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NCERT/CBSE PHYSICS CLASS 12 textbook http://www.TutorBreeze.com Answers to NCERT/CBSE PHYSICS Class 12(Class XII)textbook Exercise and Additional exercise CHAPTER TWO ELECTRIC POTENTIAL AND CAPACITANCE EXERCISES (For simplicity in numerical calculations, take g = 10 m s-2) 2.18 In a hydrogen atom, the electron and proton are bound at a distance of about 0.53 Å: (a) Estimate the potential energy of the system in eV, taking the zero of the potential energy at infinite separation of the electron from proton. (b) What is the minimum work required to free the electron, given that its kinetic energy in the orbit is half the magnitude of potential energy obtained in (a)? (c) What are the answers to (a) and (b) above if the zero of potential energy is taken at 1.06 Å separation?
2.18
Given q1== = e
r = 0.53A = 0.53 × 10−10 m
a2 = e
Key Idea U=K
q1q 2 r
1eV = 1.6 × 10 −19 J
;
Solution
U=
b)
K.E.
9 × 19 × (1.6 × 10 −19 ) 0.53 × 10 =
−10
2
{×}
1 eV = −27.2eV 1.6 × 10 −19
1 U = 13.6eV 2
Total energy = K.E. + P.E. = 13.6 + ( −27.2) = −13.6eV Energy required to free the electron is 13.6 eV. c)
Potential energy of the system Kq p q e Kq e Kq e U = q ( V1 − V2 ) = q p − = − = −13.6eV r 2r 2r
