Physics 114 Exam 1 Fall 2016

Transcription

1 Physics 114 Exam 1 Fall 2016 Name: For grading purposes (do not write here): Question Problem Answer each of the following questions and each of the problems. Points for each question and problem are indicated in red with the amount being spread equally among parts (a,b,c etc). Be sure to show all your work. Use the back of the pages if necessary.

2 Question 1. (10 points) A negatively charged rod is brought close and touches a small metal ball suspended by a thin thread. (a) Is there any movement of charge if so, how?. (b) What does the sphere do, if anything? Now, the rod is taken away from the sphere and then brought back towards the sphere, but does not touch it. (c) What do charges in the sphere do at this point? (d) What does the sphere do (move towards or away or not move), why? [Hint think about the bell experiment] Solution: (a) Negative charge transfers from the rod to the ball. The ball becomes negatively charged charged by conduction. (b) As the rod approaches, the ball moves towards it due to polarization [this not really relevant to the question at hand]. Once it touches and both are negatively charged the ball will be repelled. (c) The charges will polarize, even though there is a net charge on the sphere, more of these would be on the far side of the rod. (d) The ball will most likely be repelled. There is some chance, depending on the size of the sphere, that the charges in it will be polarized enough so there is a net positive charge on the nearer side to the rod that is attracted to the rod more strongly than the negative charges on the far side are repelled. In that case it would move towards the bar.

3 Question 2. (10 points) Consider point A located an arbitrary distance from two positive point charges in otherwise empty space as shown below. (a) Is it possible for an electric field to exist at point A in empty space? Explain. (b) Does charge exist at this point? Explain. (c) Does a force exist at this point? Explain. Solution: (a) Yes, it does in fact exist there. (b) No, only the two charges are present that are shown. (c) No, there is no charge there; there is no force

4 Question 3. (10 points) Consider a point charge q1 = 8.85 C inside of a cylindrical surface and another charge q2 = C outside the cylinder. (a) Can Gauss law be used to find the total flux on the surface of the cylinder? Why or why not? If one can, calculate it. (b) Can Gauss law be used to find the electric field on the surface of the cylinder? Why or why not? If one can, calculate it. (c) Say that q2 were taken away and the size of cylinder reduced so that its surface area was half of what it was originally? Would your answers to parts (a) and (b) change if so how? q1 q2 Solution: (a) Yes. The flux just depends on the enclosed charge, q1. 00EdA. q enc = q1/0 = C/ (C 2 /N-m 2 ) = N-m 2 /C (b) No, there is not enough symmetry. (c) No the answers would not change. The flux is the same (same number of net field lines penetrating surface). You would still not be able to use Gauss law to get the electric field. Your know the field at the surface would be greater, but you could not use Gauss law to calculate it.

5 Problem 1. (15 points) A + 10 C charge (q1) is placed at the origin and a 10 C charge (q2) is placed along the x-axis at = 1 m. Note that q2 is negative. (a) What is the electric field at the point x = 1m, y = 1m? (b) If a + 10 nc charge (q3) is placed at this point (x = 1m, y =1m), what is the force on this charge? Solution: (a) The electric field due to q1 points upward to the right 45 degrees above the x-axis (1,1) and that due to q2 points downward along y. q1 =10 C q2 = - 10 C E = E cos(45) i + (E sin(45) + E sin(270)) j (9 10 )(10 10 ) (9 10 )(10 10 ) (9 10 )(10 10 ) = 2 2 i ( 2 2 ( )j i ( j 9 10 j E = i j N/C (b) The force is just obtained by multiplying the charge and field: F = qe = ( i j) 4 4 = i j

6 Problem 2. (15 points) Find the net electric flux (a) through a square surface of area A = 2 m 2 that is parallel to the x-z plane that is in a constant magnetic field given by E = 10 i + 10 j N/C, (b) the closed spherical surface with R = 2 m in a uniform electric field of magnitude 10 N/C shown below, (c) the closed cylindrical surface shown below where the magnitude of the electric field is 10 N/C and points out of the surface on both sides as shown below. R is 2 m for the cylinder and let its length be 5 m. (d) What can you conclude about the charges, if any, inside the cylindrical surface? Solution: (a) We can solve this using Cartesian coordinates or in terms of the magnitudes of the vectors and the angles between them. The angle is 45 degrees. The magnitude of the electric field is N/C. EA. = EA cos(45) = (14.14)(2)(0.707)=20 Nm 2 /C EA. 2j ˆ(10i ˆ10j) ˆ = 20 Nm 2 /C (b) No net flux goes through this surface as all lines going in also come out. The net flux is zero. (c) No flux goes through the top and bottom. The sides have positive flux of equal magnitude through each as the electric field are parallel to the normal (theta is zero). Thus = 2EA = (2)(10)()(2 2 ) = 80 = 251 Nm 2 /C (d) Have more than one charge and the net charge is positive. qenc 0 [This question only asked what you can conclude about the charges, it was stated vaguely so you did not need to calculate the charge for full credit] = 2.22 nc

7 Problem 3. (15 points) A solid insulating sphere of radius cm has a charge of C uniformly distributed throughout its volume. A conducting spherical shell of inner radius cm and outer radius cm is concentric with the solid sphere and has no net charge. Find the electric field at (a) r = 5.00 cm, (b) r = cm, (c) r =25 cm, and (d) r = 50 cm from the center of this charge configuration. In addition (e), determine the total charge on the inner surface of the conducting shell (at r = cm) and on the outer surface of the shell (at r = cm). Solution: (a) This is inside the insulating sphere. Let the insulating sphere be that in yellow below and so draw a Gaussian surface with r = 5 cm (shown in red). 4 3 r 3 q in 0 E, 3 r 2 3 k Q ˆ 3 0AE n, Q 3 04r E, E Qr a 40 eq r, 3 4 a a a 3 E = ( )( )(0.05)/(0.1) 3 = N/C (b) Here the field is just that of the charge inside a Gaussian sphere of radius 15 cm which is the entire 10 C. We have EA = q/0. E4r 2 = q/0; E = keq/ r 2 = ( )( )/(0.15) 2 = N/C (c) This is inside the metallic shell. E = 0. (d) We just have a situation like part b. E = keq/ r 2 = ( )( )/(0.5) 2 = N/C (e) If I consider a spherical Gaussian surface of radius 25 cm, we have E = 0, so q inside must be zero. Thus the inner surface must have a charge of -10 C to balance that of the sphere. Since the inner shell has -10 C and the net charge is zero, the outer surface must have a charge of + 10 C.

8 Possibly Useful Information F q q r e = 1.6 X C E k e q 2 4 0r N m /C X ( C / Nm ) E F q EdA. r = xi + yj + zk q enc