# The Physical World I

Spring 2005
MWF 11:30

Instructor: Dr. Bruce Law, CW 327, Tel: 532-1618.

# EXCERISES OF Home Work 19

## E22.12. A penny will be slightly more massive if it is negatively charged because you have added electrons. When the penny is positively charged you have removed electrons and therefore the penny would be slightly lighter.

E22.30. By rubbing the balloon against your hair you have charged the balloon (say negatively). The balloon sticks to the door by electrical induction (i.e. the negatively charged balloon induces a separation in charge for atoms composing the door. The positively charge nucleus of the atoms is attracted to the balloon while the negative charged electrons of the atoms are repelled by the balloon. The attraction (between the balloon and the positively charge nucleus of the door atoms) is stronger than the repulsion (between the balloon and the negatively charged electrons of the door atoms) and therefore the balloon sticks to the door.

E22.31. A charged atom attracts a neutral atom by electrical induction. (See previous question for an explanation.)

PROBLEMS OF Home Work 19

P22.1. The force F is proportional to 1/d2 where d is the separation distance. If you increase d by a factor of 2 then you must decrease F by a factor of 4, i.e. the new force F = 20/4 = 5N.