Typical defibrillators have a capacitance of about 70 mu F and a potential diffe

Question

Typical defibrillators have a capacitance of about 70 mu F and a potential difference between the plates of a capacitor of 5.0 kV a) How much charge is stored between the plates of the capacitor? b) When the defibrillator is discharged within 2 ms - what is the current running through the tissue? c) The two contacts are placed 20 cm apart and the field lines are running through tissue with a relative permittivity of 50. What is the electric field between the two plates? d) Assume a flat piece of 2 times 2 mu m^2 of a cellular membrane with a relative permittivity of 9. If the membrane has a thickness of 5 nm - what is the capacitance of the membrane?

Explanation / Answer

C = 70 uF and V = 5 kV

a)We know that,

Q = CV

Q = 70 x 10^-6 x 5.5 x 10^3 = 0.385 C

Hence, Q = 0.385 C

b)Current by defination is:

I = Q/t

I = 0.385/2 x 10^-3 = 192.5 A

Hence, I = 192.5 A

c)k = 50

Ei = 5 x 10^3/20 x 10^-2 = 2.5 x 10^4 V/m

E(f) = Ei/k = 2.5 x 10^4/50 = 500 V/m

Hence, E(f) = 500 V/m

d)C = k epsilon0 A/d

A = 4 micrometer^2 = 4 x 10^-12 m^2 ; k = 9 ; d = 5 nm = 5 x 10^-9 m

C = 9 x 8.85 x 10^-12 x 4 x 10^-12/5 x 10^-9 = 637.2 x 10^-12 F

Hence, C = 637.2 pF

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