Want to use a battery?

How much volts of battery would I need to see a rise in height of water within a coaxial metal cylinder (when a battery is used to charge the inner and outer cylinders and then disconnected) which is equivalent to the rise in height of water in a coaxial glass cylinder?

If it can be approximated to x kV x \text{ kV} , where x x is a positive integer, then what is x x ?

Details and Assumptions:

  • Both cylinders have inner radius a = 3 cm a = 3 \text{ cm} , outer radius b = 6 cm b = 6 \text{ cm} and height h = 30 cm h = 30 \text{ cm} .

  • The metal plates are insulated.

  • Take surface tension of water as 0.072 Nm 1 0.072 \text{ Nm}^{-1} , relative permittivity of water to be 80 and permittivity of free space to be 8.85418782 × 1 0 12 m 3 kg 1 s 4 A 2 8.85418782 \times 10^{-12} \text{ m}^{-3}\text{ kg}^{-1} \text{ s}^4\text{ A}^2 , acceleration due to gravity as 10 m s 2 10 \ ms^{-2}

  • Angle of contact between the metal plates and water is assumed to be equal to 9 0 90^{\circ} and between the glass and water is 0 0^{\circ}

  • You should use a calculator to get the final answer.

3 5 Not possible 4 2 1

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1 solution

Kartik Sharma
Jan 22, 2017

Firstly, one must understand how can a battery lead to rise in height of water.

A coaxial metal cylinder will act as a cylindrical capacitance with a potential difference between the two cylinders. We have a formula for that -

C 0 = 2 π ϵ 0 h log ( b a ) C_0 = \frac{2\pi \epsilon_{0}h}{\log\left(\frac{b}{a}\right)}

(I am not deriving it here, if you want, just leave a comment below).

Now when I connect a battery of, let's say, V V volts, the energy stored in the capacitor in the form of electrostatic field is

U = ( C V ) 2 2 C U = \frac{{(CV)}^2}{2C}

If a dielectric of relative permittivity ϵ r \epsilon_r is put between the separation of the two cylinders upto a height x x , then the capacitance is given as a parallel combination of two capacitances -

C 1 = 2 π ϵ 0 ( h x ) log ( b a ) C_1 = \frac{2\pi \epsilon_{0}(h-x)}{\log\left(\frac{b}{a}\right)} , C 2 = 2 π ϵ r ϵ 0 x log ( b a ) C_2 = \frac{2\pi \epsilon_r \epsilon_{0}x}{\log\left(\frac{b}{a}\right)}

Therefore, C = 2 π ϵ 0 ( h + ( ϵ r 1 ) x ) log ( b a ) C = \frac{2\pi \epsilon_{0}(h + (\epsilon_r -1)x)}{\log\left(\frac{b}{a}\right)}

When this capacitor is brought close to the water surface(just touching it), the water surface feels an inward force which is due to the loss in potential energy of the capacitor if the dielectric(water) is brought into it. Neglecting edge effects, this force is equal to

F = d U d x = d d x ( Q 2 2 C ) F = - \frac{dU}{dx} = -\frac{d}{dx} \left(\frac{Q^2}{2C}\right)

since charge on the capacitor remains conserved.

= ( C 0 V ) 2 2 d d x ( log ( b a ) 2 π ϵ 0 ( h + ( ϵ r 1 ) x ) ) = -\frac{{(C_0 V)}^2}{2} \frac{d}{dx} \left(\frac{\log\left(\frac{b}{a}\right)}{2\pi \epsilon_{0}(h + (\epsilon_r -1)x)}\right)

= V 2 π ϵ 0 ( ϵ r 1 ) log ( b a ) ( 1 + ( ϵ r 1 ) x h ) 2 = \frac{V^2 \pi \epsilon_0(\epsilon_r -1)}{\log\left(\frac{b}{a}\right) \left(1 + (\epsilon_r -1)\frac{x}{h}\right)^2}

If this upward force has given rise to water upto height x x , then this must be balancing the risen water's weight.(obviously we have assumed that angle of contact of water-metal interface is equal to 9 0 90^{\circ} .

ρ π ( b 2 a 2 ) g x = V 2 π ϵ 0 ( ϵ r 1 ) log ( b a ) ( 1 + ( ϵ r 1 ) x h ) 2 \rho \pi(b^2-a^2)g x = \frac{V^2 \pi \epsilon_0(\epsilon_r -1)}{\log\left(\frac{b}{a}\right) \left(1 + (\epsilon_r -1)\frac{x}{h}\right)^2}

Now, this rise( x x ) should be equivalent to that of a glass coaxial cylinder.

For the same, force of surface tension acts perpendicularly upwards(angle of contact = 0 0^{\circ} for glass-water interface) along the inner and outer cylinder surfaces.

Using FBD of water, this must balance the weight of the risen water.

ρ π ( b 2 a 2 ) g x = 2 π ( b + a ) σ \rho \pi(b^2-a^2)g x = 2 \pi (b+a) \sigma

x = 2 σ ( b a ) ρ g x = \frac{2 \sigma}{(b-a)\rho g}

Using this in our previous formula for V V ,

V 2 = ( 1 + ( ϵ r 1 ) 2 σ ( b a ) ρ g h ) 2 2 ( b + a ) σ log ( b a ) ϵ 0 ( ϵ r 1 ) V^2 = \left(1 + (\epsilon_r -1)\frac{2\sigma}{(b-a)\rho g h}\right)^2 \frac{2(b+a)\sigma \log\left(\frac{b}{a}\right)}{\epsilon_0 (\epsilon_r -1)}

Now, just take a calculator and put in the values, we get V 3958.57 V V \approx 3958.57 V

For our purposes, that becomes 4 k V 4 kV .

And I thought batteries are always more powerful.

Kartik Sharma - 4 years, 4 months ago

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I have few concerns here, 1) Why will the water rise in the metal cylinder if we neglect the end effects. Isn't the field in the capacitor will be perpendicular to the weight?

2) The angle of contacts between metal and water is 90 and between glass and water is 0 should be mentioned in the details and assumptions part of the problem.

3) It should also be mentioned that the metal places are insulated. Otherwise, when they will be dipped in the water they will be short circuited and the charge will be neutralized.

Rohit Gupta - 4 years, 4 months ago

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  1. I shall quote B. Bukhovtsev(from where this problem was inspired, in which we had to find the force on the dielectric due to a charged capacitor). "It should be noted that when calculating energy we assumed the fields inside the capacitor was homogeneous and neglected the boundary effects, it is necessary to take into account the heterogeneity of the field near the edges to explain the action of force acting on the dielectric from the physical viewpoint " .
If energy arises from force, force can arise from potential energy as well. It was a learning for me as well.

2) Thanks, I have mentioned it.

3) Thanks, I have mentioned it.

Kartik Sharma - 4 years, 4 months ago

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@Kartik Sharma Don't you think the way the solution is done is flawed? Because at one side we are saying that the heterogeneity of the field near the edges is responsible for the force and on the other hand we are neglecting it.

Rohit Gupta - 4 years, 4 months ago

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@Rohit Gupta It is a standard solution to a standard problem, so it does have some kind of confidence behind it.

But personally, it feels a little fishy, yes. I have sufficed myself to believe that for the initial motion, the edge effects would be required but once the rise has begun, its effect becomes negligible and the work is done by an "induced kind of force(courtesy: loss in potential energy in forward motion)". Thus, the boundary effect becomes more of like an impulse (thus neglected).

Try this (an easier one).

Kartik Sharma - 4 years, 4 months ago

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@Kartik Sharma I don't really agree with you, we are talking about the water to stay in equilibrium. If the water rises and stays at a place then to balance the weight, the field has to bend. I also feel that something is fishy here.
@Josh Silverman @Steven Chase May I ask you guys to place your point of views on this problem? Thanks.

Rohit Gupta - 4 years, 4 months ago

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@Rohit Gupta Is there a causality dilemma here? How can there be a force which is due to some displacement of water into the capacitor, and yet that very force is assumed to be the cause of the water moving in to begin with? That's where I get stuck.

Steven Chase - 4 years, 4 months ago

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@Steven Chase But this idea is correct, no doubt about that. I have seen it in almost all top selling books in physics.

Kartik Sharma - 4 years, 4 months ago

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