ElectrostaticsHonors Physics

Biblical Reference

He loads the clouds with moisture; he scatters his lightning through them.

Job 37:11

Charge - Amount of electricity that can be given off by an object

Facts about Charge: 2 types - positive (protons) and negative

(electrons) LIKE charges REPEL OPPOSITE charges ATTRACT SI Unit – Coulomb (C) Symbol – q or Q

Electric Charge

Electric Charge – The specifics

Some important constants:

Particle Charge Mass

Proton 1.6x10-19 C 1.67 x10-27 kg

Electron -1.6x10-19 C 9.11 x10-31 kg

Neutron 0 1.67 x10-27 kg

A SINGLE charged particle such as 1 electron or 1 proton is an ELEMENTARY charge, e .

Charge cannot be created or destroyed - only transferred from one object to another.

Even though these 2 charges attract initially, they repel after touching.

The NET charge stays the same.

Charge is “CONSERVED”

Re-Arranging of Positive & Negative charges, but remains electrically Neutral (i.e. no loss of charges)

Polarization

The movement of charge is limited by the substance the charge is trying to pass through.Conductors: Allow charge to move readily though it.Insulators: Restrict the movement of the chargeSemi-Conductors: Slight restriction in charge movement

Conductors and Insulators

Conductor = Copper WireInsulator = Plastic sheath

Semi-Conductors make excellent electronic components.

Rubber has a much greater attraction for electrons than animal fur.

When you wipe a rubber rod with animal fur, the atoms of rubber pull electrons from the atoms of animal fur, leaving both objects with an imbalance of charge.

◦ The rubber rod has an excess of electrons (charged negatively).

◦ The animal fur has a shortage of electrons (charged positively).

Charging by Friction

Charging by Friction

Practical Use• BIONIC is the first-ever ionic formula mascara.

• The primary ingredient in BIONIC is a chain molecule with a positive charge.

• The friction caused by sweeping the mascara brush across lashes causes a negative charge.

• Since opposites attract, the positively charged formula adheres to the negatively charged lashes for a dramatic effect that lasts all day.

Requires NO PHYSICAL CONTACT

Induction and Grounding

Conduction (Two Conductors)

Force is inversely proportional to the square of the distance between the charges. Therefore, if the distance increases by a factor of 2, the force decreases by a factor of 4.

Force is proportional to the strength of each charge.

When the two charges have the same sign (positive or negative), the force between them is repulsive because like charges repel.

When the charges have opposite signs, the force between them is attractive because unlike charges attract.

Coulomb’s Law

Electric Force

LawsCoulombr

qqkF

C

Nmx.k

kr

qqF

rFqqF

E

EEE

'

10998 constant Coulomb

alityproportion ofconstant

1

221

2

29

221

221

Permittivity of Free Space (0) – Characteristic that describes how a substance affects an electric field set up on it

Another Format

2

212

0 1085.84

1

mN

C

k

20

21

4 r

qqF

Example

Nm

CCCNm

r

qqkF 29.0

)7.0(

)104)(104(1099.8

2

662

29

221

F = ?

ExampleCalculate the separation distance between a 12C charge and a 6C charge is the electric force between them is 136.5 N

2

21

r

qqkF

mN

CCCNm

F

qqkr 21776

5.136

)6)(12)(1099.8( 2

29

21

Electric Field

An electric field is a charged property of space.

An electric field is measured in N/C because it is force divided by the charge.

Find the magnitude and direction of the net force exerted by q1 and q3 on q2

More than Two Charges

More than Two Charges

Nm

CCCNm

r

qqkF 63.8

)10125(

)3)(5(1099.8

23

2

29

221

12

Nm

CCCNm

r

qqkF 25.7

)10122(

)3)(4(1099.8

23

2

29

223

32

More than Two Charges

NFnet 3.11)63.8()25.7( 22

25.7

63.8

netx

nety

F

FTan 50

- 8.63 N

- 7.25 N

Electric Field LinesA method to visualize electric field lines is to draw lines pointing in the direction of the electric field

The more concentrated the field lines, the stronger the electric field.

Force per unit charge

Electric Field Strength

N/C N

C

q

FE e

22 r

kQ

qr

Qqk

q

FE

A particle has a charge of +5.00 nC. Find the magnitude and direction of the electric field due to this particle at a point 0.4 m directly above it.

Example

C

N

r

qkE 9.280

4.0

)105)(10988.8(2

99

2

Since the charge is positive, the direction of electric field is outward (away from the charge).

Multiple ChargesFind the electric field midway between two charges of +30.0 nC and 60.0 nC with a separation distance of 30.0 cm.

C

N

r

qkE 984,11

15.0

)1030)(10988.8(2

99

21

C

N

r

qkE 968,22

15.0

)1060)(10988.8(2

99

22

+30 nC +60 nC

C

NEEETot 984,1121

Gravitational Force – Always parallel to Gravitational Field

Electrical Force – No always parallel to Electric Field If q is (+), the direction of Fe is the same as If q is (-), Fe is opposite to

Electricity vs. Gravity

221

r

qqkFe 2

21

r

mGmFg

mgFg qEFe

Video Clip

Lighting