Download Electrostatics Notetakers

Chapter 20 & 21: Electrostatics
There are four forces that exist in nature:
1. ______________________
2. ______________________
__________________________, that
is, they only act over very small
distances.
____________________________ and can
act over very large distances.
3. ______________________
4. ______________________
Rules of Electrostatics:
1. There are two kinds of charge that exist in nature (_________________ charge and
__________________ charge) and they have the property that __________ ___________________ one
another, and ______________________________ one another.
2. The ___________________ between charges varies as the inverse square of the
_____________________, and directly with the __________________.
(____________________________)
3. _______________________________________________
4. _______________________________________________ (quantized means small discrete packets that
can not be further subdivided. For example you can have one electron or 2 electrons, but never half an
electron)
Charge:

The basic unit of ________________ charge is the __________________. (Although protons are
ultimately made up of quarks)

The basic unit of ___________________ charge is the ____________________. It is almost always
electrons that are moving when charge “________________”

The SI unit of charge is the _________________ (C).

Charge of 1e- = 1 proton = 1.6x10-19 Coulombs
Picture Time: Object with:
no overall charge:
negative charge:
positive charge:
Example: Lisa rubs a piece of fur on glass rod, giving the rod a negative charge. What is the most likely thing
that happens? (a) Protons are removed from the rod. (b) electrons are added to the rod. (c) the fur is also
charged negatively. (d) the fur is left neutral.
Conductors and Insulators:

____________________ are materials in which ____________________ are free to ____________.
Example: ________________________

_______________________ are materials in which _______________________. Examples:
________________________________
Example: Which of the following best characterizes electrical conductors? (1) low mass density, (2) high tensile
strength, (3) poor heat conductors, (4) charges move freely, (5) all the above.
How to Charge Objects
Consider the “pith” ball (pith is a material kind of like cork). Why is the neutral pith ball attracted to the
negatively charged rod? Why does it bounce away?
Consider the electroscope (an electroscope measures charge):

Charge by ___________________ - a physical transfer of charge from a charged object. How does this
work?

Charge by ___________________ - A transfer of charge, but only two neutral objects touch. How does
this work?
Example: An uncharged conductor is supported by an insulating stand. I pass a positively charged rod near the
left end of the conductor, but do not touch it. The right end of the conductor will be: (1) negative, (2) positive,
(3) neutral, (4) attracted, (5) depends on the materials.
Why?
Coulomb’s Law
Where k = 9.0x109 N m2/C2 (Coulomb’s Constant)
d = Distance between charges (m)
Q = Charges (C)
Coulomb’s Law describes the forces that:
1. Binds _____________________________ to the nucleus.
2. Binds __________________________ to form molecules.
3. Binds __________________________ to form solids and liquids.
Example: Two charges, __________________ and _________________ are 5.0 m apart. Calculate the force
between them.
Example: Find the net force on the 5.0 nC charge. (Draw Picture)
Example: Two point charges are 4 cm apart. They are moved to a new separation of 2 cm. By what factor does
the resulting mutual force between them change?
Example: If the size of the charge value is tripled for both of two point charges maintained at a constant
separation, the mutual force between them will be changed by what factor?
The Electric Field:
•
The electric force is another force that is able to act at a distance. (Just like ________________)
•
Electric charges don’t need to be ___________________ to exert a force on each other.
•
This means the electric force must be a _________________________.
•
All __________________________________________________________ around themselves.
•
Any other ____________ in that electric field will experience an ________________ & will either be
attracted or repelled by the original charge.
•
It is this electric field that extends out _____________________ that allows that charge to act at a
distance on another charge.
•
Masses only attract so all gravitational fields point towards the mass creating them.
•
Charges can both ____________ and __________________. So the field might point ______________
a charge or _________________ from the charge creating the field, depending on its charge.
The Test Charge
•
To determine the direction of the electric field, a test charge is used.
•
A test charge is a pretend _________________ point charge that is infinitely small and has a very very
small positive charge so it will not disrupt the field it is in at all.
•
The test charge is always _________________, never negative.
•
The way I remember it: Pretend Positive Point charges come from your Pants Pocket
Drawing Electric Field Lines
Draw the electric field for
A positive Charge
A negative Charge:
Rules for Drawing Electric Fields:

The lines must _________ on __________ charges and _______ on _________ charges, or at infinity.

The _____________________ drawn leaving a positive charge or approaching a negative charge is
___________ to the amount of charge.

Field lines may not cross or touch each other.

Field lines must meet conductors or charges perpendicular to the surface of the conductor or charge.

Example 2 Charges:
Example Charge & Plate:
Quick Review: Back to Earth’s Gravitational Field:

Near the surface of the Earth, Earth’s gravitational field is 9.8 N/kg downwards, toward Earth’s surface.

Do larger masses experience a larger gravitational field from Earth?

What two variables does gravitational field depend on?
The electric field produced by a point or spherical charge is given by
k = Coulomb’s Constant (9.0x109 N m2/C2)
Q = The charge producing the field. Given in Coulombs
d = The distance to the point in question
The ___________________ of the _____________________ is based on the _____________ of
_________________ for a positive charge.
Another way to calculate the electric FORCE:
F = Electric Force (N)
E = Electric Field (N/C)
Q = Charge placed in the field (C)
Example: What is the electric field 20.00 m away from a (__) _________point charge?
Example: Two charges are along the x-axis. Q1 is 3.0 m from the origin and has a charge of -12.0mC. Q2 is 4.5
m from the origin and has a charge of +4.0mC. (all charges are along the positive x-axis)
a) Calculate the electric field 8.0 m from the origin. (DRAW THE PICTURE)
b) b) What force will a - 9.0 mC charge experience if it is placed 8.0 m from the origin?
Example: Two charges, +Q and –Q, are located two meters apart as shown. Which vector best represents the
direction of the electric field at the point above them? Why? (draw the picture)
Example: Two point charges, separated by 1.5cm, have charges of +2 and -4C. Suppose we determine that ___
field lines radiate out from the +2C charge. If so, what might be inferred about the -4C charge with respect to
field lines?
Measuring the Charge on an Electron – Millikan’s Oil Drop Experiment

In 1909 Robert Millikan measured the charge of an electron using an oil drop experiment

In 1923 he received the Nobel Prize for his work.

Millikan shot X-rays at a spray of oil drops, giving the oil drops a negative charge. The charged oil
drops then went into an electric field controlled by Millikan.

The oil drops were then attracted to the positively charged plate which created the field.

Millikan varied the electric field in between the plates, until the electric force on the “target” oil drop
balanced the force of gravity and the oil drop stayed suspended between the plates.
Example: A 3.2x10-8 kg oil drop is suspended in an electric field of strength __________ N/C. (a) What is the
charge on the oil drop? (b) how many extra electrons does the oil drop have?
Cool things electrostatics explains:
1. Shocking fingers & lightning rods

On a ____________ shaped object, charges are ____________ spread.

On an irregularly shaped object, charge tends to accumulate at areas of ________ curvature/smallest
radii.

In other words, charge accumulates at _____________________________.
2. Faraday Cage

The Electric Field inside a conducting surface is _________
o Conducting cup on a stand: (draw the pictures, there are 3)
Explanation: The negative charges from the polarized inside get neutralized as the positive ball comes in contact
with them. The charge from the positive ball is now left on the outside of the cup.
o Negative rod with a conducting sphere: (draw the pictures, there are 3)

A faraday Cage is a ____________ enclosure in which charge will always flow to the outside, thus
leaving the inside _____________.

Not only do Faraday cages block ______________, they more importantly block
___________________________.

Applications of a faraday cage: