1. Electric Field Physics 102 Professor Lee Carkner Lecture 11

2. Force on Charges  F y = 8.99X10 9 (4)(2)/(2 2 ) =  F x = 8.99X10 9 (2)(5)/(3 2 ) =  F 2 = F x 2 + F y 2  F =  tan  = (F y /F x )   = arctan (F y /F x )   = 3 m 2 m q 2 = 5 C q 3 = 4 C q 1 =-2 C

3. Electric Field at Origin  E y = 8.99X10 9 (4)/(2 2 ) =  E x = 8.99X10 9 (5)/(3 2 ) =  E 2 = E x 2 + E y 2  E =  Field indicates direction positive test charge will move 3 m 2 m q 2 = 5 C q 3 = 4 C

4. To charged objects attract each other with force F. If the amount of charge on one object is doubled and the distance between the objects is also doubled, what is the new force in terms of the old? A)¼ F B)½ F C)1F (force is unchanged) D)2 F E)4F

5. A charge +Q is placed in the center of a square. When a charge –Q is placed on one corner of the square the net force between them is 2 N. What is the net force on the center charge if a charge –Q is placed on all corners? A)0 N B)2 N C)4 N D)8 N E)You can’t tell from the information given

6. What information does the electric field give us about a point in space? A)The sign of the charges that produced the field B)The distance to the nearest charge C)The magnitude of the electric force at that point D)The direction a positive charge would move from that point E)The velocity of a charge at that point

7. Field Lines  We defined the electric field as:  The field fills the entire region around a charge  We would like to draw the field to give us a “map” of how the charge will effect other charges   Field lines indicate how a charge will move

8. Field Lines and the Field   Direction:   At any point the F vector is tangential to the field lines  Strength:   Density of lines proportional to field strength

9. Examples of Fields

10. Dipole   Called a dipole   Magnetic fields are always dipolar

11. Dipolar Field

12. How to Draw Field Lines  Lines determine the force experienced by a positive charge   Number of lines touching a charge proportional to the magnitude of the charge   Lines must begin and end at a charge or infinity

13. Today’s PAL  Draw below a point charge of +3q, and to the right, a point charge of –1q. Draw the electric field lines between them. Draw at least 10 lines evenly spread out.  At a very large distance away, would a small positive test charge be attracted or repelled?  If q is one electron’s worth of charge, pick a point on a line connecting the charges and compute E there.

14. Van De Graaff Generator   The positively charged belt attracts the electrons in the dome making it positively charged   Conveyer belt to remove electrons  Charge builds up until it is discharged through the air

15. Van De Graaff  Why does your hair stand up when you touch the Van De Graaff generator?   Why do you need to stand on the box? 

16. Conductors and Fields   The charges in the conductor are free to move and so will react to the field   Charge distributes itself uniformly over the surface of a conductor

17. Inside the Conductor  If we consider both positive and negative charge in an electric field,   A charge placed in the middle would feel an equal force from both sides and not move, thus,  The field inside the conductor is zero   A conductor shields the region inside of it

18. Faraday Cage  If we make the conductor hollow we can sit inside it an be unaffected by external fields   Your car is a Faraday cage and is thus a good place to be in a thunderstorm 

19. Charge Distribution  How does charge distribute itself over a surface?   e.g., a sphere   No component parallel to surface, or else the charges would move   Excess charge there may spark into the air

20. Conducting Ring Charges Pushed To Surface No E Field Inside Field Lines Perpendicular to Surface

21. Next Time  Monday, January 8  Read 17.1-17.6  Homework: Ch. 16, P 28, 31, 32, Ch. 17, P 2, 14