Electrostatics: Coulombs Law,
Equilibrium, Electric Fields
Atomic Structure
•Matter is made up of atoms.
Proton (positive charge)
neutron (neutral)
– electron (negative charge)
+
–
+
+
–
+
–
atom
nucleus
Atomic Structure and Charge
• # electrons = # protons  neutral
• # electrons > # protons  negative charge
(gain of e-)
• # electrons < # protons  positive charge
(loss of e-)
Charge is Quantized: charges must be whole
numbers and cannot exist in fraction form.
Triboelectric Series: Relative ranking of common materials
based on their electron affinity
*electron donating materials (+, glass)
* electron accepting materials (-, teflon)
•
•
•
•
•
•
•
•
•
•
•
Glass
Human Hair
Nylon
Silk
Fur
Aluminum
Paper
Cotton
Rubber
PVC
Teflon
+
+
+
+
+
-
++++
+++
++
+
----------
Electric Charge is Conserved
• When two materials interact, the amount of
electrons gained by one material must be equal to
the amount of electric charge lost by the other
material.
• Net charge remains unchanged.
• Law the Conservation of Charge
Charge and Mass of Atomic Particles
Charge
Mass
•Electron
-1.60 x 10-19 C
9.109 x 10-31 kg
•Proton
+1.60 x 10-19 C
1.673 x 10-27 kg
•Neutron
0
1.675 x 10-27 kg
Insulators
•Insulators: materials that do NOT allow electrons to
flow through them easily.
• Insulators can be easily charged by friction as the extra
electrons gained CANNOT easily escape.
• Some common insulators are glass, air, plastic, rubber,
and wood.
Conductors
•Conductors: materials that allow
electrons to flow through them
easily.
• Conductors CANNOT be easily charged by friction as the
extra electrons gained can easily escape.
• common conductors are copper, aluminum, gold, and
silver.
Charging objects
Polarization: the process of separating opposite
charges within an object, net charge of material
remain the same
• positive charge becomes separated from the negative
charge
3 Ways to Charge an Object
1. Friction: rubbing two different materials together
creates polarization
• objects made of different materials will hold onto their
electrons with different strengths.
• As objects rub past one another the electrons with
weaker bonds are “ripped” off of one material and
collected on the other material.
3 Ways to Charge an Object
2. Induction - Bring a charged object (rod) close to a neutral
one (ball) without contact
•
electrons in the ball will be repelled by the rod leaving
a positive side (polarization)
•
The now positive side of the ball will be attracted to
the negative rod
- -- ++
- - - +++
- - - +++ +
3 Ways to Charge an Object
Induction is a temporary change if no grounding occurs
 electrons are not transferred
Induction is a permanent change if grounding occurs
 electrons are transferred
• charge induced is opposite
3 Ways to Charge an Object
3. Conduction is a permanent charge with contact between 2
objects
 electrons are transferred
• Charge conducted is the same
• After conduction the balls will repel each other
Gained El
Lost
Electrons
What is grounding?
• Removing a static charge by producing a path to the ground
• Electrons move from a negatively charged objects to the
ground until the object is neutral
• Electrons move from ground to neutralize positively
charged objects
• The earth both accepts and gives electrons while
remaining overall neutral
Electric Charge: Section 2
• Symbol: Q or q
• Unit: coulomb (C)
• 1 C = the charge on 6.24 x 1018 electrons
Extra info to help you with problems
•
1 electron = -1.60 x 10-19 C
•
A coulomb is a huge charge so charge is usually stated in
µC (1x10-6 C) or nC (1x10-9 C)
Coulomb’s Law
Charged objects can exert a force on each other. This force can
be either attractive force or a repelling force.
3 factors affecting the magnitude of the force between two
charged objects:
1) Charge on the objects (magnitude of charge)
2) Distance between objects
3) Material separating objects
Coulomb’s Law
F: electrical force
Q1: charge 1
Q2: charge 2
d: distance between charges
k: constant depending on materials separating objects
- For air, k = 8.99 x 109 N·m2/C2, this is the value we
will use for calculations
How are charge and electrical force
related?
How is electrical force related to distance
between the charges?
When using this equation:
• A positive force (F) signifies repulsion
• Both charges (Qs) must be positive or both negative
• A negative force (Fe) signifies attraction
• One charge (Q1 or Q2) must be positive and the other
negative
Sample Problems
a. What is the electrostatic force between
two objects, +13 μC and -22 μC which are
0.055m apart
(μC = x 10-6 C)
b. Is it an attraction or a repulsion?
Attraction (Q1 and Q2 are opposite signs)
Problem Set 1
1) What is the electrostatic force between two objects of -14
x 10-6 C and -22 x 10-6 C that are 0.86 m apart?
2) The electrostatic force between two objects is +6.4 N
when the charges are -4.5 x 10-6C and -8.8 x 10-6C. How far
apart are the objects?
3) A force of -1.1N is exerted between two charged objects
when they are 43 cm apart. The charge on one object is -5.7
x 10 -6 C. What is the charge on the other object?
Finding Net Force with more that one charge:
Superposition
• See beige box 17 B on page 649 in your text book
Final Electricity Test Includes
• Ohms Law
• Resistance Rules
• Equivalent (total) Resistance
• Complex Circuit Resolution
• Electrostatics Concepts
• Coulombs Law (force)