# Homework Help: Triboelectrics Help please.

1. Aug 29, 2005

### TheoehT

this problem is a worked example in my textbook.. but it looks like they have just pulled numbers from thin air. I really dont understand how they got these answers.. can anyone explain?

A perspex rod is rubbed with a piece of silk. it acquires a charge of 0.02uC

a) How many electrons are transferred?
b) is this charge is now transderred to an electroscope with charge +0.01uC what will be the net state of the electroscope?

Book's Solution:
0.02 x 10^-6 x 6.25 x 10^18 electrons -or-
a) 1.25x10^-11 electrons transferred.

b) total charge on the electroscope after transfer:
0.02uC + 0.01uC = 0.03uC (+)
the electroscope leaf deflection will increase and the net charge on the electroscope is now +0.03uC = 1.87x10^11 electrons depleted

I really dont understand, can anyone help?

Last edited: Aug 29, 2005
2. Aug 29, 2005

### azeari

okay here you go.

1 uC ( micro-columb ) = 1 * 10^-6 C

Charge on 1 electron = 1.6 x 10^-19 C

Number of electrons in 1 Columb = 1/(1.6 * 10^-19)
Which = 6.25 * 10^18 -points solution-

Amount of charge transfered = 0.02 * 10^-6 (points micro columb value above)
Amount of electrons transfered = Amount of charge transfered * number of electrons/C

so...

0.02 * 10^-6 * 6.25 * 10^18 = 1.25 * 10^-11 (electrons transfered)

For b, when u rub the rod with a piece of silk, it aquires a negative charge yeah?(electrons go from the rod to the silk)

So, if u transfer this charge to the electroscope, which already has +0.01uC, its total charge becomes +0.03uC
+0.02uC +0.01uC = +0.03uC

The deflection increases cuz like-charges repel, so if i have a higher charge on the scope, the leaves both possess a higher charge, and thus experience more repulsion.

1.87 * 10^11 ?
i think u meant 1.87 * 10^-11
that'll be the number of electrons the electroscope is missing, if u calculate, that equals to 0.03uC

Thank ruby for this.. her fault

3. Aug 29, 2005

### TheoehT

thank catherine for this, her fault