How do I check if it is a positive charge or negative charge?

[CKaiL]

Homework Statement

Homework Equations

The Attempt at a Solution

[/B]
The electric field at the origin O, due to charges Q, q1 and q2, is equal to zero
Hence , EQ = Eq1
q = cos30 x 300 x10^-9/4 * 1.8^2 = 210 x10^-9 C

So I am able to find out that q = 210 x 10^-9 C and that mean that B and C cannot be the answer which left me with A and D.

How do I check if it is a positive charge or negative charge?

 

[rude man]

Homework Statement

Homework Equations

The Attempt at a Solution

The electric field at the origin O, due to charges Q, q1 and q2, is equal to zero
Hence , EQ = Eq1
q = cos30 x 300 x10^-9/4 * 1.8^2 = 210 x10^-9 C

So I am able to find out that q = 210 x 10^-9 C and that mean that B and C cannot be the answer which left me with A and D.

How do I check if it is a positive charge or negative charge?
[/B]

The x component at the origin due to Q is what polarity? So the polarity of Ex due to q1 has to be the opposite, right? So what then does the polarity of q1 have to be?

 

[CKaiL]

Why does it has to be opposite for the polarity of Ex due to q1?

 

[rude man]

Why does it has to be opposite for the polarity of Ex due to q1?

because the net E field at the origin is zero so q1 has to cancel out the x component of Q.

 

[CKaiL]

What about the q2 at the y component?

 

[rude man]

What about the q2 at the y component?

q2 does not produce an x compnent of the E field at the origin. You are summing x components of E at the origin.

By the way, you can also solve this problem with summing y components of E = 0 at the origin. I suggest you do it for increased understanding.