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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
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