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# Electric Forces and Fields

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Electric Forces and Fields
Chapter 17

Section 1 Electric charge

Chemistry Review Atoms are small particles that make up matter
Atoms are neutral when they have equal numbers of ______________and _____________ Atoms can lose electrons, because electrons are located in the _________________ Protons are in the _______________and do not leave the atom When atoms gain electrons, they become _____________ charged ions When atoms lose electrons, they become ______________ charged ions

Electric forces and charges
The electric force is one of four fundamental forces. The electric force is a field force. The electric force is the result of the interaction between electric charges. Explore- Show Van de Graff generator. After briefly demonstrating its properties, show how it exerts a force on a piece of paper. Ask, “what is VDGG doing to paper?” (A- Applying a force; paper is accelerated.) (1)Ask what fundamental forces are. Ask what kind of force electricity must be. Hint: It doesn’t have to touch the paper.

Discovery Lab: Electrostatics
Observe what happens at each step below: Step 1: Rub the rubber rod with the fur and bring it near the electroscope. Touch the rubber rod to the electroscope. Scrape the rod against the sphere. Touch the sphere with your hand. Step 2: Rub the glass rod with the silk. Bring the glass rod near the electroscope. Scrape the rod against the sphere. Touch the sphere with your hand. Step 3: Repeat Steps 1 and 2, but this time don’t touch the electroscope with your hand in between steps. Explore: Allow students to do the discovery lab. Listen to students and ask leading questions. © 2013 Mark Lesmeister/Pearland ISD

© 2013 Mark Lesmeister/Pearland ISD

Properties of Electric Charge
There are two kinds of electric charges. Protons have a positive charge; electrons negative. Opposite charges attract; Like charges repel. Bring uncharged rubber rod near electroscope. Show what happens. Charge rubber rod with fur. Bring near electroscope. Touch rod to electroscope. Ask what happens. Ask why. Answer- Something must have gotten transferred from fur to rod and then from rod to electroscope. Discharge electroscope. Then rub glass rod with silk. Touch electroscope. Show how electroscope gets charged again. Then show how charged rubber rod now reduces charge on electroscope. What does that tell us? Answer- Might be two kinds of “something.”

Properties of Electric Charge
Electric charge is conserved. When a neutral atom gives up an electron, it becomes a positive ion. The electron is the smallest unit of charge that can be isolated AKA elementary charge - + Ask if rod repelled anything before we rubbed it. Ask if silk did. Ask what that means. (2) Relate Millikan’s experiment. (3)

Insulators and Conductors
A conductor is a material that allows charges to flow easily throughout the material. Most metals are conductors. An insulators is a material that does not allow charges to flow easily throughout the material. Glass, rubber, silk and plastic are examples of insulators. Day 2: Engage- Display VDG demos. Explain- Go over above.

Transfer of Electric Charge: Contact – (friction and Conduction)
Conductors and insulators can be charged by contact. Example- Rubbing rubber rod with fur, glass rod with silk. = charging by friction Demonstrate this method of charging.

18.4 Charging by Contact and by Induction

Transfer of Electric Charge: Contact – (friction and Conduction)

Transfer of Electric Charge: Induction
Conductors can be charged by induction. A charged object is brought near the conductor. The conductor is then grounded, allowing the excess charge to flow away. The grounding is then broken. The conductor remains charged, even when the charged object is removed. Demonstrate above with electroscope. © 2013 Mark Lesmeister/Pearland ISD

18.4 Charging by Contact and by Induction
Charging by induction.

Transfer of Electric Charge: Polarization
A surface charge can be induced on an insulator by polarization. Ex. Walking across a carpet on a dry day. + - + - + - + - + - + - + - + - + - Use paper or packing peanuts to demo this with Van de Graaf. + - + - © 2013 Mark Lesmeister/Pearland ISD

LAW OF CONSERVATION OF ELECTRIC CHARGE
During any process, the net electric charge of an isolated system remains constant (is conserved).

What is happening in this diagram
There are two kinds of electric charge The two balloons must have the same charge and as a result, repel each other like charges repel A rubbed balloon and your hair do not have the same charge and as a result, they are attracted to each other Unlike charges attract - hair will have positive charges and the balloon will have a a net negative charge

III - Semiconductors These materials have properties of both insulators and conductors. Silicon and Germanium are examples of semiconductors used in electrical devises. In their pure state, semiconductors are insulators. The careful addition of specific atoms as impurities can dramatically increase the semiconductors ability to conduct electric charge.

VI - Superconductors These materials appear to show no resistance
The electrons passing through a super conductor generate no heat Ex. include metals that are perfect conductors when at or below certain temperatures. These diagrams show magnets levitating over a superconductor at extremely low temperatures This photo shows a magnet levitating above a high-temperature superconductor, cooled with liquid nitrogen. A persistent electric current flows on the surface of the superconductor, effectively forming an electromagnet that repels the magnet. The expulsion of an electric field from a superconductor is known as the Meissner Effect. (Credit: Image courtesy of DOE/Los Alamos National Laboratory

Section 2 Electric Force

Electric Force When two charged objects are brought close together, they may experience forces of attraction or repulsion. The closer the two objects are to each other, the stronger the force between them. The greater the mass (number) of the charges the stronger the force between them

Magnitude and direction of the electric force depends on:
The amount of charge carried by each object. Charge is measured in Coulombs (C). An electron (negative elementary charge) has a charge (q) of x C. A proton (positive elementary charge) has a charge (q) of 1.60 x C.

Coulomb’s Law The force between 2 charged objects can be calculated using coulomb’s Law Kc = 8.99 X 109 N.m2/C2

Fundamental forces of nature
Note the similarities and differences between gravitational force and electrical force 𝐹 𝑔 =𝐺 𝑚 1 𝑚 2 𝑟 only attractive, can’t be shielded 𝐹 𝑒 =𝑘𝑐 𝑞 1 𝑞 2 𝑟 both attractive and repulsive, can be shielded These are two of the four fundamental forces of nature

Fundamental forces of nature
Wiki article

Inverse square of distance –complete the chart
Original d New d New/orig. Change in distance reduce inverse Square Change in Fg 10 20 20/10 2/1 1/2 ¼ x 10/20 4x 50 5 8 64 25

Example 1 What is the magnitude of the electric force between a proton and an electron in a hydrogen atom? The two particles are separated by a distance of 5.3 x m on average. G: q1 = x C, q2 = 1.60 x C, r = 5.3 x m, Kc = 8.99 x 109 N.m2/C2 U: Felectric = ? E: S: S: 8.2 x 10-8 N An attractive force!!

Example 2: A Model of the Hydrogen Atom
In the Bohr model of the hydrogen atom, the electron is in orbit about the nuclear proton at a radius of 5.29x10-11m. Determine the speed of the electron, assuming the orbit to be circular. G: 𝑟=5.29𝑥10−11, 𝑞 1 =1.6𝑥10−19𝐶, 𝑘𝑐=8.99𝑥109𝑁.𝑚2/𝐶2, 𝑞 2 =1.6𝑥10−19𝐶 𝑚𝑒=9.11𝑥10−31𝑘𝑔 U: 𝑣=? E: 𝐹 𝑒 = 𝑘 𝑐 𝑞 1 𝑞 2 𝑟 2 𝑎𝑛𝑑 𝐹=𝑚 𝑎 𝑐 = 𝑚𝑣 2 𝑟 S: 𝐹 𝑒 = 8.99×109×1.6×10−19×1.6×10−19 ( 5.29×10−19) 2 = 8.22×10−8 𝑁

Class assignment Due By the end of class
will not accept late work

Section Check SECTION CHECK
Which of the following statements about charges is true? A. Charges are created by gaining electrons. B. Charges are destroyed by losing electrons. C. Charges are separated through a transfer of electrons. D. Charges cannot be created, destroyed, or separated.

Section Check SECTION CHECK
What will happen if a positively charged rod is suspended freely in air and another positively charged rod is brought near the suspended rod? A. The more you bring the rod closer to the suspended rod, the more it will attract. B. The more you bring the rod closer to the suspended rod, the less it will attract. C. The more you bring the rod closer to the suspended rod, the more it will repel. D. The more you bring the rod closer to the suspended rod, the less it will repel.

The Electric Field Section 6

Electric Field The electric field is a field force per unit charge.
An electric field exists in a region if a stationary charged object (test charge – usually positive) experiences an electric force because of its charge. Set up charge on Van de Graff, and then use pith ball to explore field. Point out that magnitude and direction of force on pith ball changes over time.

Electric Field Vector The electric field at any point is a vector.
The direction of the electric field is the direction in which a small positive charge would move if placed there. The magnitude of the electric field (E) is the force per unit charge at that point: The electric field is measured in N/C.

18.6 The Electric Field It is the surrounding charges that create the electric field at a given point.

Electric fields from different sources add as vectors.
18.6 The Electric Field Electric fields from different sources add as vectors.

Electric Field Lines Electric field lines are lines of force providing a map of the electric field in the space surrounding electric charges. Electric field lines represent the strength and direction of an electric field at any point around a charged object. Electric field lines do not really exist, however, they offer a useful means of analyzing electric fields. Not in the notes

Electric Field Lines Electric field lines run in the direction of the field at each point. Lines begin on positive charges (or infinity) and end at negative charges (or infinity). Lines do not cross. The magnitude of the field is shown by the number of lines in a given area.

Electric Field Lines – sketch this!!
Not in the notes

Electric Field Lines – sketch this!!
The number of lines leaving a positive charge or entering a negative charge is proportional to the magnitude of the charge.

Electric Field Lines – sketch this!!
Now sketch two negative charges!!

Not in the notes

Electric Field Strength
Electric field strength depends on charge and distance. 𝐸= 𝑘 𝑐 𝑞 𝑟 2 𝐸𝑙𝑒𝑐𝑡𝑟𝑖𝑐 𝑓𝑖𝑒𝑙𝑑 𝑠𝑡𝑟𝑒𝑛𝑔𝑡ℎ=𝐶𝑜𝑢𝑙𝑜𝑚𝑏𝑠 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 × 𝑐ℎ𝑎𝑟𝑔𝑒 𝑝𝑟𝑜𝑑𝑢𝑐𝑖𝑛𝑔 𝑡ℎ𝑒 𝑓𝑖𝑒𝑙𝑑 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 2

Example Problem #3 A typical thundercloud has an electric field of about 3.0 x105 N/C. If the electric field is directed toward the ground, what is the electric force on an 18 nC charge in this field? G: E = 3.0 x 105 N/C, q = 1.8 x 10-8 C U: Felectric = ? E: S: S: 5.4 x 10-3 N directed towards the ground

Class assignment Due By the end of class
Will not accept late work

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