# Homework Help: Coulomb's Laws-> Please help

1. Nov 7, 2005

### chipsdeluxe

Coulomb's Laws--> Please help!!!!

1. Object A is metallic and electrically neutral. It is charged by induction so that it acquires a charge of -2.9 x 10-6 C. Object B is identical to object A and is also electrically neutral. It is charged by induction so that it acquires a charge of +2.4 x 10-6 C. Find the difference in mass between the charged objects.

Na=(-2.9*10^-6)/(1.602*10^-19)=-1.81*10^13
Nb=(2.4*10^-6)/(1.602*10^-19)=1.49*10^13

(Nb-Na)(Mass of electron)=3.7*10^-18
what did i do wrong?

2. In a vacuum, two particles have charges of q1 and q2, where q1 = +3.6 uC. They are separated by a distance of 0.36 m, and particle 1 experiences an attractive force of 4.2 N. What is the value of q2, with its sign?

i used the equatioin F=[k(q1)(q2)]/r^2
I changed uC to C--> 3.6*10^-6 C
meters to km
F=4.2
i plugged in the knows and solved for q2 and got 1001.3 C.
please help!

thanks in advance!

2. Nov 7, 2005

### Andrew Mason

I get: 3.3e13 x 9.1e(-31) = 3.0e(-17) kg
You should show your numbers. It is difficult to figure out what you did wrong otherwise.

$$q2 = Fr^2/kq1 = 4.2*.13/9e9 * 3.6e(-6) = 17\mu C$$

AM

3. Nov 8, 2005

### andrevdh

Problem 1
Mass of A:
$$m+\delta m_a$$
Mass of B:
$$m-\delta m_b$$
Difference in mass:
$$m_A - m_B$$
$$(m+\delta m_a)-(m-\delta m_b)$$
Therefore difference is
$$\delta m_a+\delta m_b$$

4. Nov 8, 2005

### chipsdeluxe

thanks for the help :)

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