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Physics 2130 Discussion 8 worksheet NAME Learning outcomes: e Use and manipuiate vectors to represent physical quantities iike force (net force) e Describe the conceptual and mathematical models of non-touching forces: electric forces e Predict attractive and repulsive electric forces (magnitude and direction) e Predict the motion of interacting (colliding) objects using conservation of momentum, impulse, and Newton’s laws Characterize and make predictions for random motion and diffusion processes using the mean free path length, the diffusion constant, probability functions, and the rms distance 1. Charged objects can attract neutral matter through the polarization of the neutral matter — pushing the parts that have the same electric charge slightly further away. At the molecular level, neutral molecules with separated parts that are positive and ~~ negative can also attract one another by orienting properly. One example of this is hydrogen bonding of water molecules. This is the primary mechanism that creates surface tension in water, and a similar phenomenon plays a big role in a variety of biochemistry. The two hydrogensina water molecule are positive, each with a charge +e, and the oxygen is negative, with a charge —2e (e = d 2d E 1.602 x 10-19 C). Electric forces and the quantum sharing of A electrons hold the whole molecule together. Here we will explore how 8 D the electric forces between water molecules can be attractive when F properly arranged. The angle between hydrogen atoms in a water c molecule is actually 104°, but for simplicity we’lltreat them as if they were a tight angie, 90°. A skeich of how ihe waier molecules are aligned in a water-waier hydrogen bond is shown here. Ina single water molecule, the distance between the oxygen atom and a hydrogen atom (the distance between A and B iabeled d) is about 96 pm (1 pm = 1 x 1072? m). The distance between the water molecules (the distance between Band D labeled 2d) is about twice as far: 192 pm. (Note: k = 8.99 x 10? N-m2/C?) To have a stable bond between the water molecules, the net force on the DEF molecule due to the ABC molecule must be zero; this means the forces on the DEF molecule must be balanced. We will start by considering the “backbone” of the hydrogen bond composed of charges A, B, and D. a. What is the electric force between A and D? What direction is this force on D? A Chay Qe 2 \ . n chovye Qn 2 2 o . fl mere went ) (= ZW wor xwtt) (~2+ [40210 7) _ 1 Wytor? (PU WO + AVI) ype nent b. What is the electric force between B and D? What direction is this force on D? A ) -14 19 ) i, c ~ (02 KL0T A) (-2 1 G02 - f.2sxlOr FE gion = 8.47 x(07 NiO" c j(y-t2\2 (192 x10 un) 
