Fall20107CDLM11-activities - Physics 7C DLM11 Overview DLM...

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Unformatted text preview: Physics 7C DLM11 Overview DLM 11 Model: Fields and Forces Act 9.1.5 Electrical Forces and Fields (FNTs from DLM10) (50 min) Learning Goals: ° Solidify understanding of connections between charges, forces, and fields. ° Examine analogy with gravitational fields noting similarities and differences 0 Understand how an electric field affects a non-polar molecule Act 9.1.6 Forces on Polar Molecules (60 min) Learning Goals: ° Understand forces of attraction between polar molecules (thus, understand some of the basis for hydrogen bonds) ° Understand motion of an electric dipole in a uniform electric field Model: Fields, Forces, Potential & Potential Energy Act 9.2.1 Electric Forces and Bond Energies (30 min) Learning Goals: ° Startng making connections back to the energy language of 7A. ° Be able to determine forces from potential energy graphs and construct potential energy graphs from forces. ° Understand how two uncharged non-polar molecules or atoms can still have an attractive electrical interaction between them. Physics 7C Activity 9.1.5 DLM 11 Electrical Forces and Fields A) One set of charges produces the field, the other set feels the field 1) 2) 3) Put your group’s answers to FNTl on the board. A Suppose the +Q charges at A and B were left there but you could put any charge you chose at point C. Is there any charge you could put at point C so that the electric field in the center of this triangle is 0? If so, what + -Q charge? If not, why not? BQ. . C Suppose the +Q and -Q charges at A and C (respectively) were left but you could put any charge you chose at point B. Is there any charge you could put at point B so that the electric field in the center of this triangle is 0? If so, what charge? If not, why not? Whole Class Discussion B) The electric field inside a parallel plate capacitor Each group talk about FNT 2 and then draw the following three pictures on the board: 1) the negatively charged plate alone with several representative field vectors 2) the positively charged plate alone with several representative field vectors 3) the two plates neXt to each other with the field vectors for the total electric field 4) redo 3), but changing to the field-line representation Whole Class Discussion C) Analogy between electric fields and gravitational fields 1) Put your group’s answers to FNT 3 on the board. You need to show ONLY the equation with numerical values substituted in and the final answer, but not the intermediate steps. 2) This analogy between electric fields and gravitational fields is fairly good (in each there is a field, the field produces a force, the field varies as l/r2 as you get farther from the source) but there is one major difference. What is the main difference between these two models? Hint: the “things” that make the field are more complicated in the electric case. Whole Class Discussion Physics 7C Activity 9.1.6 DLM 11 A) A polar molecule in a uniform electric field The picture to the right shows a polar molecule (water) to the left of an infinite conducting plate that is positively charged.. 1) 2) Class Progress Check 3) 4) 5) Forces on Polar Molecules Draw the plate on the board and sketch the electric field lines to the left of the plate. qH : 0-333 Will the molecule, oriented as shown, be attracted to the plate, repelled by the plate, or will something else happen? If the molecule moves as you predict in (2), decide if the electric potential energy of each atom increases, decreases, or stays the same as it moves. + + + + + + + + + + + + + + For an electric dipole (such as the polar molecule shown) we can define an electric dipole moment, , which is a vector pointing from the negative side of the molecule toward the positive side of the molecule. Draw the molecule to the left of the plate and sketch a reasonable electric dipole moment vector, 13 . By the way, we will not be calculating any electric dipole moment vectors so you just need to know the direction it will point for a specific set of charges. In working with electric dipoles, we often use energy rather than forces. The potential energy of an electric dipole in an electric field is PEdipole= -pEcosB where 9 is the angle between the )3 vector and the E vector. Explain how this equation describes the lowest energy configuration and the highest energy configuration of this molecule in this field. Is your conclusion consistent with the changes in energy you found in (3)? Whole Class Discussion B) Forces between electrically neutral but polar molecules 1) 2) 3) 4) 5) 6) On the board, draw a small water molecule (~5 cm on the board) with its dipole moment pointing down. Indicate the dipole moment with a vector beside the molecule and label it. We will call this water molecule “A”. Considering water molecule A as a single dipole, draw a reasonable set of electric field lines for A out to distances of at least 5 or 6 O-H bond lengths. Ignore the field lines inside molecule A. Draw four small water molecules uniformly in the region where water molecule A's electric field has been shown. Orient them so that they would have a minimum potential energy. Hint: use the formula for their PEdipole and determine the angle, 9, between I5 and E . Consider one of the four new water molecules on the board. Call this water molecule B. If B were moved closer to A, would it feel a larger, smaller or same magnitude electric field from A? How can you tell? Assuming that B's dipole moment remained approximately constant as it approached A, and B was always oriented to minimize PEdipole would B's PEdipole increase, decrease or stay the same? Is B attracted to or repelled from A? (Hint: Forces point in the direction PE decreases the most.) Whole Class Discussion Ph sics 7C Activit 9.2.1 DLM 11 Deeper Look at the Particle-Particle Potential and Molecular Bonds The electric origins of one form of the pair- qH : 0-338 wise potential: the hydrogen bond You havejust discussed the electrical forces that help q : -0563 _. form the hydrogen bond between the two water 0 q : molecules shown to the right. You found that there is an 0H33e \ attractive force between these two molecules. ' 1) Now consider how this attractive force depends on distance. Does molecule A’s electric field change more rapidly near A or far from A? Where does PEdipole change more rapidly? 2) Next we are going to look more closely at the particle-particle or pair-wise potential you became familiar with in Physics 7A. The graph below shows this potential with a solid line, and, in addition, shows the attractive and repulsive components, that when added together, make up the potential. Our job is to make sense of these two components. '3 04 a) Copy the graphs on the board. 0 m b) You will now need to use two important Emery" ideas from Physics 7A, which you should 0 02 be quite familiar with: 00‘ i) the potential energy is defined to be 0 when the molecules are very far apart (when r is large) and ii) the force between the molecules »0 0‘ depends on the slope of the potential energy curve (that is, F = -dPE/dr). .0 oz Decide which of the three curves (solid, short dash, or long dash) corresponds to the potential energy vs r graph for the interaction you discussed in 1). Explain how that PE curve leads to an attractive force. -0 03‘ AU 0-!- c) When the molecules get too close together, a new interaction comes into play. Each water molecule is made up of 3 nuclei surrounded by electrons in the orbitals that you have learned about in a chemistry class. Suppose these water molecules get so close together that the orbitals from different molecules start to overlap each other so that the electrons in the different molecule’s orbitals start occupying the same space. Explain which curve corresponds to orbital overlap; i.e., which curve leads to a repulsive force and how you determined this. d) For the following values of r: 3.5A, 3.75 A, 4 A, and 4.5 A, explicitly show how the solid curve, the total potential energy acting between the two molecules, i.e., the pair-wise potential, is the sum of the attractive and repulsive palts. (Note: 1 A=10‘1°m. The unit is called “angstrom” and often used to measure at the atomic level). e) On the graph, indicate how far apart the two molecules are when their pair-potential energy is minimized. What is the force there? Whole Class Discussion Physics 7C FNT’S DLM 11 1. C02 (shown below) is a linear molecule and is not polar (i.e. there is no clear negatively charged side or positively charged side). Suppose this molecule is placed in a uniform electric field pointing up the page as shown. a) (solidification) What direction is the C pushed by this electric field? What direction is an O pushed by this electric field? b) (application) Model the chemical bonds as springs that bend and describe how this electric field changes the molecule. Does the molecule become polar? 2. Our electric model includes three main ideas, at this point: electric force, electric field, and electric potential energy. Summarize the relationship between these main ideas, for the case of a single charge in an electric field field (do NOT assume the electric field is from a point charge. a. (Solidification) For each arrow below, explain how one would get from one quantity (like E) to the next quantity (like F). b. (Solidification) Fill in the units for electric field and potential energy. C. (Introduction) One quantity important to electrical interactions has not yet been discussed. This box would go in the lower-left corner. Assuming the relationships for going from left to right are the same for each row, what equation would connect the new quantity (V) to PE? Assuming the relationships for going from bottom to top are the same in each column, what equation would connect the new quantity (V) to the electric field? Electric Field E Potential Energy PE 3. (Review/Introduction) In Physics 7B, you studied a particular type of electrical phenomena: current moving in a circuit. At the time, we studied it in terms of fluid flow—now we want to begin understanding it in terms of the Electric Field and Force Model. a. In the circuit shown to the right, which way does the current flow? 1 E dimly b. Around the circuit, voltage increases through some elements and decreases through others. Start at the positive terminal of the battery and travel with the current. Decide if voltage increases or decreases across each element. c. Repeat b, explaining if electrical potential energy increases or decreases. ...
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