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 , where is a positive integer, then what is ?
Details and Assumptions:
Both cylinders have inner radius , outer radius and height .
The metal plates are insulated.
Take surface tension of water as , relative permittivity of water to be 80 and permittivity of free space to be , acceleration due to gravity as
Angle of contact between the metal plates and water is assumed to be equal to and between the glass and water is
You should use a calculator to get the final answer.
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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 = lo g ( a b ) 2 π ϵ 0 h
(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 volts, the energy stored in the capacitor in the form of electrostatic field is
U = 2 C ( C V ) 2
If a dielectric of relative permittivity ϵ r is put between the separation of the two cylinders upto a height x , then the capacitance is given as a parallel combination of two capacitances -
C 1 = lo g ( a b ) 2 π ϵ 0 ( h − x ) , C 2 = lo g ( a b ) 2 π ϵ r ϵ 0 x
Therefore, C = lo g ( a b ) 2 π ϵ 0 ( h + ( ϵ r − 1 ) x )
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 x d U = − d x d ( 2 C Q 2 )
since charge on the capacitor remains conserved.
= − 2 ( C 0 V ) 2 d x d ( 2 π ϵ 0 ( h + ( ϵ r − 1 ) x ) lo g ( a b ) )
= lo g ( a b ) ( 1 + ( ϵ r − 1 ) h x ) 2 V 2 π ϵ 0 ( ϵ r − 1 )
If this upward force has given rise to water upto height 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 ∘ .
ρ π ( b 2 − a 2 ) g x = lo g ( a b ) ( 1 + ( ϵ r − 1 ) h x ) 2 V 2 π ϵ 0 ( ϵ r − 1 )
Now, this rise( 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 ∘ 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
x = ( b − a ) ρ g 2 σ
Using this in our previous formula for V ,
V 2 = ( 1 + ( ϵ r − 1 ) ( b − a ) ρ g h 2 σ ) 2 ϵ 0 ( ϵ r − 1 ) 2 ( b + a ) σ lo g ( a b )
Now, just take a calculator and put in the values, we get V ≈ 3 9 5 8 . 5 7 V
For our purposes, that becomes 4 k V .