1. Electricity and Magnetism Physics 102 Professor Lee Carkner Lecture 9

2. Electricity and Magnetism   In E&M, we will deal with forces that depend upon charge  Charged particles generate E&M forces    E&M forces only affect charged particles

3. Forces  The electric force is much stronger than the gravitational force   Why don’t we feel this strong force?   Most things have roughly equal numbers of positive and negative particles   Like charges repel, opposite attract   e.g. lightning, static electric shock

4. Plus and Minus   The basic particle of positive charge is the proton  Protons generally don’t move   Electrons are in the shells of atoms and can be removed  Adding electrons makes something negative   All E&M forces depend on what the electrons are doing

5. Using Electricity  Why is the electrical force important?   Convert electrical energy into work   Convert electrical energy into heat and light   Convert electrical energy into sound   Electrical energy can be very finely controlled

6. Making Electricity   Three ways to do this  Chemically  e.g.  Magnetically  e.g.  Physically   Four basic methods: friction, conduction, induction, polarization

7. Triboelectricity   Which way the electrons go depend on the triboelectrical properties of the material 

8. Fur Rubber e-e- Rubbing a piece of rubber with fur transfers the electrons from the fur to the rubber due to friction. Triboelectric Charging

9. How Does Charge Move?  Some types of material allow electrons to move freely (e.g. metals)   When you charge a conductor, the electrons will flow through it   Other types do not allow electrons to flow (e.g. glass, rubber)   If you charge them, the charge stays put

10. Conduction  If a charged conductor is brought in contact with neutral conductor, charge will flow   Charge can jump a short gap and form a spark  Example:

11. Neutral Metal Charged Metal e-e- Charge will move from one conductor to another Conductive Charging Both Metal Rods Now Charged

12. Induction  If you place a charged object near a conductor it will:   repel the same sign charges to the far end    Will always attract the original charged object

13. Induced Charge on Metal Charged Metal A charged conductor will split the charge on a near-by conductor Inductive Charging

14. Induction via Grounding  If you connect a conductor to the Earth, an endless amount of charge can flow from it to the ground   If you place a charged object near a grounded conductor it will repel the same sign charges to the ground   Charge is opposite that of the inductor

15. Metal becomes positiveCharged Metal A charged conductor will push out the same charge to the ground, leaving the other conductor with an opposite charge Inductive Charging with Grounding Electrons pushed to ground

16. Polarization   You have charged the balloon but not the wall   The negative charge on the balloon attracts the positive charged parts of the molecules of the wall, polarizing it

17. Polarization

18. Origins of Electricity  Moving charges by rubbing has been known since ancient times   Benjamin Franklin proposed the terms positive and negative for the two types of charge

19. Franklin’s Kite   He flew a kite in a thunderstorm and saw that charge flowed down the string 

20. Next Time  Read: 16.5-16.7  Homework: Ch 16: P 1, 5, 23, 24