Inspired from Steven Sir!

Say we have a rectangular hyperbola, x 2 y 2 = a 2 x^2-y^2=a^2 of which the right hand part is charged with linear charge density σ -\sigma and the left one with + σ + \sigma .

There is a Rod Lying on the y-axis, with mass M M , Lenght 2 L 2L and its center is kept at ( 0 , 0 ) (0,0) . It is charged with, say surface charge density, + λ +\lambda .

Check out the image.

Find the magnitude acceleration of the rod, as soon as we release it.

If the acceleration of the rod is A A , Input 20 A × 4 π ϵ 0 20A \times 4 \pi \epsilon_{0} .

Details And Assumption

  • Take M = 1 k g M=1 kg , L = 2 m L=2m and a = 2 m a=2m , λ = 0.25 C / m \lambda =0.25 C/m and σ = 10 C / m \sigma= 10 C/m

  • Hyperbola is indefinite.

  • Find the acceleration in ( m / s 2 ) (m/s^2)

  • You can use Wolfram Alpha.

  • Orginial

  • Inspired from Steven Sir


The answer is 410.372632.

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1 solution

Karan Chatrath
Feb 7, 2020

Let the position vector of a point on the vertical wire be:

r p = y j ^ \vec{r}_p = y \hat{j}

And that on the upper half of the right branch of the hyperbola be:

r h = x i ^ + x 2 4 j ^ \vec{r}_h = x \hat{i} + \sqrt{x^2-4} \hat{j}

The vector joining these two points directed away from the wire is:

r = r h r p \vec{r} = \vec{r}_h-\vec{r}_p

We consider a line element on the wire. The length of the element near the given position vector has charge λ d y \lambda dy . We also consider an arc length element on the hyperbola. The charge on that element near the given position vector is σ d s \sigma ds where d s ds is the arc length magnitude which is:

d s = d x 1 + ( d y d x ) 2 ds = dx\sqrt{1 + \left(\frac{dy}{dx}\right)^2}

For the upper half of the right lobe:

d s = d x 2 x 2 4 x 2 4 ds = dx\sqrt{\frac{2x^2-4}{x^2-4}}

Applying Columb's law to find electrostatic force at wire element due to the hyperbolic element:

d F = K ( λ d y ) ( σ d s ) r r 3 d\vec{F} = \frac{K \left(\lambda dy\right)\left(\sigma ds\right)\vec{r}}{\lvert \vec{r} \rvert^3}

K = 1 4 π ϵ o K = \frac{1}{4\pi \epsilon_o}

d F = d F x i ^ + d F y j ^ d\vec{F} = dF_x\hat{i} + dF_y \hat{j}

Substituting all expressions and simplifying gives:

d F x = ( K λ σ 2 x 2 x 2 4 2 ( x 2 4 ) ( ( y x 2 4 ) 2 + x 2 ) 3 / 2 ) d x d y dF_x = \left(\frac{K \lambda \sigma\sqrt{2}\,x\,\sqrt{\frac{2\,x^2-4}{2\,\left(x^2-4\right)}}}{{\left({\left(y-\sqrt{x^2-4}\right)}^2+x^2\right)}^{3/2}}\right)dx \ dy

Similarly an expression for d F y dF_y can be computed. Now, the total force on the wire due to the upper half of the right lobe of the hyperbola is:

F x = 2 2 2 ( K λ σ 2 x 2 x 2 4 2 ( x 2 4 ) ( ( y x 2 4 ) 2 + x 2 ) 3 / 2 ) d x d y F_x = \int_{-2}^{2} \int_{2}^{\infty}\left(\frac{K \lambda \sigma\sqrt{2}\,x\,\sqrt{\frac{2\,x^2-4}{2\,\left(x^2-4\right)}}}{{\left({\left(y-\sqrt{x^2-4}\right)}^2+x^2\right)}^{3/2}}\right)dx \ dy

Therefore, by symmetry, the force due to the entire right lobe of the hyperbola will be:

F R = 2 F x F_R = 2F_x

Again, by symmetry, the force due to both lobes along the x-direction will be:

F T = 2 F R = 4 F x F_T = 2F_R = 4F_x

Therefore, the acceleration A A is:

A = F T M A = \frac{F_T}{M}

All forces along Y will evaluate to zero. One can check this by even evaluating the integrals. This step is left out here.

Having obtained F T F_T , the answer comes out to be:

20 A K 410.373 \boxed{20\frac{A}{K} \approx 410.373}

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