Electrostatics Notes (614)

Charge!

n  Have you ever walked across the carpet and gotten “shocked” when you touched the doorknob?

n  What about static cling? Have you ever gotten to school only to be embarrassed when someone points out the sock sticking to your back?

What’s going on in these cases? Why did they occur?

Review: Atomic Structure

3 Basic Particles make up Atoms:

1. Protons 2. Neutrons 3. Electrons

Charge

n Protons & Electrons have a property called electric charge – Protons: positive electric charge (+) – Electrons: negative electric charge (-) – The strength of the positive charge on a

proton is the same as the strength of the negative charge on the electron

Charge cont. n Neutrons do not have charge

– Neutrons: neutral

Neutrons must be from Switzerland

Can you tell the difference? n Can you tell the difference between an

electron in an oxygen atom and an electron in a sodium atom?

n The electrons of ALL atoms are identical. Each has the same quantity of charge and the same mass.

Protons & Neutrons

n Protons and neutrons compose the nucleus. Protons are about 1800 times more massive than electrons but carry an amount of charge equal to the negative charge of electrons. Neutrons have slightly more mass than the protons and have no charge.

Particles Summary: n Particles may be charged (positive or

negative) or neutral (not charged)

Particle Charge Relative Masses

Protons positive (+) massive

Neutrons neutral massive

Electrons negative (-)

almost massless!

How do charged particles behave?

Opposites Attract!

Basically… n  Oppositely charged particles attract each

other –  Ex: Protons (+) and Electrons (-) attract

n  Particles with the same charge repel each other –  Ex: 2 Electrons (-) would repel each other –  Ex: 2 Protons (+) would repel each other

Continued…..

n Particles with neutral charge do not interact – Neutrons do not attract or repel each other – Neutrons do not attract or repel electrons

or protons

In the real world, we rarely deal with individual charged particles n Everyday objects are made of lots of

atoms n Most atoms have an equal number of

protons and electrons n Therefore, atoms are neutral

– Remember, even though atoms are neutral, they are still made of charges

What does it mean to say that an object is neutral? n A neutral object has no net (overall)

charge n A neutral object has equal amounts of

positive and negative charge

What does it mean to say that an object is charged? n A charged object has a net charge n A positively charged object has a

greater quantity of positive charge than negative charge

n A negatively charged object has a greater quantity of negative charge than positive charge

Charged Tape Demonstration

n Explain the demonstration you see with the charged tape. – Step 1: Charging one Tape

•  Explain what happens to your tape after you charged it and why?

– Step 2: Charging two tapes •  Explain what happens to the two pieces of

tapes after you charged them and why? •  Explain different combinations of interaction

and why.

What does it mean for an object to be polarized? n When an object is polarized, its charges

have shifted so that one side of the object has a net negative charge and the other side of the object has a net positive charge.

n A polarized object might be neutral (no net charge) or have a net charge.

Electrons move, Protons don’t!

n Protons don’t move! – Protons are very massive. They have too

much inertia. – They are in the center of the atom.

n Electrons are outside the nucleus. –  It is easier to move particles on the

perimeter.

Re-examine Lab from last class… n Find at least 3 situations where

polarization occurs. n Explain how polarization occurred in

each station. DRAW a before and After Picture to demonstration polarization.

So, a charged sock can stick to my shirt… Does that mean that my shirt is charged?

n Not necessarily n Remember that a neutral object is made

up of innumerable positively and negatively charged particles.

n A charged object (positive or negative) will be attracted to a neutral object. – We’ll discuss exactly why later…

What do you have to do to make an object positively charged?

n You need to take electrons away from the object

What do you have to do to make an object negatively charged?

n You need to transfer electrons to the object.

So what is happening when you rub a balloon on your head and it becomes charged? Is friction creating charge? n  No! Charge cannot be

created or destroyed. –  Conservation of Charge

n  Charges are being exchanged…

Conductors vs. Insulators

n Conductors: – Loosely bound

electrons – Allow the flow of

electrons – Examples: metals

n  Insulators: – Tightly bound

electrons – Slow the flow of

electric charge – Examples:

rubber, plastic, and styrofoam

There are 3 Charging Methods

n Objects can be charged by – Conduction –  Induction – Friction (triboelectricity)

Conduction n  Requires the objects to be in contact

(or close enough for a spark) n  Electrons are exchanged n  Works best from conductor to conductor

Positively Charged

Neutral (equal numbers of + and – charges)

• When the objects touch (or get really close) electrons move from one object onto the other

Positively Charged

Neutral (equal numbers of + and – charges)

• When the objects touch (or get really close) electrons move from one object onto the other • The two objects share the charge equally

Charging by Conduction Suppose you have two identical metal spheres, imaginatively

named A and B. Each sphere is on an insulating stand. Originally, A has 10 units of charge, and B is neutral. If the two

spheres are briefly touched together, then separated, what is the charge on each?

A.  A has 10 units, B has zero. B.  B has 10 units, A has zero. C.  A has 10 units, and B also has 10 units. D.  A and B each have 5 units. E.  A and B each have zero charge.

Induction n  A charged object charges a neutral conductor

without contact n  The conductor does make contact with a

neutral object (often the ground) n  The conductor ends up with a charge opposite

that of the charged object brought near

Positively Charged

Neutral (equal numbers of + and – charges)

• As the positive object moves toward the neutral one, the positive object attracts the electrons in the neutral one.

Positively Charged

Neutral (equal numbers of + and – charges)

• As the positive object moves toward the neutral one, the positive object attracts the electrons in the neutral one. • The positive object induced the electrons to move. • The neutral object is now polarized.

Positively Charged

Neutral (equal numbers of + and – charges)

• As the positive object moves toward the neutral one, the positive object attracts the electrons in the neutral one. • The positive object induced the electrons to move. • The neutral object is now polarized.

Charging by Induction Notice that we are

charging by induction, grounding the side of the sphere next to the positive object.

What will the final charge on the sphere be?

A.  Positive B.  Negative C.  Neutral D.  It depends

Triboelectric (friction)

n  Two dissimilar materials are brought into contact and charge is exchanged

n  The contact often involves friction

n  Works best with insulators

Neutral: no charge

Charging by Friction

Charged

More on Triboelectricity

n Charge separation occurs when two insulators are rubbed together

n One of the insulators is more likely to grab electrons and the other insulator is more likely to donate electrons

n CHARGE IS NOT CREATED! Electrons are simply being transferred.

ç Electron Donors (objects that give electrons become positive)

(objects that take electrons become negative) ç Electron Grabbers

The Triboelectric Series

You rub a balloon against your hair, and the hair becomes positively charged. This means that

A.  Electrons moved from the balloon to your hair.

B.  Protons moved from the balloon to your hair. C.  Protons moved from your hair to the balloon. D.  Electrons moved from your hair to the

balloon. E.  The rubbing destroyed electrons in your hair,

leaving it positive.

Triboelectric Charging

A Triboelectric Sequence"ELECTRON GRABBERS " " " "ELECTRON DONORS"Rubber Amber Cotton Silk Cat fur Wool Glass Rabbit fur" If you rub cotton with amber, which becomes positive?

A. Amber B. Neither C. Cotton Which of the following can make glass negative?

A. Amber B. Cat fur C. Rabbit fur

Triboelectric Series

Neutralizing/Grounding Objects

n  When a charged object comes in contact with a very large, neutral conductor, the object becomes neutralized.

n  Earth itself is a large, neutral conductor, so it neutralizes charged objects quite well.

Review n  Charged objects exert forces:

Like repels like Opposites attract Charged objects and neutral objects attract

n  How objects get charged or neutralized: Conduction Induction Triboelectric Grounding

Coulomb’s Law

n Two charged objects exert a force on each other.

n The magnitude of the force is directly proportional to the product of the quantities of charge.

n The magnitude of the force is inversely proportional to the distance between the objects squared.

More Coulomb’s Law (We’ll come back to this later…)

!

Fe =kq1q2R2

k = constantq = quantity of chargeR = distance between the centers of the objects.

Consider two identical charged particles near each other as shown. The force exerted by

q1 on q2 points A.  Left B.  Right C.  Up D.  Down E.  Nowhere; there is

no force.

Two air pucks each carry a charged sphere.

The sphere on the right carries three times as much charge as the sphere on the left.

Which force diagram correctly shows the direction and magnitude of the electrostatic forces?

What if one object is neutral?

A charged object will polarize a neutral (or weakly charged) object.

Animation of Polarization

Illustrations of Polarization

Note that a neutral object and a charged object will attract each other.

Three pith balls hang from threads…

Wait a minute! What’s a “pith ball”?

A pith ball is a small, light sphere coated with conductive material.

The coating allows it to be easily charged or grounded, and its small mass allows it to respond easily to small forces.

May we go on?

Three pith balls each hang from an insulating thread. n  PB 1 and PB 2 repel

each other. n  PB 2 and PB 3 repel

each other. Which of these

statements is true about the charges on the PBs?

A.  1 and 3 have charges of opposite sign.

B.  1 and 3 have charges of the same sign, 2 has opposite charge.

C.  All three have charges of the same sign.

D.  One of the objects carries no charge.

E.  We need more data to determine the signs of the charges.

And now for a similar, yet different, problem…

Three pith balls each hang from an insulating thread. n  PB 1 and PB 2

attract each other. n  PB 2 and PB 3 repel

each other. Which of these

statements is true about the charges on the PBs?

A.  1 and 3 have charges of opposite sign.

B.  1 and 3 have charges of the same sign.

C.  All three have charges of the same sign.

D.  One of the objects carries no charge.

E.  We need more data to determine the signs of the charges.

The End!

HOORAY!!!