People experience static electricity everyday.

Transcription

1 Electrostatics Electrostatics/static electricity is the branch of physics that deals with electrical charges or charged objects, and their interactions. The charges are stationary. People experience static electricity everyday. Examples Clothes tumble in the dryer and cling together. You walk across a carpet and get a shock when you touch a door knob. You pull off a wool sweater and see sparks of electricity. Sparks of electricity can be seen when you pull a wool blanket off the sheets of your bed. You rub your dog's fur and observe it standing up on it ends. Bolts of lightning dash across the evening sky during a spring thunderstorm. Your hair stands on end when you pull off a hat. 1

2 Our Changing View of the Atom Early Greek Philosophers (~400 B.C.) Atoms are the building blocks of matter. J. J. Thomson ( ) Negative electrons are embedded in a sea of positive charge. Ernest Rutherford (1911) Positive charge is located within a central nucleus. Neils Bohr (1913) Electrons are in circular orbits with quantized energy. Quantum Mechanics (Modern Model) Electrons occupy regions of space whose shape is described by complex mathematical equations. 2

3 Atomic Theory All objects contain atoms. An atom consists of protons, electrons and neutrons. The protons and neutrons are found in the nucleus of the atoms. Atoms do not change their number of protons and neutrons. Each proton carries one positive electrical charge. This is called the positive elementary charge (+e). +e = 1.6 x C Neutrons carry no electrical charge, and contribute only to the mass of the the atom. Electrons are found in each atom outside the nucleus. Each electron carries one negative charge. This is called the negative elementary charge ( e). e = 1.6 x C On the whole, atoms are electrically neutral, meaning that the number of protons equals the number of electrons. 3

4 Charged versus Neutral Objects Atoms can change their electron count. Electrons can be removed or added to atoms. If an atom gains electrons, we say that is is a negative ion. If an atom loses electrons, we say that it is a positive ion. (deficit of electrons) net "+" charge (excess of electrons) net " " charge no net charge Net charge is simply the sum of the total + and total charges. 4

5 Law of Conservation of Electrical Charge In a closed system, the net amount of electrical charge remains constant. This basically tells us that charge is neither created nor destroyed, merely redistributed. 5

6 Methods of Charging Objects can be charged by a number of ways. by friction (rubbing): one object loses electrons and the other gains electrons by conduction: a neutral object is touched by a charged object. Both objects will share the net charge, therefore both will have the same type of charge. by induction; through this process, a neutral object can be charged with the opposite sign of the charging object. 6

7 Electrostatic Series An electrostatic series can be used to determine the type of net charge an object will acquire. Common Substances gold brass rubber copper ebonite wax silk lead fur wool glass holds electrons tightly ( ) holds electrons loosely (+) If two materials are rubbed together, the material that is higher on the list will collect electrons and the material lower on the list will give away electrons. Frictional Charging Frictional charging results in the transfer of electrons between two objects that are rubbed together. The two objects will become charged with opposite charges as a result of the transfer of electrons from the least electron loving material to the most electron loving material Example A balloon rubbed on human hair will become negatively charged and the hair will become positively charged. 7

8 Electroscopes An electroscope is a device that not only detects the presence of an electric charge, but it will also determine the nature of the charge (+ or ). The most common kind is the split leaf electroscope. It is comprised of an insulated (usually glass) container and a central conductor ending in two thin metallic leaves at the bottom. When the knob of the electroscope is touched with an electrically charged object, electrons are transferred to or from the electroscope. Charging by Conduction Transferring charge from one object to another by touching is called charging by conduction. If a positively charged glass rod is rubbed with silk, the glass rod may be used to charge and electroscope positively by conduction. The electroscope is charged positively since some electrons transferred from the electroscope to the positive glass rod. 8

9 Charging by Induction When a charged object is brought near a neutral object, the charged object can induce, or force, a net charge to appear on one side of the object. When the negatively charged object is moved away, the object will revert back to being neutral as the electrons on the far side move back to be with the protons. The induced charge is present only while the negative charge is near! Example 9

10 It is because of induction that charged objects attract neutral objects. Polarization Polarization is the process of separating opposite charges within an object. By inducing the movement of electrons within an object, one side of the object is left with an excess of positive charge and the other side of the object is left with an excess of negative charge. 10

11 Electric Forces Any charged object can exert a force on another charged or uncharged object. Electric forces can be either attractive or repulsive. They are noncontact forces. According to the third law of motion, the forces are equal in magnitude and opposite in direction. AF B = B F A According to the third law of motion, the forces are equal in magnitude and opposite in direction. DF C = C F D 11

12 12

13 Conductors versus Insulators Electric charge can be transferred from object to object, and from atom to atom within an object. Therefore, all materials are separated into two categories based on their ability to transfer electrons. Conductors these are materials in which electrons can move easily from atom to atom most metals are conductors when a conductor is charged, the net charge will be evenly distributed over the surface of the object Insulators these are materials where the electon movement is restircited. Electrons are not free to move from atom to atom wood, plastic, glass, cork and rubber are insulators when an insulator is charged, the net charge will be localized to the region of the surface that was rubbed 13

14 Measuring Electric Charge In 1916, Robert Millikan determined the amount of charge carried by an electron. He discovered that the net charge on every electrically charged object that he studied carried a charge of 1.6 x C, or some whole number multiple of this charge. We use the symbol "q" to represent the net charge on an object. Charge is measured in Coulombs, C. Therefore, charge of 1 electron q e = 1.6 x C charge of 1 proton q p = 1.6 x C ( e) (+e) Since each electron carries 1.6 x C of charge, if we gather together 6.25 x electrons, the net charge would be (6.25 x )( 1.6 x C) = 1 C 14

15 Charles Augustin de Coulomb ( ) Coulomb's Law When two electric charges come near each other, an electric force, F e, will act between them. It is an attractive force between opposite charges and a repulsive between like charges. The force is directly proportional to the product of the charges and inversely proportional to the square of the distance between the charges. This is another example of an inverse square law. F e electrostatic force (N) q 1 charge (C) q 2 charge (C) k Coulomb's constant 9.00 x 10 9 Nm 2 C 2 d distance between the charges (m) d d 15

16 16

17 Grounding Grounding is the process of removing the excess charge on an object by transferring electrons from the object to a second object of substantial size. A ground is an object which serves as a seemingly infinite reservoir of electrons; it is capable of transferring electrons to or receiving electrons from a charged object in order to neutralize that object. 17

18 Review A gravitational force holds a satellite in orbit about a planet. An electrical force holds an electron in orbit about a proton. 18

19 Electric Fields An electric field is a force field that surrounds an electric charge or group of charges. Region of Influence Region of Influence Q (source charge) Region of Influence Region of Influence The magnitude of an electric field is defined as the force per charge on any given test charge located within the electric field. where E is the strength/intensity of the electric field (N/C) F e is the magnitude of the force experienced by the test charge (N) is the magnitude of the test charge (C) A test charge is a mathematical creation it does not actually exist. The electric field strength is NOT dependent on the quantity of charge on the test charge. 19

20 Note The field's strength varies inversely with the square of the distance from the center of Q to the point in the field where the strength is to be measured. One feature of the above formula is that it illustrates an inverse square relationship between electric field strength and distance. 20

21 Electric Field Lines (Force Lines) The field lines around a single positive charge extend to infinity. For a pair of equal but opposite charges, the field lines emanate from the positive charge and terminate on the negative charge. Field lines are evenly spaced between two oppositely charged metal plates. 21

22 Force of Repulsion Between Like Charges 22

23 Textbook A. page Practice Problems Questions B. page 655 Practice Problems Questions 20 22, 24, 25 23

24 Electrical Potential Energy To push a positive test charge closer to a positively charged sphere, we will expend energy to overcome electrical repulsion. Work is done in pushing the charge against the electric field. This work is equal to the energy gained by the charge. The energy a charge has due to its location in an electric field is called electrical potential energy. If the charge is released, it will accelerate away from the sphere and electrical potential energy transforms into kinetic energy. 24

25 Electric Potential Difference The concept of electrical potential energy per charge has the name, electric potential difference. difference difference NOTE Electric potential difference is not the same quantity as electrical potential energy!! 25

26 Electric Potential Difference The SI unit of measurement for electric potential difference is the volt, named after the Italian physicist Allesandro Volta. The symbol for volt is V. Potential energy is measured in joules and charge is measured in coulombs, Since electric potential is measured in volts, it is commonly called voltage. E 26

27 Uniform Electric Field The strength of an electric field between two oppositely charged, parallel plates is uniform. The electrical potential difference between two locations in a uniform electric field can be calculated using the equation: where, V = Ed V voltage V E strength of the electric field N/q d distance between the locations m 27