1. Electric Charges and Induction
(HW1 due in a week)

2. What’s Electricity?
Certain (related) interactions between objects: forces and sparks
EXPERIMENT →
Electric interaction seems to be governed by some quantity of stuff present
or absent in matter.
A) Matter showing no electric effects is neutral (‘stuff’ =0)
Name this stuff to be electric charge.
B) The ‘stuff’ seems to come in two different types.
Name them + and -.

3. Electrically neutral objects- + + - + - - +
Neutral atom
Nothing (vacuum)
Filled with as much - as + charge.

4. Charging a neutral object- - + + - + - + + - + + - + - -
Net charge of one + .

5. + − Some demos: Pith-balls Stu’s tape trick RUB-er RUB-ee CHARGE Silk
Glass rod +
Acrylic
Fur −
PVC
Green wool
Brown paper towel
Plexi
Gray PVC
Some demos:
Pith-balls
Stu’s tape trick

6. EXPERIMENT →
A) Charges of the same type repel; Opposites attract.
B) Matter seems divided up into either electric…
Conductors:
Insulators:
metals
water
wet string
‘ionized’ gas ….
air
dry wood
rubber
plastic
dry silk string …..

7. Conductors
Coke can demo

8. Inductive attraction:
Coke can
electrically neutral
‘atom’ + −

9. Inductive attraction:
mobile charge (conductor)
fixed charge + −

10. Inductive attraction:− + − + − + − − + − + +

11. Inductive attraction:
new fact: interactions are stronger if they’re closer! − + − + − + − − + − + +

12. Inductive attraction:
Just about any object can be attracted by induction! − + − + − + − − + − + +

13. Demo: Balloon!
Balloon simulation!
The moral:
If two things attract, might be opposite charge or (most likely) induction
If two things repel, sure sign of like charges!

14. Another example: Water molecule− +
Do water demo

15.
− − − − −
− − − − −

16. Electroscope (demo)

17. Electroscope

18. Electroscope − − − − − + + + + + + + + + + +

19. Electroscope − − − − − − + + + + + + + + + + +

20. Electroscope
+ + + + + + + + + + + + +

21. Electroscope
+ + + + + + + + + + + +

22. Charging an object by Induction:
Electrophorus ‘machine’

23. Finally: the Van de Graff !
(pie plates!)