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Electric Charges and Forces
25.1
I)
Developing a Charge Model
Experimenting with Charges
A) Neutral
B) Attraction/repulsion
C) Like/unlike
II) Electric Properties of Materials
A) Charge can be transferred – only by contact, discharging
B) Conductors and Insulators
(1) Conductors move charge easily
(2) Insulators can be charged, but charge cannot move easily
25.2 Charge
I) Convention for positive and negative began with Ben Franklin
A) Positive
B) Negative
II) Atoms and Electricity
A) Electron Cloud
(1) Only know its size and shape – consists of negatively charged electrons
(a) Diameter = 10-10 m
B) Nucleus
(1) Positively charged protons, neutral neutrons
C) Charge is an inherent property of electrons and proton
(1) Like mass
(2) Comes in discrete values (+𝑒 and – 𝑒)
III) The Micro/Macro Connection
A) Charge is represented by the variable 𝑞 (or 𝑄)
B) 𝑞 = 𝑁𝑝𝑟𝑜𝑡𝑜𝑛𝑠 𝑒 − 𝑁𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑛𝑠 (𝑒)
C) Neutral doesn’t mean no charges, rather it means no NET charge
D) Quantization
(1) Positive charge is established by the absence of electrons, not an abundance of protons
IV) Conservation and Charge Diagrams
A) Charge is neither created nor destroyed
B) As one object looses electrons the other in contact will have to gain the same number of
electrons as lost by the first
25.3
I)
Insulators and Conductors
Charging (contact)
A) Insulators – charged by rubbing
B) Conductors – allow charge to spread out on their surfaces (electrostatic equilibrium)
II) Discharging (contact)
III) Charge Polarization (momentary)
IV) Electric Dipole (established/maintained)
V) Charging by Induction (established/maintained)
25.4
I)
Coulomb’s Law
Force between two charges can be quantified
A) Cavendish experimentation similar to that used to determine 𝐺
B) Equation: 𝐹1 𝑜𝑛 2 = 𝐹2 𝑜𝑛 1 =
C) 𝐾 = 9.0 𝑥 109
𝐾|𝑞1 ||𝑞2 |
𝑟2
N∙m2
C2
D) Forces exist in pairs, as always in relation to Newton’s third law
E) Multiple forces can be summed for one individual charge, Superposition Principle
1. May be written in vector form
2. May be written in component form
II) Units of charge
A) Coulomb: 1.6 𝑥 10−19 𝐶 = 1𝑒
B) Force may also be described in terms of Permittivity constant
(1) 𝜖0 =
(2) 𝐹 =
1
= 8.85 𝑥
4𝜋𝐾
1 |𝑞1 ||𝑞2 |
4𝜋𝜖0
𝑟2
10−12
C2
N∙m2
(3) Examples 25.3 and 25.4
25.5 The Field Model
I) The Concept of a Field
A) Mathematically, field describes a function that assigns a vector to every point in space
B) fields as a model explain many physics phenomena
II) The Electric Field
A) Two postulates
(1) Source charges alter the space around them by creating an electric field, 𝐸⃗ .
(2) A separate charge in the field experiences electric force 𝐹𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐
B) 𝐸⃗ (𝑥, 𝑦, 𝑧) ≡
𝐹𝑜𝑛 𝑞 𝑎𝑡 (𝑥,𝑦,𝑧)
𝑞
III) The Electric Field of a Point Charge
A) 𝐹𝑜𝑛 𝑞′ =
B) 𝐸⃗ =
C) 𝐸⃗ =
𝐹𝑜𝑛 𝑞′
1 𝑞𝑞′
4𝜋𝜖0 𝑟 2
, away from 𝑞
1
𝑞
= 4𝜋𝜖 𝑟2
𝑞′
0
1
𝑞
𝑟̂
4𝜋𝜖0 𝑟 2
, away from 𝑞
IV) Representing the Field
A) Finding a multitude of forces and field strength at various positions, allows us to draw a
Field Diagram
B) The arrow shows the direction and the strength at various positions
C) Though drawn as rays, the values of electric field do not extend forever, rather exist only at
one point.