Question

A long uniformly charged thread (linear charge density λ = 2.4C/m ) lies along the x axis in the figure.(Figure 1) A small charged sphere (Q = -2.2C ) is at the point x=0cm, y=−5.0cm.
A)What is the direction of the electric field at the point x=7.0cm, y=7.0cm? E⃗ thread and E⃗ Q represent fields due to the long thread and the charge Q, respectively.

B)What is the magnitude of the electric field at the point x=7.0cm, y=7.0cm?

Figure 1 : http://session.masteringphysics.com/problemAsset/1083950/4/GIANCOLI.ch21.p31.jpg

Answer 1

The first thing to figure out is what field is produced by the line of charge, this can be got from Gauss's law i think, if you don't know it already

Answer 2

Hmm ok. I've never seen a problem like this so I don't know how to apply Gauss's Law

Answer 3

Gauss's law says that the flux of E thru a surface is equal to the enclosed charge divided by epsilon0, you could apply that to a cylindrical surface around the wire - symmetry makes things quite simple - or you could just look up the field due to a line charge !

Answer 4

Then it is just a matter of adding the E vectors due to the line charge and the point charge to find the resultant electric field

Answer 5

Hmmm I'm just generally confused about this problem as a whole. My professor has not gone over Gauss's Law, but has assigned homework pertaining to it...

Answer 6

Well if you look it up or work it out, the field due to the wire is radial and has magnitude lambda/(2pi epsilon0 r). In your case you can just use the value of y in place of r in the formula

Answer 7

how do you use epsilon?
Is it a constat?

Answer 8

*constant

Answer 9

epsilon zero, or epsilon nought, yes is a constant, numerical value 8.85 x 10^-12

Answer 10

I got - 8.6x10^9. does that sound correct?

Answer 11

no idea, i haven't worked it out - yet

Answer 12

Let me know what your answer is when you work it out?

Answer 13

i will - what does that number you calculated represent ?

Answer 14

the magnitude

Answer 15

your answer does not agree with mine - to do this problem you need firstly to have a clear idea of the field produced by each source on its own, (the point charge, and the line charge) then you need to vectorially add the two fields from each source to find the resultant field

Answer 16

what answer did you get?

Answer 17

so i can compare it to mine so I can see where I went wrong

Answer 18

can you calculate the size of the electric field due to the point charge at the point (7,7) ?

Answer 19

i just used the formula you mentioned earlier. one with the lambda

Answer 20

that formula is for the field from the line charge, but you also need the field from the point charge that sits at (0, -5)

Answer 21

hmm ok so what answer did you get when you use that point?

Answer 22

*point charge