# Electrostatic Forces.

## Presentation on theme: "Electrostatic Forces."— Presentation transcript:

Electrostatic Forces

When you comb your hair and…
“Static” Electricity When you comb your hair and… … bring your comb over a pile of paper bits

Why did this happen? What will happen? A. B. “Static” Electricity

“Static” Electricity 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.

Match the diagram below with its correct description.
Excess of Electrons Net Charge Negative Deficiency of Electrons Net Charge Positive Balanced atom Net Charge Zero Volunteers with separate colors: Red,Yellow, Green A. B. C.

“Static” Electricity 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.

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

Results in a transfer of charges
Methods of Charging Direct contact Induction Usually results in a temporary rearrangement of charges Results in a transfer of charges Direct contact label replaced. Picture swapped.

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

Electric charge in cloud

Possible lightning paths

Charge and image

Main Stroke

Lightning striking tree

Lightning striking Empire State Building

Shoes of man struck by lightning
Man not hurt!!

Charges flow from less negative to more negative areas.
Self Check? True False Charges flow from less negative to more negative areas. True False Like charges repel.

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.

Making a Magic Tape Electroscope
Third Step: Pick up the tape “handles” and pull the tapes apart.

Making a Magic Tape Electroscope
The tapes separate as shown. What do you think causes this to occur? 1. 2. 3.

Making a Magic Tape Electroscope
Charges on the surface of the tapes shown are alike Do we know whether these charges are positive or negative? How could we tell? Discuss these questions with your group and record answers in your notebook

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 positive 3. both are negative 4. both are positive or both are negative

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 positive 3. both are negative 4. both are positive or both are negative The fact that the balls repel each other only can tell you that they have the same charge, but you do not know the sign. So they can be either both positive or both negative.

Concept Check - Electrostatics
From the picture, what can you conclude about the charges? have opposite charges have the same charge all have the same charge 4. one ball must be neutral (no charge)

Concept Check - Electrostatics
From the picture, what can you conclude about the charges? have opposite charges have the same charge 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. positive 2. negative 3. neutral 4. positive or neutral 5. 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. positive 2. negative 3. neutral 4. positive or neutral 5. negative or neutral Clearly, the ball will be attracted if its charge is negative. However, even if the ball is neutral, the charges in the ball can be separated by induction (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? ? 3. – + 5. – –

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 blue to the green ball due to polarization. Once disconnected, the charges will remain on the separate conductors even when the rod is removed. ? 3. – + 5. – –

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.

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. Answer: 4.The same result is achieved regardless of whether the charge carriers are positive or negative.

Electromagnetic Charge
Charge is fundamental and cannot be described in terms of simpler, more basic concepts. we know it by what it does, not by what it is. Like charges repel; unlike charges attract.

Positive and Negative Charge
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 (-) We follow Ben Franklin’s lead and arbitrarily call the two type of charge positive and negative. In ordinary solid matter, the (+) charge is locked up in the nuclei of essentially stationary atoms. The (-) charge is mobile in the form of electrons. Neutral means equal (+) and (-) amount of charge. Charge is Quantized and Conserved. There is no way to “peel off” the charge on an electron. The charge of an electron seems to be the fundamental quantity of charge. All charge seems to come in whole number increments of the charge on an electron. An object becomes charged by losing or gaining charge. The total charge within an isolated system is always constant. This doesn’t mean that charge cannot be created; it just means that, if it is, as much positive as negative will be produced. Charging by Rubbing. Atoms are neutral, but the outermost electrons can shed. Different materials have different affiinities for electrons. There are degrees of grabbers, an object taking electrons away from one material might find itself serving as a donor to a more potent grabber. By the behavior of various materials, a sequence can be established known as the Triboelectric Sequence.

Insulators and Conductors
+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + A conductor allows charge introduced anywhere within it to flow freely and redistribute. Nonconductor Conductor

Insulators and Conductors
+ Q + + + + + + + + + + + + + + + + + + A conductor allows charge introduced anywhere within it to flow freely and redistribute. + + + + + + + + + + Q/2 Q/2 + + + + + + + +

Concept Check – Coulomb’s Law
What is the magnitude of the force F2? N N N N N Q F1 = 3N F2 = ?

Concept Check – Coulomb’s Law
What is the magnitude of the force F2? N N N N N The force F2 must have the same 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 of a pair is the same as the reverse. Note that this sounds suspiciously like Newton’s 3rd Law!! Q F1 = 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: Answer: 5. The magnitude of the electrostatic force exerted by 2 on 1 is equal to the magnitude of the electrostatic force exerted by 1 on 2. If the charges are of the same sign, the forces are repulsive; if the charges are of opposite sign, the forces are attractive.

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: Answer: 5. The magnitude of the electrostatic force exerted by 2 on 1 is equal to the magnitude of the electrostatic force exerted by 1 on 2. If the charges are of the same sign, the forces are repulsive; if the charges are of opposite sign, the forces are attractive.

Concept Check – Coulomb’s Law (2)
If we increase one charge to 4Q, what is the magnitude of F1? 1. 3/4 N N N N N Q F1 = 3N F2 = ? 4Q Q F1 = ? F2 = ?

Concept Check – Coulomb’s Law (2)
If we increase one charge to 4Q, what is the magnitude of F1? 1. 3/4 N N N N N Originally we had: Now we have: which is 4 times bigger than before. Q F1 = 3N F2 = ? 4Q Q F1 = ? F2 = ?

Concept Check – Coulomb’s Law (3)
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? F F 3. F 4. 1/3 F 5. 1/9 F Q F r ? 3r

Concept Check – Coulomb’s Law (3)
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? F F 3. F 4. 1/3 F 5. 1/9 F Originally we had: Now we have: which is 1/9 as big as before. Q F r F/9 3r

Electric Charge Model p158
Summary 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 1 5 1

TWO kinds of electric energy
Static Electricity Current Electricity

A few thousand volts

About 100,000 or 150,000 volts Van de Graaff Born 1901
Invented static electricity generator in 1929 This is the generator we will use today In air, about 2500 volts per inch. So about 4-6 inch sparks, depending on humidity. About 100,000 or 150,000 volts

Here is a bigger Van de Graaff generator

An even bigger one!

A giant Van de Graaff generator

The biggest--25 Million Volts Oak Ridge National Lab in Tennessee

So, how does our Van de Graff Generator work?

Electrons jump off the belt at the top
Electrons jump onto the belt at the bottom

One of the properties of matter
Where the electrons go when two things are rubbed together

Volunteers?

Why does your hair do this??

Because all the (-) charges (electrons) repel
Because all the (-) charges (electrons) repel! Finish: Lab handout Give me a charge(van de graf) CDP