Exercise 21.56 The ammonia molecule (NH3) has a dipole moment of 5.0 x 10 30 C m

Question

Exercise 21.56 The ammonia molecule (NH3) has a dipole moment of 5.0 x 10 30 C m. Ammonia molecules in the gas phase are placed in a uniform electric field E with magnitude 1.6x105 N/C. What is the change in electric potential energy when the dipole moment of a molecule changes its orientation with respect to Efrom parallel to perpendicular? Express your answer using two significant figures AE AU Submit My Answers Give Up incorrect; Try Again; 2 attempts remaining Part B At what absolute temperature T is the average translational kinetic energy GHT of a molecule equal to the change in potential energy calculated in part (a)? (Note: Above this temperature, thermal agitation prevents the dipoles from aligning with the electric field.) Express your answer using two significant figures

Explanation / Answer

Part A:

Here, the change in electricpotential energy when the dipole moment of a molecule changes itsorientation with respect to E from parallel to perpendicular -

              U = pEcos0o - pEcos90o

                    = pE - 0

                    = pE

                    = 5.0 x 10^-30 x1.6 x 10^6 = 8.0 x 10^-24 J

Part B:

Again write the formula -

             U = (3/2)kT

               T = (2 / 3)(U / k)

                  = (2 / 3)(8.0 x 10^-24J / 1.38x 10^10-23J/K)

                  = 0.386 K

So, answer in two significant figures, T = 3.9 x 10^-1 K

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