Question

unit 3: electricity
lesson 3.2 conductivity & charging by friction
7 benjamin franklin rubbed a glass rod on the fur on the back of a cat, then rubbed a rod of amber on the back of a different cat. use the electrostatic series table above to explain what he would have observed.

1. describe the charges of each object for the following cases:

a) a wool sweater is pulled off, rubbing against a cotton shirt.
wool sweater becomes ____________ cotton shirt becomes ____________
b) a silk scarf is removed from your neck, rubbing against a cotton shirt.
silk scarf becomes ____________ cotton shirt becomes ____________
c) the silk scarf is left on top of a wool sweater and is rubbed as you pick it up.
wool sweater becomes ____________ silk scarf becomes ____________

Explanation:

Step1: Recall charging by friction principle

When two materials are rubbed together, electrons are transferred from one to the other based on their position in the electrostatic series. The material higher in the series gives up electrons and becomes positively - charged, while the lower one gains electrons and becomes negatively - charged.

Step2: Analyze case a

In the electrostatic series, wool is higher than cotton. When a wool sweater is rubbed against a cotton shirt, wool gives up electrons. So the wool sweater becomes positively - charged ($+$) and the cotton shirt becomes negatively - charged ($-$).

Step3: Analyze case b

Silk is higher than cotton in the electrostatic series. When a silk scarf is rubbed against a cotton shirt, silk gives up electrons. So the silk scarf becomes positively - charged ($+$) and the cotton shirt becomes negatively - charged ($-$).

Step4: Analyze case c

Wool is higher than silk in the electrostatic series. When a silk scarf is rubbed against a wool sweater, wool gives up electrons. So the wool sweater becomes positively - charged ($+$) and the silk scarf becomes negatively - charged ($-$).

Answer:

a) Wool Sweater becomes $+$, Cotton Shirt becomes $-$
b) Silk Scarf becomes $+$, Cotton Shirt becomes $-$
c) Wool Sweater becomes $+$, Silk Scarf becomes $-$