Quiz 3 Info. Average 6.40 St. Dev Quiz 3 Scores. 2/25/2014 Physics 132 1

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1 Quiz 3 Info Quiz 3 Scores Average 6.40 St. Dev /25/2014 Physics 132 1

2 1. (3 pts) A particular reaction has a negative enthalpy change AND a negative entropy change. What can you say about whether it is takes place spontaneously? A. It will never take place spontaneously. B. It will always take place spontaneously, at any temperature. C. It will only take place spontaneously at a high enough temperature. D. It will only take place spontaneously at a low enough temperature. E. You cannot tell from the information given. 2/25/2014 Physics 132 2

3 2. (3 pts) Suppose a small amount of heat Q flows from a system A at high temperature (350K) to a system B at low temperature (250K). If Q = 0.5 J, m A = 1.2 kg, and m B = 0.6 kg, what will the total entropy change of the system be as a result? SS = QQ TT = QQ 1 TT CC 1 TT HH = 0.5J 1 250K 1 350K = J K 2/25/2014 Physics 132 3

4 3. (4 pts) A gas of diatomic molecules is at STP (standard temperature and pressure). The two atoms in each molecule interact with each other with the potential shown at the right. Because of the rules of quantum physics, the molecule can t have any arbitrary binding energies, but has to have only the particular energies shown as E 0 to E 4. Most of the time the molecule is in it s ground state (lowest allowed state) E 0. It can be excited to a vibrational state at energy E 1 if the right amount of energy is added. Energy can be added to the molecule as it undergoes a collision with other molecules. If E 1 E 0 = 15 mev = ev, then at equilibrium, what will be the probability to find a molecule in the state E 1 compared to in the state E 0? (Note: The following numbers might be of use: At STP k B T = 1/40 ev/molecule; RT = 2.4 kj/mole) A. Very, very small B. About 0.5 C. About 1.0 D. You can t tell without more information Which foothold idea(s) did you use? Boltzmann Factor ee = /25/2014 Physics 132 4

5 Exam 1 on Thursday! For those who took PHYS131 similar format Multiple choice, problems, essay Link to sample exam on schedule 2/spring2014/ScheduleB.html 2/25/2014 Physics 132 5

6 Your Questions What is the difference between the force on a test charge and electric field calculations? If we are treating the electric force as acting in space, thus creating an electric field, why do we not look at gravity the same way, acting in space? Actually, we do! Every time a person refers to the acceleration due to gravity g, they are also referring to the gravitational field as well. The units just work out easier with gravity since both gravitation and acceleration deal with mass Why for Coulomb's law do we have to consider both the source and the test charge, but this is not the case for field? That s the point of the E-field to eliminate the need of looking at how a source charge exerts force on a test charge, but rather how the presence of the source charge alters the space around it I don t get the Cheshire Cat analogy I was a little confused by the ending of the writing where they astericked the point that there are correction to the concept such as coupling the fields to the velocity of the object? Nature is a little more complicated once charges move (magnetic effect)

7 E is defined everywhere in space not just in places where charges are present Foothold idea: Electric Forces and Fields When we focus our attention on the electric force on a particular object with charge q 0 (a test charge ) we see the force it feels depends on q 0. Define quantity that does not depend on charge of test object test charge -> Electric Field E

8 Foothold ideas: Fields A field is a concept we use to describe anything that exists at all points in space, even if no object is present. A field can have a different in magnitude at different points in space. (and if it s a vector field, direction). Examples: temperature, wind speed, wind direction A gravitational, electric, or magnetic field is a force field. Fields allow us to predicts the force that a test object would experience. The field does not depend on what test object is used. gg rr = FF acting on mm ( rr) mm EE rr = FF acting on qq ( rr) qq Field is the value at a position in space r assuming that the force is measured by placing the object at r. 10/17/12 Physics 131 8

9 See the system below. A B and C are positive charges, q is a negative charge. How many interactions do we need to add to compute the force exerted on a test charge? Test Charge 9

10 See the system below. A B and C are positive charges, q is a negative charge. How many terms do we need to add to compute the electric field? 10

11 The electric field at a particular point in space A. Depends only on the magnitude of the test charge used to measure it. B. Depends only on the sign of the test charge used to measure it. C. Depends on both the sign and magnitude of the test charge used to measure it. D. Does not depend on the test charge used to measure it. E. None of the above Depends only on the sign of... Depends only on the magni... 20% 20% 20% 20% 20% Depends on both the sign a.. Does not depend on the test... None of the above 1/23/13 Physics

12 POTENTIAL ENERGY 2/25/2014 Physics

13 Foothold ideas: Energies between charge clusters Atoms and molecules are made up of charges. The potential energy between two charges is The potential energy between many charges is 2/15/13 Physics

14 A positive charge might be placed at one of three spots in a region. It feels the same force (pointing to the left) in each of the spots. How does the electric potential energy, U elec, on the charge at positions 1, 2, and 3 compare? A. U is greatest at 1 B. U is greatest at 2 C. U is greatest at 3 D. U = 0 at all three spots E. U 0 but same at all three spots F. Not enough information 1/23/13 Physics

15 Does the potential energy of the system change when I add a test charge? Test Charge 2/25/2014 Physics