Electric Charges and Forces

Transcription

1 Lecture 1 Chapter 25 Physics II Electric Charges and Forces Course website:

2 Course Website Everything you need to know about the course can be found on the course website:

3 Lectures/Textbook Physics II meets 5 times a week Lectures : 8:00-8:50 Mo-Fr, Olney 218 Lecture slides will be posted on-line The textbook: Physics for Scientists and Engineers, a Strategic Approach, Third Edition by Randall Knight (Pearson, 2012)

4 Weekly Homework Online HW is on Course title: 95144Summer2016 Course ID: DANYLOVSUMMER2016 The online HW is typically due midnight on Wednesday and Sunday (You are penalized 25% for each day late)

5 Course Grading Item Points HW 100 Quizzes 100 Exam Exam Final Exam Lecture Attendance Total 650 Range Grade >80% A 75-80% A % B % B 60-65% B % C % C 45-50% C % D % D <35% F

6 Electricity Electricity and magnetism is all around us. microphones, calculators, televisions, radio, computers. The colors of the rainbow in the blue sky are there because of electricity Your nerve system is driven by electricity. You could not see without electricity.

7 Electrostatics Charges are at rest Electrical forces arise from particles in atoms Electrons negatively charged Protons positively charged (by convention) Observations: charges interact Like charges repel Opposite charges attract Coulomb found an expression to describe these interactions

8 Coulomb s law F 1on2 When two charged particles are a distance, r, apart, they each experience a force. F 2on1 r q 2 q 1 In SI units K = N m 2 /C 2. Enormous!!! Let s define a new constant, called the permittivity constant 0 : Rewriting Coulomb s law in terms of 0 gives us:

9 The force between two charges separated by a distance d is F. If the charges are pulled apart to a distance 3d, what is the force on each charge?? Q F ConcepTest 1 Q Q d F? Q A. 9F B. 3F C. F D. (1/3)F E. (1/9)F 3d Originally we had: F before = k(q)(q)/d 2 = F Now we have: F after = k(q)(q)/(3d) 2 = 1/9F

10 Principle of superposition If multiple charges are present, the net electric force on a charge q due to all other charges is q 1 q 2 q F 1 F 2 F 3 q 3 F 4 q 4

11 ConcepTest 2 Forces in 2D Which of the arrows best represents the direction of the net force on charge +Q due to the other two charges? d +2Q 1 +Q A. 1 B. 2 C. 3 D. 4 The charge +2Q repels +Q d E. 5 toward the right. The charge +4Q repels +Q +4Q upward, but with a stronger force. Therefore, the net force is up and to the right, but mostly up. +2Q +4Q

12 Usually an atom is neutral Charged ions If an atom loses one or more electrons, it becomes positively charged called a positive ion If an atom gains one or more electrons, it becomes negatively charged called a negative ion electrons

13 Insulators and conductors The electrons in an insulator are all tightly bound to the positive nuclei and not free to move around. In metals, the outer atomic electrons are only weakly bound to the nuclei. These outer electrons become detached from their parent nuclei and are free to wander about through the entire solid.

14 Charging When a plastic rod is rubbed with a towel, the plastic acquires a negative charge and the towel acquires an equal amount of positive charge. (The charges are separated but the sum is zero) It leads to CONSERVATION OF CHARGE The net amount of electric charge produced in any process is zero Demo:rod/paper

15 The figure shows how a conductor is charged by contact with a charged plastic rod. Charging metal Electrons in a conductor are free to move. Once charge is transferred to the metal, repulsive forces between the electrons cause them to move apart from each other. Demo:rod/electroscope

16 Induced charge in conductor Although the metal as a whole is still electrically neutral, we say that the object has been polarized. Charge polarization is a slight separation of the positive and negative charges in a neutral object. Metal Demo:rod/electroscope/induction and rod/baloon

17 Channel 61 ConcepTest X and Y are two uncharged metal spheres on insulating stands, and are in contact with each other. A positively charged rod R is brought close to X as shown in Figure (a). Sphere Y is now moved away from X, as in Figure (b) What are the final charge states of X and Y? Polarization/Dipole A) Both X and Y are neutral B) X is positive and Y is neutral C) X is neutral and Y is positive D) X is negative and Y is positive E) Both X and Y are negative _ ++ _ X Y _ X _ Y One piece of metal Two pieces of metal So we created a dipole.

18 Charge induction in an insulator The figure shows how a neutral atom is polarized by an external charge, forming an electric dipole. Center of negative charge electric dipole F negatively charged surface positively charged surface Demo:rod/paper

19 Demonstrations Electroscope A Van de Graff generator (named after its inventor) is a high voltage generator. It basically loads a spherical hollow conductor with static charges which brings the conductor to a very high (or very low) potential.

20 Thank you See you tomorrow