electrostatic forces. “static” electricity when you comb your hair and… … bring your comb...

Electrostatic Forces

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“Static” Electricity

When you comb your hair and…

… bring your comb over a pile of paper bits

http://nsdl.org


What will happen? A. B.

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Why did this happen?


The force felt by the paper bits is due to a difference in charge on the comb compared to the paper.

This “force of attraction” was first observed by the Greeks who found that piece of amber (“elektron”) attracted other objects when rubbed.

http://nsdl.org

Match the diagram below with its correct description.

1. 2. 3.

A. B. C.

Deficiency of Electrons

Net Charge Positive

Balanced atom Net Charge

Zero

Excess of Electrons

Net Charge Negative

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Usually charges balance each other out, and nothing happens. But when two objects with like charges (all positive or all negative) come together, the charges repel and the objects move away from each other.

Objects with opposite charges attract each other because the different charges want to balance each other.

Objects can get a negative charge by picking up electrons from other objects.

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Common Misconceptions about “Static” ElectricityActually, the thing we call static electricity is an imbalance in the amounts of positive and negative charges found on the surface of an object.

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Methods of ChargingDirect contact Induction

Results in a transfer of charges

Usually results in a temporary rearrangement of charges

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Common Misconceptions about “Static” ElectricityLightning is like static electricity, except on a much bigger scale. Both lightning and static electricity happen because of the attraction between the opposite charges.

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Electric charge in cloud

Possible lightning paths

Charge and image

The “Leader”

Main Stroke

Lightning striking tree

Lightning striking Empire State Building

Shoes of man struck by lightning

Man not hurt!!

Self Check?

True False

True FalseCharges flow from less negative to more negative areas.

Like charges repel.

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Pt II: Making a Magic Tape Electroscope

First Step:Bend back one end on each of two tapes to make a handle.Second Step:Lay each piece of magic tape on top of each other on a smooth surface.

Third Step:

Pick up the tape “handles” and pull the tapes apart.

Making a Magic Tape Electroscope

http://nsdl.org


The tapes separate as shown. What do you think causes this to occur?

1.

2.

3.


Charges on the surface of the tapes shown are alikeDo we know whether these charges are positive or negative? How could we tell?

Discuss these questions with your

group and record answers in your

notebookhttp://nsdl.org

Concept Check - Electrostatics

Two charged balls are repelling each other as they hang from the ceiling. What can you say about their charges?

1. one is positive, the other is negative 2. both are positive3. both are negative4. both are positive or both are negative


Two charged balls are repelling each other as they hang from the ceiling. What can you say about their charges?

1. one is positive, the other is negative 2. both are positive3. both are negative4. both are positive or both are negative

The fact that the balls repel each other only can tell you that they have the same chargesame charge, but you do not know the sign. So they can be either both positive or both negative.


From the picture, what can you conclude about the charges?

1. have opposite charges

2. have the same charge

3. all have the same charge

4. one ball must be neutral (no charge)


From the picture, what can you conclude about the charges?

1. have opposite charges

2. have the same charge

3. all have the same charge

4. one ball must be neutral (no charge)

The PERIWINKLE and BLACK balls must have the same charge,

since they repel each other. The RED ball also repels the

PERIWINKLE , so it must also have the same charge as the

PERIWINKLE (and the BLACK).

Concept Checks – Conductors

A metal ball hangs from the ceiling by an insulating thread. The ball is attracted to a positive-charged rod held near the ball. The charge of the ball must be:

1. positive2. negative3. neutral4. positive or neutral5. negative or neutral

Concept Checks – Conductors

A metal ball hangs from the ceiling by an insulating thread. The ball is attracted to a positive-charged rod held near the ball. The charge of the ball must be:

1. positive2. negative3. neutral4. positive or neutral5. negative or neutral

Clearly, the ball will be attracted if its charge is negativenegative. However, even if the ball is neutralneutral, the charges in the ball can be separated by inductioninduction (polarization), leading to a net attraction.

remember the ball is a conductor!

Concept Checks – Conductors (2)

Two neutral conductors are connected by a wire and a charged rod is brought near, but does not touch. The wire is taken away, and then the charged rod is removed. What are the charges on the conductors?

1. 0 0

2. + –

3. – +

4. + +

5. – –

0 0

? ?

Concept Checks – Conductors (2)

Two neutral conductors are connected by a wire and a charged rod is brought near, but does not touch. The wire is taken away, and then the charged rod is removed. What are the charges on the conductors?

While the conductors are connected, positive charge will flow from the positive charge will flow from the blue to the green ball due to polarizationblue to the green ball due to polarization. Once disconnected, the charges will remain on the separate conductorscharges will remain on the separate conductors even when the rod is removed.

1. 0 0

2. + –

3. – +

4. + +

5. – –

0 0

? ?

Concept Check – Charging by Induction

A positively charged object is placed close to a conducting object attached to an insulating glass pedestal (a). After the opposite side of the conductor is grounded for a short time interval (b), the conductor becomes negatively charged (c). Based on this information, we can conclude that within the conductor

1. both positive and negative charges move freely.2. only negative charges move freely.3. only positive charges move freely.4. We can’t really conclude anything.

Electromagnetic Charge

Positive and Negative Charge

Positive(+)

Air

Human Hands

Asbestos

Rabbit's Fur

Glass

Human Hair

Mica

Nylon

Wool

Lead

Cat's Fur

Silk

Aluminum

Paper

Cotton

Steel

Wood

Lucite

Sealing wax

Amber

Polystyrene

Polyethylene

Rubber balloon

Sulphur

Hard rubber

Nickel, Copper

Brass, Silver

Gold, Platinum

Sulfur

Acetate, Rayon

Polyester

Celluloid

Polyurethane

Polyethylene

Polypropylene

Vinyl

Silicon

Teflon

Saran Wrap

Negative(-)

Insulators and Conductors

++

+

++

++

+

+

+++

+ + +

+

+

Conductor

++

+

++

++

+

+++

+

+ ++

+

+

Nonconductor

Insulators and Conductors

++

+

++++

+

+

+++

+ + +++

+

+

++

+ + +

++

+

+

++

+ + +

++

Q

+

+

++

+ + +

++

+

+

++

+ + +

++

Q/2 Q/2

Concept Check – Coulomb’s Law

What is the magnitude of the force F2?

1. 1.0 N2. 1.5 N3. 2.0 N4. 3.0 N5. 6.0 N

QQ QQF1 = 3N F2 = ?

Concept Check – Coulomb’s Law

What is the magnitude of the force F2?

1. 1.0 N2. 1.5 N3. 2.0 N4. 3.0 N5. 6.0 N

The force F2 must have the same magnitudesame magnitude as F1. This is due to the fact that the form of Coulomb’s Law is totally symmetric with respect to the two charges involved. The force of one on the other force of one on the other of a pair is the same as the reverseof a pair is the same as the reverse. Note that this sounds Note that this sounds suspiciously like Newton’s 3rd Law!!suspiciously like Newton’s 3rd Law!!

QQ QQF1 = 3N F2 = ?

Concept Check – Electric Force

Two uniformly charged spheres are firmly fastened to and electrically insulated from frictionless pucks on an air table. The charge on sphere 2 is three times the charge on sphere 1. Which force diagram correctly shows the magnitude and direction of the electrostatic forces:

Concept Check – Coulomb’s Law (2)

If we increase one charge to 4Q, what is the magnitude of F1?

1. 3/4 N2. 3.0 N3. 12 N4. 16 N5. 48 N

4Q4Q QQF1 = ? F2 = ?

QQ QQF1 = 3N F2 = ?


If we increase one charge to 4Q, what is the magnitude of F1?

1. 3/4 N2. 3.0 N3. 12 N4. 16 N5. 48 N

Originally we had:

Now we have:

which is 4 times bigger4 times bigger than before.

4Q4Q QQF1 = ? F2 = ?

QQ QQF1 = 3N F2 = ?

1 21 2

3Nq q

F Kr

1 2 1 2

1 12 2

44 =4 =12N

q q q qF K K F

r r


The force between two charges separated by a distance r is F. If the charges are pulled apart to a distance 3r, what is the force on each charge?

1. 9 F2. 3 F3. F4. 1/3 F5. 1/9 F

QQFF

QFF

rr

QQ??

QQ??

33rr


The force between two charges separated by a distance r is F. If the charges are pulled apart to a distance 3r, what is the force on each charge?

1. 9 F2. 3 F3. F4. 1/3 F5. 1/9 F

Originally we had:

Now we have:

which is 1/9 as big as1/9 as big as before.

QQFF

QQFF

rr

QQF/9F/9

QQF/9F/9

33rr

1 22

q qF K

r

1 2 1 2

2 2

1=

9 93

q q q qF K K F

rr

Electric Charge ModelSummary of things we know:

• There is a property of matter called electric charge. (In the metric system its units are Coulombs.)

• Charges can be negative (like electrons) or positive (like protons).

• In matter, the positive charges are stuck in place in the nucleus. Matter is negatively charged when extra electrons are added, and positively charged when electrons are removed.

• Like charges repel, unlike charges attract.• Charges travel in conductors, not in insulators• Force of attraction or repulsion F=Kqq / r2

p158

TWO kinds of electric energy

Static Electricity

Current Electricity

A few thousand volts

Van de Graaff

Born 1901

Invented static electricity generator in 1929

This is the generator we will use today

Here is a biggerVan de Graaff generator

An even bigger one!

A giant Van de Graaff generator

The biggest--25 Million VoltsOak Ridge National Lab in Tennessee

Electrons jump onto the belt at the bottom

Electrons jump off the belt at the top

One of the properties of matter

Where the electrons go when two things are rubbed together

Why does your hair do this??

Because all the (-) charges (electrons) repel!

Finish:Lab handout Give me a charge(van de graf)

CDP 32-1 32-2

electrostatic forces. “static” electricity when you comb your hair and… … bring your comb...

Documents

negative slide

cloud slide

leader slide

image slide

negative charges

main stroke slide

like charges

lightning striking tree