# JEE Class main Answered

**please tell me how can i finde the charge which has been cutt off**

Figure shows a wire in the form of an circular arc AB obtained from circle of radius r.

The arc is obtained by cutting angular portion θ at center as shown in figure.

Let charge Q is distributed uniformly over the circumference of circular wire.

Let λ be the linear charge density , i.e. charge per unit length .

λ = Q / ( 2 π r ) .......................(1)

( It is to be noted here , charge distributed uniformly to circular wire and then the

angular portion is cut . Hence above expression for linear density is valid )

Let us consider a small charge element dq in the arc at angular position φ .

Let dφ be the angle subteneded by this small charge element .

Charge content of this small charge element is

dq = λ r dφ

electric field dE due to this small charge element is

dE = K × ( dq / r^{2} )

where K = 1/(4π ε_{o} ) is Coulomb's constant

This electric field dE is resolved along x-axis as

dE_{x} = K × ( λ / r ) cosφ dφ .........................(2)

If we consider similar charge element dq over the full length of arc, sum of resolved components dE_{y}

of electric field dE over the full length of arc is zero due to symmetry.

Hence the net electric field due to full length of arc is determined by integrating the eqn.(2)

E = K ( ( λ / r ) 2 sin(φ/2)

By substituting λ from eqn.(1) and substituting K = 1/(4π ε_{o} ) , we get electric field from above expression as