Ch 18 Static Electricity

Conductors & Insulators

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Charging by Contact

Charging by Induction

Summarize the 3

Steps of Induction:

1.

2.

3.

Summarize the 3

Steps of Induction:

1.Induce a Dipole

2.

3.

Summarize the 3

Steps of Induction:

1.Induce a Dipole

2.Ground “Excess

Charge”

3.

Summarize the 3

Steps of Induction:

1.Induce a Dipole

2.Ground “Excess

Charge”

3.Remove Bridge

What’s going on here?

What’s going on here?

If you stick a piece of transparent

tape on your desk and then

quickly pull it off, you will find that

the tape is attracted to other

areas of your desk that are not

charged.

Why does this happen?

Why is an electrostatic

spray gun more efficient

than an ordinary spray

gun?

Electroscope:

• Detects charge &

charge type.

• Stores charge.

• Detects charge

magnitude.

Forces Between Charges

Coulomb’s Law of Electrostatic

Attraction/Repulsion

Fkc q1 q2

r2

kc 9 109

N m

2

C2

The Coulomb

• Fundamental, smallest unit of charge

found in nature.

• Denoted by e, positive or negative.

• e = 1.6 x 10-19 C

• Microcoulomb = 1 x 10-6 C

• Nanocoulomb = 1 x 10-9 C

• Charge is said to be quantized.

Find the ratio of the

Coulombic

Electrostatic Force Fe

to the gravitational

force Fg between 2

electrons

Fek q

2

r2

FgG m

2

r2

Fe

Fg

k q2

r2

G m2

r2

k q2

G m2

Fe

Fg

8.99109

N m

2

C2

1.61019

C 2

6.671011

N m

2

kg2

9.11031

kg 2

Fe

Fg

4.17 1042

1.00

4000000000000000000000000000000000000000000

1

FBD Subscript Convention

F12

Fon 1 from 2

F13

Fon 1 from 3

The Electric Field

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Compare E field strength (density)

for regions A and B.

The Test Charge Concept

• Fictitious charge, q0

• Behaves like real charges but exerts no forces on other charges in the vicinity.

• Approximated by a real charge of nearly negligible value.

Electric Field Concept

• The space about a charged object

in which a second charged object

would experience a force of

attraction or repulsion.

• Field vector direction same as

direction experienced by + test

charge.

Electric Field Concept • Sketched with electric field

lines. The closer the spacing

between lines, the stronger the

electric field and resulting

force.

Electric Field Concept

Electric Field Strength, E

• Also called Electric Intensity.

• Equal to the force experienced by a + test

charge, +q0, placed at that point.

• If q is negative, then F is opposite in

direction.

E F

qso F

q E

Superposition Principle • The net force experienced by

a charge due to other charges is the vector sum of the coulombic forces acting on it due to these other charges.

• Similarly, the electric field intensity, E, at a point due to several charges is the vector sum of the intensities due to these individual charges.

Electron in an Electric Field

(a) What is the direction of the

electrostatic force on the electron

due to the electric field shown?

(b) In which direction will the

electron accelerate if it is moving

parallel to the y axis before it

encounters the electric field?

(c) If, instead, the electron is

initially moving rightward, will its

speed increase, decrease, or

remain constant?

Lightning

strikes a 20-m-

high sycamore.

Because the

tree was wet,

most of the

charge traveled

through the

water on it and

the tree was

unharmed.

Ch18 Page 582 Prob *43

Ch18 Page 583 Prob **46

Ch18 Page 583 Prob **47

Ch18 Page 584 Prob *68

Ch18 Page 584 Prob *69

Ch18 Page 584Prob **71

When a rubber rod is rubbed with

wool, the rod becomes

negatively charged, what can

you conclude about the

magnitude of the wool’s charge

after the rubbing process?

Why?

The electron and proton of a

hydrogen atom are separated,

on average, by a distance of

about 5.3 × 10-11 m. Find the

magnitudes of the electric

force and the gravitational

force that each particle exerts

on the other.