Help me out{Electric field intensity}

A semicircular ring of mass m is kept in x-y plane with centre at origin.It
is charged in such a way such that charge/unit length varies as λ=λocosθ .
Find accelration of ring when electric field E=Eoxi+Eoj is switched on.

17 Answers

1
sanchit ·

i dont agree wid aveek nd his method......especiaallly cant understand how u got total charge on the ring = 2 λ0 R

49
Subhomoy Bakshi ·

well i gave another question!!!

A semicircular ring of mass m is kept in x-y plane with centre at origin. It
is charged in such a way such that charge per unit length varies as λ=λocosθ (θ is angle measured w.r.t. positive x-axis!).
Find torque about the centre of ring when electric field E=Eoxi+Eoj is just switched on.

49
Subhomoy Bakshi ·

ADDITION:: Find torque about centre of half ring!!

49
Subhomoy Bakshi ·

Let me try this...

the applied field= Eoxi + Eoj

so constant field applies to y direction, and by symmetry will exert NO NET FORCE on the half ring of charge...

so the applied field is equivalent to Eo . x . i only!!

referring to Sanchit's diagram,

considering a small arc subtending angle d@ as in figure, and consider the charge distribution is uniform in this region...

length=Rd@

linear charge density=λ=λocos@

so charge on the small element=Rλocos@d@

x coordinate=R cos@

so, field=EoRcos@

so force on small element = EoλoR2cos2@d@

so net force=\int_{0}^{\pi }{E_o \lambda _oR^2cos^2\theta d\theta }=\frac{E_o \lambda _oR^2}{2} \int_{0}^{\pi }{(cos2\theta+1) d\theta} = \frac{E_o \lambda _oR^2}{2}.\left[0+\pi -0-0 \right]=\frac{\pi E_o \lambda _oR^2}{2}

thus, acceleration=\frac{\pi E_o \lambda _oR^2}{2m}

1
ajoy abcd ·

Answer given is πR2E0ε0/2M

1
Bicchuram Aveek ·

@ Sanchit : Yup this was a mistake :
following ur own figure :

the charge distribution : charge is located at the ends of the wire and not the upper part.....

as charge at the ends = λ0 and charge on top = 0

so by symmetry the centre of charge will lie on the lower end of the axis of the semicircular wire . as this is a point charge where the entire charge is concentrated look at the field vector :

in x direction acceleration is ax = qE/m

and in y direction acceleration is once again the same as

→ →
a = q E / m

1
sanchit ·

yaar shudnt it be like this:::::

dq = λ dl
dq = λ0 cosθ R dθ
Ï€
Q = λ0 R ∫cosθ dθ
0
Q=0

1
Bicchuram Aveek ·

dq = λ dl

dq = λ0 cosθ R dθ

Ï€/2
Q = 2 λ0 R ∫cosθ dθ (by symmetry)
0

so Q = 2 λ0 R

....i think this clears ur doubt.

1
sanchit ·

"in x direction field will be perpendicular to the charge .... hence accn. = 0" can u explain... how field ll be prependicular to the charge????????

1
Manmay kumar Mohanty ·

use a = qEm

i thnk that shuld help [1]

1
Bicchuram Aveek ·

not sure of the answer..i'm getting : ( 2√2 λ0 R E0 )/m

Now if ajoy is not speaking of a unform field...i can't say anything more than the thing that my entire process is wrong...........

@ Kaymant Sir : Isn't he trying to say that the field in the X-direction will vary with x ?

1
Bicchuram Aveek ·

also note that the total charge on the ring is constant above.

1
Bicchuram Aveek ·

i think he has made the mistake with this x thing in the expression.

else as far as i can say :

total charge on the ring = 2 λ0 R

where R is the radius

now there is a centre of charge where the entire charge is concentrated.........we can work out from this :

in x and y - both dir. apply a= qE/m

I think this can be the method...........(not so sure)

66
kaymant ·

Are you sure about the given field? It seem dimensionally inconsistent.

1
sanchit ·


soo ye rotate nahi karegi kyaa if m not pretendin it wrongly??????

1
Aishwarya Maurya ·

i will try and post the solution....it seems a bit tricky...

1
ajoy abcd ·

I know that,but I'm not getting how to apply that in the problem.
Pleae post at least 60 percent of soln.....

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