,

A. the rod and fur both gain mass.

B. the rod and fur both lose mass.

C. the rod gains mass and the fur loses mass.

D. the rod loses mass and the fur gains mass.

E. none of the above

Q21.1

A. electrons are less massive than atomic nuclei.

B. the electric force between charged particles decreases with increasing distance.

C. an atomic nucleus occupies only a small part of the volume of an atom.

D. a typical atom has many electrons but only one nucleus.

Q21.2

A positively-charged piece of plastic exerts an attractive force on an electrically neutral piece of paper. This is because

Three point charges lie at the vertices of an equilateral triangle as shown. All three charges have the same magnitude, but Charge #1 is positive (+q) and Charges #2 and #3 are negative (–q).

The net electric force that Charges #2 and #3 exert on Charge #1 is in

A. the +x-direction. B. the –x-direction.

C. the +y-direction. D. the –y-direction.

E. none of the above

Q21.4

Charge #1

Charge #2

Charge #3

+q

–q

–qx

y

A positive point charge +Q is released from rest in an electric field. At any later time, the velocity of the point charge

A. is in the direction of the electric field at the position of the point charge.

B. is directly opposite the direction of the electric field at the position of the point charge.

C. is perpendicular to the direction of the electric field at the position of the point charge.

D. is zero.

E. not enough information given to decide

Q21.5

Two point charges and a point P lie at the vertices of an equilateral triangle as shown. Both point charges have the same negative charge (–q). There is nothing at point P.

The net electric field that Charges #1 and #2 produce at point P is in

Q21.7 Charge #1

Charge #2

–q

–qx

y

P

A. the +x-direction. B. the –x-direction.

C. the +y-direction. D. the –y-direction.

E. none of the above

The illustration shows the electric field lines due to three point charges. The electric field is strongest

A. where the field lines are closest together.

B. where the field lines are farthest apart.

C. where adjacent field lines are parallel.

D. none of the above

Q21.8

Q21.9

A. the +x-direction.

B. the –x-direction.

C. the +y-direction.

D. the –y-direction.

E. none of the above

Positive charge is uniformly distributed around a semicircle. The electric field that this charge produces at the center of curvature P is in

Q21.10

Charge #1

Charge #2

Charge #3

+q

+q

–qx

y

A. clockwise.

B. counterclockwise.

C. zero.

D. not enough information given to decide

Three point charges lie at the vertices of an equilateral triangle as shown. Charges #2 and #3 make up an electric dipole.

The net electric torque that Charge #1 exerts on the dipole is