Electrostatics

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• Electricity come from the Greek word Elektron Meaning Amber.

• Electrostatics is the study of electricity at rest.

Electric Force and ChargesProtons• Positive electric charges• Repel positives, but attract negatives

Electrons• Negative electric charges• Repel negatives, but attract

positives

Neutrons• Neutral electric charge

When a balloon is rubbed against your hair and pulled away, your

hair clings to it because _________.

A. Rubber is very stickyB. The balloon is filled with heliumC. The hair and balloon are both positively

chargedD. The hair is positively charged and the balloon is

negatively charged

Rubbing your hair with a balloon increases the static charge because

____________.

A. Protons are moving from your hair to the balloon.

B. The balloon is getting charge from your hair.

C. The rubber in the balloon is non-conducting so it allows the electricity to gather

D. Hair and rubber are compatible

But what is Charge?

A Charge is an imbalance of protons and electrons.

Its important to remember that atoms lose or gain electrons not protons.

Electric Force and ChargesElectric Force and ChargesCHECK YOUR ANSWERCHECK YOUR ANSWER

A Positive Charge

-Has more Protons then electrons

Electric Force and ChargesElectric Force and ChargesCHECK YOUR ANSWERCHECK YOUR ANSWER

A Negative Charge

-Has more Electrons than Protons

Electric Force and ChargesElectric Force and ChargesCHECK YOUR ANSWERCHECK YOUR ANSWER

When thinking of electric force remember The old dating saying thatOposites attract!

That’s how charges are, Opposite charges attract and like charges repel each other.

Electric Force and ChargesElectric Force and ChargesCHECK YOUR ANSWERCHECK YOUR ANSWER

Conductors and Insulators• Conductor: Materials in which one or more of the

electrons in the outer shell of its atoms free to wander in the material– Example: Metals such as copper and aluminum

• Insulators: Materials in which electrons are tightly bound and belong to particular atoms and are not free to wander

-Example: Rubber, glass

Electric wires are wrapped with rubber or plastic in order to keep you from being electrocuted. This is because

the rubber or plastic is a good _____________.

A. InsulatorB. Conductor C. Transfer pointD. Bridge for the electrons

Triboelectricity• Charging by friction and contact.

Example:

Stroking cats fur, combing your hair, rubbing your shoes on a carpet

• Electrons transfer from one material to another by simply touching. For example, – when a negatively charged rod is placed in

contact with a neutral object, some electrons will move to the neutral object.

+ -

+-

+

+-

-

-+

+

+

-

-

+

+

+-

-

-

Charging Due to FrictionGla

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Charging• Charging by induction

– If you bring a charged object near a conducting surface, electrons are made to move in the surface material, even without physical contact.

– Example: The negative Example: The negative charge at the bottom of charge at the bottom of the cloud induces a the cloud induces a positive charge on the positive charge on the buildings below.buildings below.

Charge Polarization

Neutral charges are separated due Neutral charges are separated due to a charged objectto a charged object

ChargingInduction: Consider two insulated metal spheres A and B.a. They touch each other, so in effect they form a single

uncharged conductor. b. When a negatively charged rod is brought near A, electrons in

the metal, being free to move, are repelled as far as possible until their mutual repulsion is big enough to balance the influence of the rod. The charge is redistributed.

c. If A and B are separated while the rod is still present, each will be equal and oppositely charged.

Charge Polarization• If the charged rod is negative, If the charged rod is negative,

then the positive part of the then the positive part of the atom or molecule is tugged in a atom or molecule is tugged in a direction toward the rod, and direction toward the rod, and the negative side of the atom or the negative side of the atom or molecule is pushed in a molecule is pushed in a direction away from the rod. direction away from the rod.

• The positive and negative parts The positive and negative parts of the atoms and molecules of the atoms and molecules become aligned. They are become aligned. They are electrically polarized.electrically polarized.

Charge Polarization• Rub an inflated balloon on your hair, Rub an inflated balloon on your hair,

and it becomes charged. and it becomes charged.

• Place the balloon against the wall, Place the balloon against the wall, and it sticks. and it sticks.

• This is because the charge on the This is because the charge on the balloon induces an opposite surface balloon induces an opposite surface charge on the wall. charge on the wall.

• Again, closeness wins, for the Again, closeness wins, for the charge on the balloon is slightly charge on the balloon is slightly closer to the opposite induced closer to the opposite induced charge than to the charge of same charge than to the charge of same signsign

Conservation of charge

• Charge is never lost it is just transferred to other objects.

• The total Charge will always remain constant.

What type of charging occurs in a wall when a balloon sticks to the wall?

A. Charging by contact

B. Charging by friction

C. Triboelectric charging

D. Polarization

Electric Field

Electric field direction• Same direction as the force on a positive charge• Opposite direction to the force on an electron

The Electric FieldThe Area around a charged object where it can exert a force on other objects.

Positive Plate

Negative Plate

Neutral Point

The field is drawn from + to -

Coulomb’s Law

Coulomb’s law• Relationship among electrical force, charge, and

distance discovered by Charles Coulomb in the 18th century

• States that for a pair of charged objects that are much smaller than the distance between them, the force between them varies directly, as the product of their charges, and inversely, as the square of the separation distance

Coulomb’s Law

Coulomb’s law (continued)• If the charges are alike in sign, the force is

repelling; if the charges are not alike, the force is attractive.

• In equation form:

k = 9x109 Nm2/C2

• Unit of charge is coulomb, C• Similar to Newton’s law of gravitation for masses• Underlies the bonding forces between molecules

F = kq

1q

2d2

According to Coulomb’s law, a pair of particles that are placed twice as far apart will experience forces

that are

A. half as strong.

B. one-quarter as strong.

C. twice as strong.

D. 4 times as strong.

Electric Field

Answer the following question using the picture below. If you placed an proton between plates

A and B, in which direction would the proton move?

A. Towards plate A

B. Towards plate B

C. It would not move.

The Superposition Principle

• When a number of separate charges act on the charge of interest, each exerts an electric force. The electric forces can be calculated separately and added as vectors.

The Superposition Principle

• Each charge exerts a force on the other.• The net force on a charge is the sum of all the forces acting on it by other charges.

• Fnet = F12 – F23

Remember: if vectors are

pointed in the same

directio

n, you add them. B

ut

if they are pointed in

opposite dire

ctions, you

subtract!

Practice

1. Three charges are arranged in a line as shown.

What is the net force on q2?

Electric Potential

Electric potential energy• Energy possessed by a charged particle due to

its location in an electric field. • Work is required to push a charged particle

against the electric field of a charged body.

Grounding

• Since the earth has an massive supply of charges electricity will always want to discharge into the earth.

Lightning

• Water molecules bumping around in clouds create an enormous charge.

• These clouds induce a charge on the earth and when enough potential builds up there is a discharge ( lightning)

Electric Shielding

• Unlike gravitational fields electric fields can be shielded against.

• When you are fully inside of a conductor the net charge is zero because the charges builds up on the outer surface.

• This is why if your car gets struck by lighting you will be ok.

Lightning

A car being hit by lighting is a great example of electric shielding.

Which of the following reasons explains why you will not be injured if you are inside your car when it is struck by

lightning.

A. The rubber tires insulate the car.

B. The charges applied to the car mutually repel each other and spread around the frame of the car.

C. Nonsense! You will be injured when your car is struck by lightning.

D. Lightning is repelled by the fuzzy dice hanging from your mirror.

Static Powered Home: the Static Powered Home: the future?future?

Uses of Static ElectricitySpraying a Car

Positive Car

Negative Spray gun

The paint spreads out as each negative drop repelsNo paint is wasted as the positive car attracts the negative paint

Removing Smoke from Power Station Chimneys

Positive Plate

Negative Plate

Charge up the Smoke

No smoke leaves the chimney

Earthing Fuel Tankers

Fuel rubbing against the pipe can build up a static charge which could cause an explosionThe tanker is joined to the ground with a wire to stop a charge building up