Download Chapter 15: Electrostatics

Chapter 15: 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 charge at the origin. (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:
A field is an _____________________________ that has a _______________, representing some
_______________________, assigned to every location.
A scalar field has a number associated to each position.
A vector field has an amount and a direction associated with each position.
The Electric Field (E) is a ___________________________.

Any ________________ will set up an _______________________ around itself.

Exerts an electric ____________________ on any other ____________________ within the
________________________.

It is the ___________________________________________ so the units are Newton per Coulomb
(N/C)

Electric fields are always tested with _________________________________
o (Remember: Positive point charges are in 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:
Consider a positive charge in space - The Electric Field is used to describe the effect of this charge at some
point in space.
+

If _________________________________ (______) is placed at that point, a force will be exerted on it
by the ___________________ charge.
o Let’s say that force is _____ N

Then we can say, whenever a ___________________ is placed at that point, for every
____________________________, a force of _____ N act on it.
o Therefore the electric field at that point is 10 N per coulomb or 10 N/C

If a __________________ is placed _______________, the force on it would be
_________________________ or 30 N.
So in general …
(Electric Field Equations #1):
The force is equal to the amount of charge times the electric field strength.
The electric field produced by a point or spherical charge is given by…. (Electric Field Eqn #2)
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.
Example: What is the electric field 20.00 m away from a (__) _________point charge?
Example: The electric field produced by a point charge is 16 N/C at a distance of 10. m. At what distance will
the field be 4.0 N/C?
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 charges are along the x-axis. Q1 is 3.0 m from the origin and has a charge of -12.0C. Q2 is 4.5
m from the origin and has a charge of +4.0C. (all charges are along the positive x-axis)
a) Calculate the electric field 8.0 m from the origin. (remember 1C = 1x10-6C)
b) What force will a - 9.0 C charge experience if it is placed 8.0 m from the origin?
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:
Chapter 16: Electrical Energy and Capacitance
Electric Potential Energy

When going for a hike there are two things to consider:
1. How high up are you going? (What is your change in elevation?)
2. How much energy will it take to get there? (How much potential energy will you have once you get
there?)

Even though they are climbing the same mountain the hiker with the __________
(______________________) load will be doing more work (or will have more potential energy at the top)

If the hiker trips, his potential energy will convert into __________ energy and he will fall down the hill.

Just like hikers, charges can have potential energy too. That is called ELECTRIC potential energy.

Electric potential energy has to do with where the object is (V) (compared to where it wants to be) and
how much charge it has (Q).
Where: V = voltage (units volts, V or J/C)
Q = Charge (units Coulombs, C)

Defined as the ____________________________ a charge has as a result of its being acted upon by
an _____________________.


We are usually interested in ____________________________, or the change of potential energy
Gravitational potential energy has to do with where the hiker is (y) (how high up are they) and how hard
gravity pulls on them (mg) (how heavy their pack is).
Electric Potential

In general, no one really talks about how much work someone did climbing a mountain or how much
potential energy they had once they got there. Instead everyone talks about the
_______________________ of the peak.

The same is true for charged objects
Electric Potential is __________________________________________________
For a point charge or spherical charge the potential is given by:

V = Electric potential (Volts, V or J/C)
K = Coulomb’s constant
Q = Charge
d = distance
Electric Potential describes properties ____________________________ as the result of being in the
____________________ of a charge.

Just as height describes properties of a point in space as the result of being in the proximity of a
gravitational field.
Recap:

The electric field describes what an ______________ in the proximity of a charge is feeling
____________________________ on the object.

The electric potential describes where an object in the proximity of a charge ___ compared to where the
object ____________________________________________.

The height describes where an object is in the proximity of another mass compared to where the object
wants to be, regardless of the amount of mass.
Charges move to decrease their potential energy (PE = QV)

Masses will always try to move to lower their ________________________. (This is why meatballs roll
off the table and onto the floor…)

Charges will always move to lower their _____________________. (high PE = +34J  low PE = -34J)

(Based on the picture)
o Close to a negative charge it is a ___________ potential.

Positive charge want to be close to ___________________ charges (+Q x –V = -PE)
o Close to a positive charge it is _____________ potential

Negative charge want to be close to ___________________ charges (-Q x +V = -PE)
Example: If the distance from a negative ______ charge is tripled, the electric potential changes by a factor of…
Another equation for Electric Potential Energy:
Where: k = Coulomb’s Constant
Q1 & Q2 = Charges
d = distance
Example: Two point charges of +____ and +______ micro (10-6) respectively, are separated by 0.20m. What is
the potential energy of this 2 charge system?
Equipotential Lines & Electric Field Lines

When going hiking, it is helpful to bring a map. Hikers use topographical maps.

Each contour (line) on the map is at the same height. They could be called equi-elevational lines.

Equipotential Lines & Electric Field Lines (Draw the two pictures)
Example: The lamp will not glow when it is held with both ends equidistant from the charged Van de Graaff
generator. But when one end is closer to the dome than the other, a current is established and it glows. Why?
Why does anyone care about electric potential? ________________

What happens when two chemicals are mixed together? _______________

Why? All elements strive to have ____________________________
o Some elements do this by giving away valence electrons
o Some elements do this by gaining valance electrons

When a _______________ atom (K) is placed next to a ________________ atom (S), they react and
___________________ from the potassium atom to the sulfur atom.

This means, there must be an electric potential difference between potassium & sulfur.

Batteries harness this flow of electrons
o In a battery, elements are placed close to each other, without touching, so they do not react.
o A ____________ connects the two elements.
o The wire allows the ________________________ from one element to the other.
o This movement of electrons generates _______________________!

As the zinc (Zn) _____________ electrons, it goes
from ____________ zinc into an ________
dissolved in water.

As the dissolved hydrogen ions (H+) __________
electrons, they become hydrogen gas and leave the
container.

Eventually the chemicals run out, and the battery is
considered ________________.

Rechargeable batteries can be “reset”
o A dead rechargeable battery can be plugged into the wall.
o The __________________________ (voltage) from the ____________ returns the electrons and
chemicals to their original starting position to be used again.
o This process is not perfect so rechargeable batteries eventually die as well.
Parallel Conducting Plates & Uniform Electric Fields.

There exists between them a uniform Electric Field.
o The Equation for the Electric Field between two plates:
Where: E = Electric Field (N/C)
V = Potential (Voltage, V)
d = plate separation

Consider a positive charge between the plates.
o The force on the charge is:
Where: F = Force
Q = Charge (C)
V = Potential (Voltage, V)
d = plate separation
How does a Cathode Ray Tube work?
Electrostatic Equations:
Force
Electric Field
Electric Potential
Electric Potential Energy
F (N)
E (N/C)
V (V, volts)
PE (J)
* Parallel plate/Uniform electric field equations: d = distance between plates