Chapter 19. Electric Charges, Forces and Electric Fields

Transcription

1 Chapter 19 Electric Charges, Forces and Electric Fields

2 Agenda Atom History Induction Static Conductors Insulators Coulomb s Law Observation/ question Quiz

3 The electrons in an atom are in a cloud surrounding the nucleus, and can be separated from the atom with relative ease.

4 First Observations Greeks Observed electric and magnetic phenomena as early as 700 BC/ 600 BC

5 First Observations : Greeks They saw that when they rub a piece of fur, an amber rod acquires a charge and can attract small objects.

6 First Observations Greeks -Cont Whenever ladies were bored in a party, they will rub the amber jewelry and touch a frog. Ladies and gentlemen were amused when they saw the frog jumping after being touched by the amber.

7 Benjamin Franklin Printer, author, founding father, inventor, diplomat Physical Scientist 1740 s work on electricity changed unrelated observations into coherent science Benjamin w/o knowledge of proton or electron

8 Benjamin Franklin Franklin also noticed that the closer the objects were the stronger the forces. He believed that there were substances that conduct better than others and he called those substances conductors.

9 Properties of Electric Charges Two types of charges exist They are called positive and negative Named by Benjamin Franklin Like charges repel and unlike charges attract one another

10 Properties of Electric Charges Charging both amber and glass rods shows that there are two types of electric charge; like charges repel and opposites attract.

11 Properties of Electric Charges Nature s basic carrier of positive charge is the proton Protons do not move from one material to another because they are held firmly in the nucleus Nature s basic carrier of negative charge is the electron Gaining or losing electrons is how an object becomes charged

12 More Properties of Charge Electric charge is always conserved Charge is not created, only exchanged Objects become charged because negative charge is transferred from one object to another

13 Charging by Induction A neutral sphere has equal number of electrons and protons Let s say that this is a conductor. That means that the electrons will be easy move.

14 Charging by Friction I take an amber rod and I rub it with fur. Some of the electrons on the atoms in the fur are transferred to the amber. The amber rod is negative charge and the fur is positive charge. The rod was charged by friction.

15 Charging by Induction, 1 Now, let s take that rod and brought it near an uncharged sphere The charges in the sphere are redistributed. The sphere has been charged by induction.

16 Charging by Induction, 2 The region of the sphere nearest the negatively charged rod has an excess of positive charge because of the migration of electrons away from this location If the sphere is connected to a conducting wire or pipe buried in the earth, it is said to be grounded, the electrons will migrate to earth.

17 Charging by Induction, final The wire to ground is removed, the sphere is left with an excess of induced positive charge The positive charge on the sphere is evenly distributed due to the repulsion between the positive charges Charging by induction requires no contact with the object inducing the charge

18 Charging by Conduction A charged object (the rod) is placed in contact with another object (the sphere) Some electrons on the rod can move to the sphere When the rod is removed, the sphere is left with a charge The object being charged is always left with a charge having the same sign as the object doing the charging

19 Conductors Conductors are materials in which the electric charges move freely in response to an electric force Copper, aluminum and silver are good conductors When a conductor is charged in a small region, the charge readily distributes itself over the entire surface of the material

20 Insulators Insulators are materials in which electric charges do not move freely Glass and rubber are examples of insulators When insulators are charged by rubbing, only the rubbed area becomes charged There is no tendency for the charge to move into other regions of the material

21 Polarization In most neutral atoms or molecules, the center of positive charge coincides with the center of negative charge In the presence of a charged object, these centers may separate slightly This results in more positive charge on one side of the molecule than on the other side This realignment of charge on the surface of an insulator is known as polarization

22 Examples of Polarization The charged object (on the left) induces charge on the surface of the insulator A charged comb attracts bits of paper due to polarization of the paper

23 DEMONSTRATION # 1 Conductor

24 DEMONSTRATION # 2 Non- Conductor

25 DEMONSTRATION # 3 Friction

26 DEMO # 6 (if we have time) I need a volunteer for the good of science. Someone with nylon or polyester shirt.

27 DEMO # 7 Van De Graff Generator confetti (at the end of class)

28 Charles Coulomb Studied electrostatics and magnetism

29 Vector Nature of Electric Forces If I take two charges that are separated by a distance r, we will see that: The like charges produce a repulsive force between them The force on q 1 is equal in magnitude and opposite in direction to the force on q 2

30 Vector Nature of Forces, cont. What about unlike charges separated by a distance r? The unlike charges produce a attractive force between them The force on q 1 is equal in magnitude and opposite in direction to the force on q 2

31 Coulomb s law (continued)

32 Coulomb s Law Coulomb found the relationships between those charges is: q1 q2 F ke 2 r k is called the Coulomb Constant k = 8.99 x 10 9 N m 2 /C 2 Remember that force is a vector quantity Applies only to point charges

33 Coulomb s Law Cont Coulomb shows that an electrical force has the following properties: It is directed along the line joining the two particles and inversely proportional to the square of the separation distance, r, between them It is proportional to the product of the magnitudes of the charges, q 1 and q 2 on the two particles It is attractive if the charges are of opposite signs and repulsive if the charges have the same signs

34 Class participation1: 1) Two charges have a force of attraction between them. If you triple the distance between the charges what happens to the force? 2) Two charges are a distance r apart and exert a force F on each other. If you triple one or the charges what happens to the force?

35 Coulomb s Law Cont Magnitude of an electron s charge q = e = 1.60 x C We can see that 1C = 6.25 X electrons Typical charges can be in the µc range µc = 10-6 C

36 Characteristics of Particles

37 Example1: Electrical Force Question: The electron and proton of a hydrogen atom are separated (on the average) by a distance of about 5.3x10-11 m. Find the magnitude of the electric force that each particle exerts on the other.

38 Question: The electron and proton of a hydrogen atom are separated (on the average) by a distance of about 5.3x10-11 m. Find the magnitude of the electric force that each particle exerts on the other. Observations: We are interested in finding the magnitude of the force between two particles of known charge, and a given distance of each other. The magnitude is given by Coulomb s law. q 1 =-1.60x10-19 C q 2 =1.60x10-19 C r = 5.3x10-11 m F k e q 1 r 2 q 2 38

39 Question: The electron and proton of a hydrogen atom are separated (on the average) by a distance of about 5.3x10-11 m. Find the magnitude of the electric force that each particle exerts on the other. Observations: The magnitude is given by Coulomb s law. q 1 =-1.60x10-19 C q 2 =1.60x10-19 C r = 5.3x10-11 m Solution: e C 9 2 Nm 8 Fe ke N C 2 r m Attractive force with a magnitude of 8.2x10-8 N. 39

40 9.21e-4 Class participation # 2