Physics 8.0 Exam Two Equation Sheet Spring 004 closed surface EdA Q inside da points from inside o to outside I dsrˆ db 4o r rˆ points from source to observer V moving from a to b E ds 0 V b V a b E ds a B ds oi contour through where I through is the current flowing through the open surface bounded by the contour V many pointcharges N i 1 4 r r o q i i B center of circle of radius a oi a C Q V Q V C I through J da open surface ds right-handed wrt da U = 1 C V = Q C I qna wire v drift I J A wire E J qnv drift where is the resistivity J E where 1/ the conductivity V = i R L R A L A 1 R parallel = 1 R 1 + 1 R R series = R 1 + R V Pohmic heating iv i R (joules/sec) R q o vrˆ B 4 r 0 U EE dv all space vol F q v B ext df I ds B ext Cross-products of unit vectors: ˆi ˆi ˆjˆjkˆ k ˆ 0 ˆi ˆj k ˆ ˆi kˆ ˆj ˆj kˆ ˆi Useful integrals: d 1/ ( c f ) ( c f ) f d 1 ln( c f ) ( c f ) f d 1 1 c f f c f ( ) ( ) 1/ Useful Small Argument Approximations (1 ) n 1 n for <<1 ln(1 ) for <<1
Problem 1 (5 Points) Circle your choice for the correct answer to the eight questions below. A: A current I is uniformly distributed over the cross-section of a conducting wire of radius a. For distances r from the center of the wire with r > a, 1. The magnitude of the magnetic field does not depend on r.. The magnitude of the magnetic field is proportional to 1/r 3. The magnitude of the magnetic field is proportional to r 4. The magnitude of the magnetic field is proportional to 1/r 5. The magnitude of the magnetic field is proportional to r B: A segment of a wire of length dl carries a current I (see sketch below). The magnitude of the magnetic field due to this wire segment at the location P shown in the sketch is 1.. 3. 4. 5. oi a dl 4 b oi a 3 / a dl 4 a b oi b dl 4 b oi a 3 / b dl 4 a b oi dl 4 a b Page
A: Eight parallel wires cut the page perpendicularly at the points shown. All the wires carry the same magnitude of current I 0, but some carry current into the page and some carry current out of the page. For wires 1 to 4, the current flows up out of the page; for the rest, the current flows down into the page. If we valuate B d s along the closed path (see figure) in the direction indicated by the arrowhead, we get (a) 4 o I o (b) 3 o I o (c) o I o (d) oio (e) 0 (f) oio (g) o I o (h) 3 o I o (i) 4 o I o F: A wire loop carries current I 1, and is located near an infinite wire carrying current I. The currents flow in the directions shown. The net force on the wire loop due to the presence of the infinite wire is (a) upwards (b) downwards (c) 0 (d) to the left (e) to the right G: The coil below has current flowing clockwise as seen from above. It sits in an external magnetic field shown by the field lines below. The coil will feel a magnetic force that is Page 3
(a) Upwards (b) Downwards (c) Zero Page 4
Problem (5 points): A Bainbridge mass spectrometer is shown in the figure. A charged particle with mass m, charge q and speed V enters from the bottom of the figure and traces out the trajectory shown in the fields shown. The only electric field is in the region where the trajectory of the charge is a straight line. (a) Is the sign of the charge positive or negative? To get credit for your answer you must give a logical reason to justify it. (b) When the particle is moving through the first (straight-line) segment of its trajectory, what is its speed V, in terms of E and B? Justify your answer. Page 5
(c) When the charge is moving through the second (curved) segment of its trajectory, derive a relation between the radius r of the arc through which it moves, its charge q, its speed V, and B o. To get credit for your answer, you must show the details of your derivation. (d) The charge hits the left wall of the spectrometer at a vertical distance h above where it entered the upper region and a horizontal distance L to the left of where it entered the upper region (see sketch). Given this information and your result from above, what is the speed of the particle in terms of h, L, q, m, and B o? Page 6
Problem 3 (5 points): A wire segment is bent into the shape of an Archimedes spiral (see sketch). The equation that describes the curve in the range 0 (see sketch) is b r( ) a, for 0 where is the angle from the x-axis in radians ( radians is 180 degrees). The point P is located at the origin of our xy coordinate system. The vectors e ˆ r and e ˆ are the unit vectors in the radial and azimuthal directions, respectively, as shown. The wire segment carries current I, flowing in the sense indicated. (a) A infinitesimal line segment ds of the spiral is located at a distance r from the origin and at an angle from the x-axis. What is the magnetic field db at the point P due to this current carrying segment of the wire? You may use the expression ds dr eˆ r d eˆ. Give both the magnitude and direction. r Page 7
(b) What is the total magnetic field B at point P due to the entire Archimedes spiral segment between 0? You will find useful one of the integrals on the formula sheet. (c) Suppose the distance b goes to zero. What does your expression above become in this limit? Is this what you expect? Justify your answer. You will find useful one of the small argument approximations given on the formula sheet. Page 8
Problem 4 (5 points): A coaxial cable consists of a solid inner conductor of radius a, surrounded by a concentric cylindrical tube of inner radius b and outer radius c. The conductors carry equal and opposite currents I 0 distributed uniformly across their crosssections. Determine the magnetic field at a distance r from the axis for the following ranges of radii. On the figure below, draw the amperean loop you use in each case. (a) r < a (b) a < r < b Page 9
(c) b < r < c (d) Plot your answers for the magnitude of B above on the graph below. Label your vertical axis or you will lose points. Page 10
Problem (5 points): A current I flows around a continuous path that consists of portions of two concentric circles of radii R and R/, respectively, and two straight radial segments. The point P is at the common center of the two circle segments. (a) Use the Biot-Savart Law, to calculate db at P due only to that segment of the path ds shown in the sketch. Indicate on the sketch the vector rˆ you use and the direction of db. Give the magnitude of db in terms of I, R, dl, and o. (b) Using your expression in (a), find the magnetic field at P due to the larger circle segment only. Give its magnitude and direction. (c) What is the total field B at P? Give its magnitude and direction. Page 11
Problem 4 (5 Points) Circle your choice for the correct answer to the five questions below. A: A particle with known charge q and known mass m is performing circular motion in a uniform magnetic field of known magnitude B (as usual, the orbit is perpendicular to the magnetic field). With q, m, and B known, examine the following statements: (a) The radius R of the orbit can be determined uniquely. (b) The speed v of the particle can be determined uniquely. (c) The angular velocity ω of the particle can be determined uniquely. Circle the one correct statement below 1. Only (a) is correct.. Only (b) is correct. 3. Only (c) is correct 4. Only (a) and (b) are correct. 5. Only (a) and (c) are correct. 6. Only (b) and (c) are correct. 7. All are correct. 8. None is correct B: Consider an infinitely long cylindrical solenoid of radius R with n turns per unit length and a current I. The magnetic field due to this solenoid satisfies: 1. B 0 for r < R and B 0nI for r > R.. B 0nI for r < R and B 0 for r > R. 3. 1 B 0nI for r < R and B 0 for r > R. 4. 1 B 0 for r < R and B 0nI for r > R. 5. B 0nI for all r. Page 1
C: Consider a charged circular loop of radius R and linear charge density λ (charge per unit length) glued down to the loop. Suppose the loop is rotating with angular velocity ω about the axis normal to the loop and going through its center. The loop acts as a magnetic dipole with magnetic dipole moment given by 1.. 3. 4. 5. R R R 3 R R 3 D: An infinitely long wire carries a current I. What is the magnitude B of the magnetic field a distance r away from the wire? 1. 0I B r. I 0 B r 3. I 0 B r 4. None of the above. A: A charged particle of mass m and charge q has a speed v and moves in a circular orbit in a magnetic field of strength B. 1. a noticeable force and no noticeable torque. no noticeable force and a noticeable torque 3. a noticeable force and a noticeable torque 4. no noticeable force and no noticeable torque B: A current I is uniformly distributed over the cross-section of a conducting wire of radius a. For distances r from the center of the wire with r < a, 6. The magnitude of the magnetic field is constant. 7. The magnitude of the magnetic field varies as 1/r 8. The magnitude of the magnetic field varies as r 9. The magnitude of the magnetic field varies as 1/r 10. The magnitude of the magnetic field varies as r Page 13
C: A segment of a wire of length L carries a current I (see sketch above). The magnitude of the magnetic field a distance z from the center of the wire segment along its perpendicular bisector (see sketch) is given by the expression 6. 7. 8. 9. 10. oi 4 oi 4 I o 4 oi 4 oi 4 L / L / 3 / x dx x z x dx L / L / x z z dx L / L / 3 / x z z dx L / L / x z dx L / L / x z I. The sketch shows a current carrying element of wire. The element has length ds, cross-sectional area A, and carries current I. It sits in a constant external magnetic field B ext. The charge carriers in the wire have number density per unit volume n, positive charge q > 0, and drift speed v drift. Starting from the Lorentz force law for the force on a single charge carrier, F q v drift B ext, derive an expression for the total magnetic force on this current carrying line element in terms of I, ds, and B ext. You must show the steps you use in arriving at this formula to be given credit. Page 14
II. The sketch shows a resistor with resistance R, length L and cross-sectional area A. It is made out of material with resistivity. Starting from Ohm s Law in microscopic form, E = J, derive the relationship between R, L, A, and You must show the steps you use in arriving at this formula to be given credit. III. The sketch shows two co-axial current-carrying rings. The rings have current flowing in them in the same sense, as shown (clockwise as viewed from the top). Are these rings attracted to each other, repelled by each other, or do they feel no mutual force? You must explain how you arrived at this answer to be given credit. Page 15
E: Two wires run parallel to the z-azis, which is out of the page. The wire on the right carries a current I 1 > 0 out of the page. The wire on the left carries a current of I = I 1 / also out of the page. Which of the four iron filings representation of the magnetic field of these two wires shown below is correct? In an iron filings representation, the magnetic fields are parallel (or anti-parallel) to the streaks. (a) (b) (c) (d) Page 16
Problem 3 (5 points): For a finite wire carrying a current I the contribution to the magnetic field at point P is 0 Br () (cos1 cos ) r where B is the magnitude of the magnetic field and 1 and are the angles indicated in the figure: Consider a current I on a piece of wire of length a: (see figure below). (a) Find the magnitude B of the magnetic field at point P located a distance r away from the wire and equidistant from the endpoints of the wire. On the above diagram indicate with an or whether the field points in or out of the page. Page 17
Consider a square loop of side a lying on the (x, y) plane as illustrated on the figure below (corners at (x, y) = ( a, a) and ( a, a) ). We are interested in the magnetic field at the point P on the z axis, a distance z from the origin. (b) What is the magnitude B 1 of the magnetic field at P contributed separately by each of the sides of the loop. Give your answer in terms of a, z, and other constants ( 0, I,,...) [You may wish to use the result of (a).] Page 18
(c) Find the magnitude B t of the total magnetic field at P. What direction does point? B t (d) Calculate the leading value of the magnetic field B t far away from the loop, namely, when z >> a. Write this magnetic field in terms of z and the dipole moment m of the current loop (as well as constants 0,, ). Page 19
Problem 4 (5 points): A long cylindrical cable consists of a conducting cylindrical shell of inner radius a and outer radius b. The current density J in the shell is out of the R page (see sketch) and varies with radius as Jr () Jo for a r b and is zero outside r of that range. Find the magnetic field in each of the following regions, indicating both magnitude and direction. Show your work and your Amperean loops. (a) r < a (b) a < r < b Page 0
(c) r > b. Give also B( r b), the value of the magnetic field at r b. (d) Plot your previous answers for the magnitude of B on the graph below. Page 1
Problem 1: Three Short Questions (5 points total): Page
Problem : Biot-Savart Law and Magnetic Dipole Moment (5 points) A current I flows around a continuous path that consists of portions of two concentric circles of radii R and 3R, respectively, and two straight radial segments, as shown. The point P is at the common center of the two circle segments. (a) What is the magnitude and direction of the magnetic field B at P due to the outer circle segment? Use the Biot-Savart Law to calculate this field. Show your work. (b) What is the magnitude and direction of the magnetic field B at P due to the two straight line segments of the current loop? Use the Biot-Savart Law to calculate this field. Show your work. (c) What is the magnitude and direction of the magnetic field B at P due to the inner circle segment? (d) What is the total field B at the point P due to the entire current loop. Be sure to specify the direction of B. (e) What is the magnetic dipole moment of this current loop? Give its magnitude and direction. Page 3