1. consider this example, What happens if the potential is changed to ?100V?

EXAMPLE 25.5 | Rotating a molecule The water molecule is a permanent electric dipole with dipole mo- AU dipole = Uf - U; = -PE cos 90 - (-PE cos 0" ) ment 6.2 X 10 3 Cm. A water molecule is aligned in an electric field with field strength 1.0 X 10'N/C. How much energy is needed = PE = 6.2 X 10-23 J to rotate the molecule 90? MODEL The molecule is at the point of minimum energy. It won't This is the energy needed to rotate the molecule 90". spontaneously rotate 90". However, an external force that supplies ASSESS AUdipole is significantly less than kgT at room temperature. energy, such as a collision with another molecule, can cause the Thus collisions with other molecules can easily supply the energy water molecule to rotate. to rotate the water molecules and keep them from staying aligned SOLVE The molecule starts at ; = 0" and ends at of = 90. The with the electric field. increase in potential energy is