HI I dont understand the method of image.(adsbygoogle = window.adsbygoogle || []).push({});

I have this problem:

A uniform charged disk is suspended ( at a distance h) above an infinite conducting plane ( disk is parallel to the plane). The disk carries a charge Q and has a diameter d. Assume d>>h.

1- Calculate the electric field vector near the center and below the disk.

2- Calculate the capacitance of the system using the formula C=Q/V.

3- Calculate the capacitance of the system using the energy formulas.

I attach a picture.

I get lost at some point

the electric fiel for a disk is:

E=Qy/(2*pi*epsilon)*(1/y-1/(sqrt(y^2+R^2))y (unit vector)

to answer the question 1-:

electric field from charge Q:

E_{Q}=Q(h-y)/(2*pi*epsilon)*(1/(h-y)-1/(sqrt((h-y)^2+R^2))(-y) (unit vector)

electric field from charge -Q ( the image):

E_{-Q}=-Q(h+y)/(2*pi*epsilon)*(1/(h+y)-1/(sqrt((h+y)^2+R^2))(-y) (unit vector)

Here is my first problem: the direction of the two electric fields E_{Q}and E_{-Q}.

I am not sure about the electric field of the expressions.

(I suspect I get some mistakes because I tried a similar problem but with a more simple expression for the 2 fields and when I add them, I get zero)

I would really have some help with this problem.

please can someone help me?

Thank you

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# Homework Help: Please help with image method

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