Engineering Finding the force acting on a point charge with a disk around it

AI Thread Summary
The discussion focuses on calculating the force acting on a point charge surrounded by a disk, with the final answer being F = -99.4 az μN. Participants clarify the meaning of variables, particularly the radius (R) and the disk's position in space, which remains ambiguous. The calculations involve integrating electric field equations derived from the disk's surface charge density and geometry. The integration process leads to a final electric field value of E = 3.313269 V/m, which is then used to compute the force. The conversation emphasizes the importance of clear definitions and accurate calculations in physics problems.
falyusuf
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Homework Statement
Attached below.
Relevant Equations
Given below.
Question:
20089301620236335838842342050563270.png

Here's my attempt with the relevant equations:
1637708550496.png

The correct answer is F = -99.4 az μN. Could someone please figure out my mistake?
 
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Let's consider the second equation (E=...). What does R stand for?
 
Gordianus said:
Let's consider the second equation (E=...). What does R stand for?
Radius.
 
Hmmm...Try again
 
The position of the disk in space is not clear. What does it mean "a disk located 0<ρ<1 " that is, the center of the disk is in the z axis or is displaced by 1 m and the radius of the circle is 1 m or only 1/2 m. It is not clear at all.
 
I think I got how was calculation done. The circle of radius 1m with the center located on z axe. If we consider an infinite small surface dS=r*dα*dr [as in attached sketch] then:
dE=ρs/(4/PI()/εo/a^3*dS*ā
E=ρs/(4/PI()/εo*ʃdS*ā/sqrt(r^2+d^2)/(r^2+d^2)
E=ρs/(4/PI()/εo*ʃdS*ā/(r^2+d^2)^3/2
E=ρs/(4/PI()/εo*ʃʃdr*r*dα*ā(r^2+d^2)^3/2 [one integral for dr and another for α]
The component radial ȓ is cancel out, because of all direction of component radial ȓ around z
E=2*pi()*ρs/4/PI()/εo*ʃdr*r*ž/(r^2+d^2)^3/2 |ž|=d
E=ρs/2/εo*d*ʃdr*r/(r^2+d^2)^3/2
Let’s put x=r^2+d^2 then dx=2*r*dr or r*dr=dx/2 ; r=0 x=1 r=1 x=2
ʃdx/2/x^3/2=1/2/(-3/2+1)*x^(1-3/2)=-1/x^0.5|x=1 to x=2|
-1/sqrt(2)+1/sqrt(1)= 0.292893
E=ρs/2/εo*d*0.292893=3.313269 V/m
F=3.313269*30=99.39808 µN
 

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  • Electrostatics with charged circle.jpg
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