Answer
Verified
425.7k+ views
Hint: To solve this question, we simply have to find the electric field outside the disc. We just have to use the formulae of electric field at $x$ from the centre for a small surface and then integrate it to get the answer.
Complete step by step answer:
Let’s consider the disc of the radius $r$. It is given that $\sigma $ is the uniform charge density. Let $E$ be the electric field at the required point on the axis of the disc at a distance from its centre.
We have to assume the charge distribution as a collection of concentric rings of charge. Let’s consider one such ring of radius $r$ and charge $dq$.
Let a small element of area = $dA = \left( {2\pi r} \right)dr$
And the charge distribution of the ring = $dq = \sigma dA = 2\pi r\sigma dA$
Because of symmetry, there is no vertical component of the electric field at point P. So, there is only a horizontal component. We know that for a ring:
$ \Rightarrow dE = \dfrac{{k\left( {dq} \right)x}}{{{{\left( {{x^2} + {r^2}} \right)}^{\dfrac{3}{2}}}}}$
Here $x$ is the distance from the surface to the point P, $E$ is the electric field, $q$ is the charge, $r$ is the radius of the earth.
Putting the value of $dq$, we get
$ \Rightarrow dE = \dfrac{{k\left( {2\pi r\sigma dA} \right)x}}{{{{\left( {{x^2} + {r^2}} \right)}^{\dfrac{3}{2}}}}}$
Integrating both sides,
$ \Rightarrow \int\limits_0^E {dE} = \int\limits_0^a {\dfrac{{k\left( {2\pi r\sigma dr} \right)x}}{{{{\left( {{x^2} + {r^2}} \right)}^{\dfrac{3}{2}}}}}} $
Let this be 1
Putting and differentiating we get,
$ \Rightarrow {x^2} + {r^2} = {t^2}$
$ \Rightarrow 2rdr = 2tdt$
At $r = 0$, $t = x$
At $r = a$, $t = \sqrt {{a^2} + {x^2}} $
Using this in 1, we get,
$ \Rightarrow \int\limits_0^E {dE} = \dfrac{\sigma }{{2{\varepsilon _0}}}\int\limits_x^{\sqrt {{a^2} + {x^2}} } {\dfrac{{tdt}}{{{t^3}}}} $
$ \Rightarrow E = \dfrac{\sigma }{{2{\varepsilon _0}}}\left[ { - \dfrac{1}{t}} \right]_x^{\sqrt {{a^2} + {x^2}} }$
$ \Rightarrow E = \dfrac{\sigma }{{2{\varepsilon _0}}}\left[ {\dfrac{1}{x} - \dfrac{1}{{\sqrt {{a^2} + {x^2}} }}} \right]$
$ \Rightarrow E = \dfrac{\sigma }{{2{\varepsilon _0}}}\left[ {\dfrac{1}{x} - \dfrac{1}{{\sqrt {{a^2} + {x^2}} }}} \right]$
$ \Rightarrow E = \dfrac{\sigma }{{2{\varepsilon _0}}}\left[ {1 - \dfrac{1}{{\sqrt {\dfrac{{{a^2}}}{{{x^2}}} + } 1}}} \right]$
For a very small $\dfrac{{{a^2}}}{{{x^2}}} \approx 1$
$ \therefore E = \dfrac{\sigma }{{2{\varepsilon _0}}}$
So option (A) is the correct option.
Note: Thus from the above derivation we can say that the electric field at a point due to a charged circular disc is independent from the distance of the point from the center. It depends on the surface charge density of the disc. Just like here we assumed the disc to be made up of many infinitesimally thin discs, we can use the same iea to calculate the electric field at a point due to a charged hollow cylinder.
Complete step by step answer:
Let’s consider the disc of the radius $r$. It is given that $\sigma $ is the uniform charge density. Let $E$ be the electric field at the required point on the axis of the disc at a distance from its centre.
We have to assume the charge distribution as a collection of concentric rings of charge. Let’s consider one such ring of radius $r$ and charge $dq$.
Let a small element of area = $dA = \left( {2\pi r} \right)dr$
And the charge distribution of the ring = $dq = \sigma dA = 2\pi r\sigma dA$
Because of symmetry, there is no vertical component of the electric field at point P. So, there is only a horizontal component. We know that for a ring:
$ \Rightarrow dE = \dfrac{{k\left( {dq} \right)x}}{{{{\left( {{x^2} + {r^2}} \right)}^{\dfrac{3}{2}}}}}$
Here $x$ is the distance from the surface to the point P, $E$ is the electric field, $q$ is the charge, $r$ is the radius of the earth.
Putting the value of $dq$, we get
$ \Rightarrow dE = \dfrac{{k\left( {2\pi r\sigma dA} \right)x}}{{{{\left( {{x^2} + {r^2}} \right)}^{\dfrac{3}{2}}}}}$
Integrating both sides,
$ \Rightarrow \int\limits_0^E {dE} = \int\limits_0^a {\dfrac{{k\left( {2\pi r\sigma dr} \right)x}}{{{{\left( {{x^2} + {r^2}} \right)}^{\dfrac{3}{2}}}}}} $
Let this be 1
Putting and differentiating we get,
$ \Rightarrow {x^2} + {r^2} = {t^2}$
$ \Rightarrow 2rdr = 2tdt$
At $r = 0$, $t = x$
At $r = a$, $t = \sqrt {{a^2} + {x^2}} $
Using this in 1, we get,
$ \Rightarrow \int\limits_0^E {dE} = \dfrac{\sigma }{{2{\varepsilon _0}}}\int\limits_x^{\sqrt {{a^2} + {x^2}} } {\dfrac{{tdt}}{{{t^3}}}} $
$ \Rightarrow E = \dfrac{\sigma }{{2{\varepsilon _0}}}\left[ { - \dfrac{1}{t}} \right]_x^{\sqrt {{a^2} + {x^2}} }$
$ \Rightarrow E = \dfrac{\sigma }{{2{\varepsilon _0}}}\left[ {\dfrac{1}{x} - \dfrac{1}{{\sqrt {{a^2} + {x^2}} }}} \right]$
$ \Rightarrow E = \dfrac{\sigma }{{2{\varepsilon _0}}}\left[ {\dfrac{1}{x} - \dfrac{1}{{\sqrt {{a^2} + {x^2}} }}} \right]$
$ \Rightarrow E = \dfrac{\sigma }{{2{\varepsilon _0}}}\left[ {1 - \dfrac{1}{{\sqrt {\dfrac{{{a^2}}}{{{x^2}}} + } 1}}} \right]$
For a very small $\dfrac{{{a^2}}}{{{x^2}}} \approx 1$
$ \therefore E = \dfrac{\sigma }{{2{\varepsilon _0}}}$
So option (A) is the correct option.
Note: Thus from the above derivation we can say that the electric field at a point due to a charged circular disc is independent from the distance of the point from the center. It depends on the surface charge density of the disc. Just like here we assumed the disc to be made up of many infinitesimally thin discs, we can use the same iea to calculate the electric field at a point due to a charged hollow cylinder.
Recently Updated Pages
How many sigma and pi bonds are present in HCequiv class 11 chemistry CBSE
Mark and label the given geoinformation on the outline class 11 social science CBSE
When people say No pun intended what does that mea class 8 english CBSE
Name the states which share their boundary with Indias class 9 social science CBSE
Give an account of the Northern Plains of India class 9 social science CBSE
Change the following sentences into negative and interrogative class 10 english CBSE
Trending doubts
Fill the blanks with the suitable prepositions 1 The class 9 english CBSE
Which are the Top 10 Largest Countries of the World?
Difference Between Plant Cell and Animal Cell
Difference between Prokaryotic cell and Eukaryotic class 11 biology CBSE
Differentiate between homogeneous and heterogeneous class 12 chemistry CBSE
Give 10 examples for herbs , shrubs , climbers , creepers
How do you graph the function fx 4x class 9 maths CBSE
The Equation xxx + 2 is Satisfied when x is Equal to Class 10 Maths
Change the following sentences into negative and interrogative class 10 english CBSE