Electrostatics Notes

Electrostatics

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Today's notes are about electrostatics, the study of electrical charge. I hope your day is going strong. Let's charge in!

Electrostatics

● Electrostatics - study of electrical charges that

can be collected and held in one place

Electrical Basics

● All atoms have a positively charged

nucleus (made up of protons (+) and
neutrons (0) that are surrounded by
electrons (-))

● When the positive charge equals the

negative charge the atom is neutral

● Electrons can be removed or transferred

by the addition of energy.

Conservation of Charge

● Electric charge is never created or

destroyed, just transferred by the
separation of positive and negative charge

● Like charges repel

● Opposite charges attract

● A neutral object will be attracted to a

charged object

Electrical Materials

● Conductors – materials through which

charges will move about easily because they have loosely bound electrons; Ex: most metals

● Insulators - materials through which

charges will not move about easily
because they have tightly bound electrons;
Ex: glass, wood, plastic

Charging a neutral object

● Conduction – charging a neutral object by

touching it with a charged object

● Induction – charging a neutral object

without touching it, but by the separation of charges

Coulomb’s Law

● The electrical force between 2 charged objects varies

directly with the product of the charges and inversely
with the square of the distance between them
F = kq₁q₂/d²

k = 9x10⁹ Nm2/C2
k is called Coulomb's Constant

● F = Force in Newtons

● q₁ and q₂ are charges measured in Coulombs

● d = distance in meters

● A repulsive force is positive because both charges are

alike

● An attractive force is negative because the charges are

opposites

Multiple Choice

How is the electrical force changed if one of the charges is tripled? Hint: Make everything else 1!

• The electrical force will be tripled.
• The electrical force will remain the same.
• The electrical force will be doubled.
• The electrical force will be halved.

Multiple Choice

How will the electrical force change if the distance is tripled?

• Decrease by a factor of 9
• Decrease by a factor of 2
• Remain the same
• Increase by a factor of 3

Multiple Choice

Two charges are separated by .7 m. One charge is 3 x 10^-4 C and the other is 8 x 10^-4 C. What is the force between them? Don't forget Coulomb's Constant is 9 x 10⁹ !

• 4.4 x 10³ N

• 1.75 x 10^-2 N

• 1.03 x 10² N

• 2.45 x 10⁷ N

Electric Fields

Electric Fields

● A charge creates an electric field in all

directions

● The electric field intensity is the force per

unit charge

units are N/C

​E = F/q

Multiple Choice

1. A positive charge of 1.0 x 10^-5 C experiences a force of 0.20 N What is the electric field intensity ?

• 1.0 x 10^4 N/C
• 0.10 N
• 1.0 x 10^-6 C
• 2.0 x 10^4 N/C

Electric Field Lines

● Electric Field Lines

provide a model for
the electric field. The
strength of the field is
indicated by the
spacing between
lines. The closer the
lines the stronger the
field. The field always
points away from the
positive towards the
negative charge.

Draw

Draw field lines from a POSITIVE charge going to a NEGATIVE charge. The field is weak- think about how your field lines should look.

Draw

Draw field lines from a POSITIVE charge going to a NEGATIVE charge. The field is strong- think about how your field lines should look.

Electric Potential Difference

● Electric Potential Difference - Change in

Potential Energy per unit charge; this is often
called voltage difference

ΔV = ΔPE/q or ΔV = Work/q

units are J/C = volts

● Also ΔV = Ed

● ΔV = potential difference or voltage difference

● E = Electric Field

● d = distance

Math Response

What is the voltage if the electric field intensity is 16 N/C over a distance of 4 m?

Type answer here

Correct Answer: 64

Deg^°

Rad

Grounding

● Grounding -

Eliminating the
excess charge by
touching a charged
object to the earth which is
low voltage. The
charge moves from
higher potential to
lower potential until
they are equal.

​The Earth is capable of absorbing tremendous
amounts of electricity due to its large size.

● All charged objects

are on the surface of a solid conductor

● The charge will move

to the pointed end of
a conductor

● Excess charge will

move to the outer surface of a hollow conductor

​How Charges behave on a Conductor

Capacitor

● Capacitor - Stores

charge; made up of two
oppositely charged
conducting plates
separated by an insulator

● Capacitance – ratio of

charge stored to potential difference

C = q/V

units are C/V = 1 Farad (F)

Multiple Choice

Find the Capacitance of a capacitor that stores 3 E – 6 C of charge when it is given
60 V of potential difference.

• 2E-8 F
• 4E-8 F
• 6E-8 F
• 5E-8 F

Millikan Experiment

● Milikan determined

the charge of an
electron to be

1.6 x 10 ^-19 C

● Eq = mg

● E = Electric Field

● q = Charge

● m = mass

● g = gravity

Multiple Choice

What is the charge of a particle if it has a field intensity of 10 N/C and a mass of 1.7 x 10^-11 kg? Don't forget g=9.8!

• 1.7 x 10^-10 C
• 1.666 x 10^-12 C
• 1.5 x 10^-11 C

• 1.666 x 10^-11 C

Multiple Choice

A 25 C lightning bolt strikes the Earth. How many electrons are transferred? Remember, the charge of an electron is 1.6 x 10^-19 C.

• 1.25 x 10^20 electrons
• 3.125 x 10^20 electrons
• 2.6 x 10^20 electrons

• 1.5625 x 10^20 electrons