Electrostatics 1

Electric Charge & Force Interactions

Electric charge history

• First documented by ancient greeks (600 BC) that saw amber with built up charge attract things and even create a spark. Called it Elektron.
• 1600 AD Electricity became an English word
• 1700’s conductors (allow charge to flow) and insulators (block flow of charge) defined.

What are examples of conductors and insulators?

Ben Franklin

Tons of experiments by Ben led to

• 2 types of charge: “positive” and “negative” (experiments)

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Side note: Net charge and Objects

All objects are made of lots of charges, but most objects are balanced and “neutral”

The net charge determines the behavior, so we often ignore the balanced charges

We usually treat the net charge as being a point charge at the center of the object in order to make it more simple. ur welcome.

Ben Franklin

Tons of experiments by Ben led to

• 2 types of charge: “positive” and “negative” (experiments)
• conservation of charge
• lightning was electricity
• invented lightning rods and a “battery” that stored charge (the truth about the Kite)

Conservation of charge

1. When 2 neutral insulators are rubbed, they may create a charge, but that charge is equal and opposite.

Simulation

2. When conductors are able to exchange charge, the total net charge is distributed evenly among identical objects.

- must be same size and shape for it to be this simple

Try it

1. A neutral plastic rod is rubbed on a neutral piece of wool. If the plastic rod ends up with a total charge of -3, what is the charge on the wool?
2. A metal sphere with a charge of +5 is touched to an identical +9 metal sphere. What is the charge on each after touching?
3. The same spheres, but this time +5 and -9 in charge are touched. What is the charge on each after touching?

Electroscopes

Can be used to “see” electric charge

Explain how an electroscope works when it contains a net charge.

What are its limitations?

Charging by conduction

touching conductors allows charges to flow from one object to another.

Result: shared same charge � (reduced on rod)

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Neutral objects with uneven charge distribution

have charges separated, causing the effect of charged sides, but they are still balanced.

Result: source unaffected,�conducting object has opposite sides (charge separation) �insulators have shifted charge (polarization)

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2 Ways to charge by induction

How can you define one of these to be an open system and one to be an isolated system?

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Charging by induction:

Without touching a charged object, a neutral conductor is polarized, allowed to exchange charges with a 3rd object �(or ground), separated from 3rd object and then charged object is removed.

Result: source unaffected,

object receives opposite charge

Talk about it

What advice can you give about lightning and safety based on your understanding of Physics?

Consider the idea of standing under a tree, not swimming in a lightning storm and being in a car?

Faraday’s Cage

“As the negatively charged rod

nears the sphere, it causes the electrons in the sphere to move away from the rod. The side of the sphere nearest to the rod becomes positively charged while the other side becomes negatively charged. So the sphere will be attracted toward the rod. If they touch, the sphere will swing back since they will both become neutral.”

What, if anything, is wrong with this statement? If something is wrong, explain the error and how to correct it. If the statement is valid, explain why it is valid.

A student observes a demonstration involving an interaction between a neutral metallic sphere suspended from a string and a negatively charged insulating rod. The student makes the following observation:

Fundamental Charge

Electric Charge is Quantized

(comes in multiples of a basic chunk)

Millikan’s oil drop experiment found smallest increment of charge observable

• this “fundamental charge” is sometimes represented by e
• e = the charge on an electron
• e = 1.6x10^-19 C

SI unit for charge is C (Coulomb)

What’s my charge?

A Calcium ion has the following properties:

20 protons, 18 electrons and 20 neutrons

What is its charge?

Elementary Particles

• all atoms are composed of the following particles

note that a proton has a mass 2000x that of an electron

Quarks determine charge

• We used to think protons, neutrons and electrons were the smallest building blocks of matter, but have since discovered quarks and leptons. (and bosons) �
• We still can’t isolate less charge, �but quarks have fractions of e that�combine to determine charge on�protons and neutrons. �
• An electron is a type of lepton, so it is �not made of anything else

Talk it out:

Someone claims to have discovered a new technique of charging cell phones by harnessing small stray charges normally lost in typical electrical plugs. “It takes several thousand charges to add up to a single electron, which is why no one has utilized them before.”

Explain how you know this is false. Give a supported reason.

Effects of charge: Electrostatic force

• Like charges repel, opposites attract
• How are friction, contact forces and tension all macroscopic examples of electric forces?

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Coulomb’s Law: �describes the magnitude of the force between 2 charged objects

Ignore the signs of the charges and assign the direction after solving the magnitude.

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k= 9.0 x10⁹ N-m²/C²

µ = x10^-6 and pronounced micro, so 5µC = 5x10^-6C

Try it

What is the force �experienced by each charge if q_blue= 5µC, �q_red= 1.2µC and the charges are separated by a distance of 0.5 m?

• What if the charge on q₂ is doubled?
• What should the graph of F v q look like?
• What if the distance is doubled?
• What should the graph of F v r look like?

Another inverse square law?

Compare Universal gravity with Coulomb’s Law

• What is similar and what is different?
• Consider effect of distance, mass and charge on each.
• Consider value of the constant.
• Consider direction of the force
• Consider what the graphs could look like

1. Draw FBD for 1 ball
2. Derive q in terms of θ, m, d, and physical constants

θ

Charge = q

Mass = m

for both

d

Sum of forces

Multiple charges can affect the same charge. You simply draw FBD and combine the magnitude of each force to find net force.

• Find the net force on q₁
• Compare the net force on q1, q2, and q3

Where on the line could you put a positive charge to be in equilibrium?

•

-1C

•

+4C

1m

3m

x=0