Chapter 32. Electrostatics

Transcription

1 Chapter 32 Electrostatics

2 Chapter Warm Up What are the name, symbol, charge, and location in the atom of the three major atomic particles? What is the rule of charges? What is static electricity? What are some ways to charge objects? What is the difference between a conductor and an insulator? What is electric force?

3 Section 1: Atomic Theory What is an atom? Smallest particle of an element that has all the properties of that element.

4 Parts of an atom( Current Theory) Nucleus In the center of the atom Made of protons and neutrons Positively charged Contains almost all the mass of the atom Discovered by Ernst Rutherford (1911)

5 Electron Cloud Outside the nucleus, but doesn t touch the nucleus Most probable place for the electrons to be Amorphous shape Negatively charged Essentially massless Described by Erwin Schrodinger and others (1926)

6 Proton Symbol: p, p +1 Atomic Particles Charge: +1, positive, or 1.60 x Coulombs Mass:~1 amu ( atomic mass unit) or x kg Location in atom: nucleus Function: number of protons identifies the atom Ease of removal/addition: very difficult(nuclear reaction)

7 Neutron Symbol: n, n 0 Charge: 0 or neutral Mass: ~ 1 amu or x kg ( largest particle) Location in atom: nucleus Function: Stabilizes nucleus/ responsible for isotopes(2 atoms of the same element with differing numbers of neutrons) Ease of removal/addition: very difficult (nuclear reaction)

8 Electron Symbol: e, e -1 Charge: -1, negative, or x Coulombs Mass: 0 amu, or x kg Location in atom: electron cloud Function: the number and arrangement of electrons determines the properties of the atom Ease of removal/addition: very easy

9 Section 2: Electric Charge Properties of charge Symbol: q Units: Coulomb ( C) a fundamental unit Types of charges Positive(protons) Negative(electrons)

10 Rule of charges Like charges repel, unlike charges attract Negative charge can be transferred from one object to another Charge is quantized Charge is always a multiple of the unit charge(e) e = 1.60 x C charge on the electron(q e = x C) charge on the proton(q p = 1.60 x C) 1.0 C = 6.2 x electrons or protons

11 Example: The charge on an electron is 1.6 x C. How many electrons make a charge of -2.00C? (just a domino)

12 Section 3: Conservation of Charge In a closed system, electric charge is conserved. Total negative charge in a neutral system = total positive charge Equation: q 1i + q 2i = q 1f + q 2f

13 Example: A pair of coins is electrically charged, one with a charge of +15 C, and the other with a charge of -25 C. When they are brought together, they become equally charged. What is the charge on each after contact?

14 Properties of charged objects Charge can build up on an object Static electricity Most objects are naturally electrically neutral Number of negative charges(electrons) = number of positive charges(protons)

15 An object is electrically charged when there is an excess of one type of charge If more negative charges than positive charges will be negatively charged Object has gained electrons from someplace else If fewer negative charges than positive charges will be positively charged Object has lost electrons to someplace else

16 Section 4: Transferring Electric Charge Two main types of materials based on how well charge is transferred Conductors Electric charge moves easily through the material metals Material does not hold on to electrons very well

17 Insulators Electric charge does not move easily through the material Nonmetals, rubber, glass, plastics, etc. Material holds on to electrons tightly

18 Other types of materials Semiconductors Insulators that have been modified to improve electrical conductivity Frequently use silicon and add small amounts of phosphorus, arsenic, boron, or gallium Superconductors Material that allows infinite charge flow at or below a certain temperature Conduct electricity without heating up

19 Section 5: Charging Materials Contact(objects must touch) Friction Works with insulators or conductors Electrons transfer between the two objects One object transfers electrons to the other object Both objects can start as neutral objects Objects end up with opposite charges

20 Conduction Both objects must be conductors Electrons transfer between objects One object transfers electrons to the other object One object must start out with a charge, the second object starts out neutral Objects end up with same charge

21 Proximity(objects do not touch) Induction Conductors only No electron transfer between objects electrons are rearranged in the conductor One object must start with a charge; second object starts out neutral; second object is grounded Objects end up with opposite charges

22 Polarization Insulators No electron transfer between objects electrons are shifted in the electron cloud of the atoms of the insulator One object must start with a charge; second object starts out neutral Objects end up with opposite charges

23 Section 6: Electric Force Properties The force between two charged objects Field force Can be either attractive or repulsive (Rule of charges) Stronger than gravitational force Strength depends upon the size of the charges and distance between them

24 Symbol: F elec or F e Units: Kgm/s 2 or Newtons (It s a force) Equation: F e = k c q 1 q 2 r 2 q 1 = charge on object 1 q 2 = charge on object 2 r = distance between the 2 objects k c = Coulomb Constant (8.99 x 10 9 Nm 2 /C 2 )

25 Examples: Two balloons are 0.40 m apart. Each balloon has a charge of 6.5 C on it. What is the electrostatic force between the two balloons? 2.4 N repulsive

26 A dog s fur is combed, and the comb gains a charge of 8.0 nc. If the electric force between the fur and the comb is x 10-3 N, how far apart are the dog and the comb? 2.0 x 10-2 m

27 Proportional Reasoning: Two charged objects are near each other. How will the force on each object change if the charge on one object doubles, but the distance between them and charge on the other object remain the same?

28 Two charged objects are near each other. How will the force on each object change if the distance between the objects is tripled, but the charges on the objects remain the same?