# Physics

22x Ld

d 1

dL Qk

E

b) 9322 N/C, EAST c) 0 4) a) 1.46 x 106 N/C b) 45º c) Because Ex = Ey 5) a) 0 b) 1.79 x 104 N/C, SOUTH 6) 1.35 x 106 N/C, NORTH

7) 5.93 x 104 N/C, 13.6º 8) 2.37 x 104 N/C, 59.8º

9) a) R k2Ey

b) 4.85 x 105 N/C, 22.0º

10) a) R k2E y

b) 2.05 x 106 N/C, 74.8º 11) a) 5.53 x 105 N/C, WEST b) 2.26 x 106 N/C, WEST c) 0 12) 1.01 x 105 N/C, WEST 13) a) 2.17 x 106 m/s2 b) 2.45 x 10–14 J 14) a) 4.9 s b) 3780 m 15) 2.6 x 106 N/C, 22.5º

Physics 226 Fall 2013

Problem Set #5

NOTE: Any answers of zero must have some kind of justification. 1)

A uniform electric field of strength 300 N/C at an angle of 30º with respect to the x-axis goes through a cube of sides 5 cm. (a) Calculate the flux through each cube face: Front, Back, Left, Right, Top, and Bottom. (b) Calculate the net flux through the entire surface. (c) An electron is placed centered 10 cm from the left surface. What is the net flux through the entire surface? Explain your answer.

2)

A right circular cone of height 25 cm and radius 10 cm is enclosing an electron, centered 12 cm up from the base. See Figure G. (a) Using integration and showing all work, find the net flux through the cone’s surface. The electron is now centered in the base of the cone. See Figure L. (b) Calculate the net flux through the surface of the cone.

3) Using the cube in #1, you place a 4μC charge directly in the center of the cube. What is the flux through the top face? (Hint: Consider that this problem would be MUCH more difficult if the charge was not centered in the cube.)

4) Using the cube in #1, you place a 4μC charge at the lower,

left, front corner. What is the net flux through the cube? (Hint: Think symmetry.)

5) You have a thin spherical shell

of radius 10 cm with a uni- form surface charge density of – 42 μC/m2. Centered inside the sphere is a point charge of 4 μC. Find the magnitude and direction of the total electric field at: (a) r = 6 cm and (b) r = 12 cm.

6) You have a solid sphere of radius 6 cm and uniform volume charge density of – 6 mC/m3. Enclosing this is a thin spherical shell of radius 10 cm with a total charge of 7 μC that is uniformly spread over the surface. (a) What is the discontinuity of the E-field at the surface of the shell. (b) What is the discontinuity of the E-field at the surface of the solid sphere? Also, find the magnitude and direction of the total electric field at: (c) r = 4 cm, (d) r = 8 cm, and (e) r = 13 cm.

x 30º

y

7) Use the same set-up in #6 with the following exceptions:

The solid sphere has a total charge of 5 μC and the shell has uniform surface charge density of 60 μC/m2. Answer the same questions in #6, (a) – (e).

8) You have a thin infinite

cylindrical shell of radius 8 cm and a uniform surface charge density of – 12 μC/m2. Inside the shell is an infinite wire with a linear charge density of 15 μC/m. The wire is running along the central axis of the cylinder. (a) What is the discontinuity of the E-field at the surface of the shell? Also, find the magnitude and direction of the total electric field at: (b) r = 4 cm, and (c) r = 13 cm.

9) You have a thin infinite

cylindrical shell of radius 15 cm and a uniform surface charge density of 10 μC/m2. Inside the shell is an infinite solid cylinder of radius 5 cm with a volume charge density of 95 μC/m3. The solid cylinder is running along the central axis of the cylindrical shell. (a) What is the discontinuity of the E-field at the surface of the shell? (b) What is the discontinuity of the E-field at the surface of the solid cylinder. Also, find the magnitude and direction of the total electric field at: (c) r = 4 cm, (d) r = 11 cm, and (e) r = 20 cm.

–

–

G L

+

10) You have a thick spherical shell of outer diameter 20 cm and inner diameter 12 cm. The shell has a total charge of – 28 μC spread uniformly throughout the object. Find the magnitude and direction of the total electric field at: (a) r = 6 cm, (b) r = 15 cm, and (c) r = 24 cm.

11) You have a thick cylindrical shell

of outer diameter 20 cm and inner diameter 12 cm. The shell has a uniform volume charge density of 180 μC/m3. Find the magnitude and direction of the total electric field at: (a) r = 6 cm, (b) r = 15 cm, and (c) r = 24 cm.

12)

You have an thin infinite sheet of charge with surface charge density of 8 μC/m2. Parallel to this you have a slab of charge that is 3 cm thick and has a volume charge density of – 40 μC/m3. Find that magnitude and direction of the total electric field at: (a) point A which is 2.5 cm to the left of the sheet, (b) point B which is 4.5 cm to the right of the sheet, and (c) point C which is 1 cm to the left of the right edge of the slab.

13)

You have an infinite slab of charge that is 5 cm thick and has a volume charge density of 700 μC/m3. 10 cm to the right of this is a point charge of – 6 μC. Find that magnitude and direction of the total electric field at: (a) point A which is 2.5 cm to the left of the right edge of the slab, (b) point B which is 6 cm to the right of the slab, and (c) point C which is 4 cm to the right of the point charge.

14) You have two infinite sheets of charge with equal surface charge magnitudes of 11 μC/m2 but opposite signs. Find the magnitude and direction of the total electric field, (a) to the right of the sheets, (b) in between the sheets, and (c) to the left of the sheets.

15)

A hydrogen molecule (diatomic hydrogen) can be modeled incredibly accurately by placing two protons (each with charge +e) inside a spherical volume charge density which represents the “electron cloud” around the nuclei. Assume the “cloud” has a radius, R, and a net charge of –2e (one electron from each hydrogen atom) and is uniformly spread throughout the volume. Assume that the two protons are equidistant from the center of the sphere a distance, d. Calculate, d, so that the protons each have a net force of zero. The result is darn close to the real thing. [This is actually a lot easier than you think. Start with a Free-Body Diagram on one proton and then do F = ma.]

ANSWERS:

NOTE: Units for 1 – 4

are CmN 2 1) a) 0 for F/B, 0.375 for L/R, 0.65 for T/B

b) & c) 0 2) a) – 1.81 x 10–8

b) – 9.05 x 10–9 3) 7.54 x 104 4) 5.66 x 104 5) a) 9.99 x 106 N/C, OUTWARD [O] b) 7.99 x 105 N/C

INWARD [I] 6) a) 6.29 x 106 N/C

b) 0 c) 9.04 x 106 N/C, I d) 7.63 x 106 N/C, I e) 8.36 x 105 N/C, O 7) a) 6.78 x 106 N/C

b) 0 c) 4.99 x 105 N/C, O d) 7.03 x 106 N/C, O e) 6.67 x 106 N/C, O

8) a) 1.36 x 106 N/C b) 6.74 x 106 N/C, O c) 1.24 x 106 N/C, O 9) a) 1.13 x 106 N/C b) 0 c) 2.15 x 105 N/C, O d) 1.22 x 105 N/C, O e) 9.15 x 105 N/C, O 10) a) 0 b) 2.94 x 106 N/C, I c) 4.37 x 106 N/C, I 11) a) 0 b) 5.49 x 105 N/C, O c) 1.09 x 106 N/C, O 12) a) 3.84 x 105 N/C, L b) 5.20 x 105 N/C, R c) 4.30 x 105 N/C, R 13) a) 3.84 x 105 N/C, R b) 3.57 x 107 N/C, R c) 3.18 x 105 N/C, L 14) a) 0 b) 1.24 x 106 N/C, R c) 0 15) 0.794R

10 cm

A B C

10 cm

A B –

C

R

+ +

d d

Physics 226 Fall 2013

Problem Set #6

NOTE: Any answers of zero must have some kind of justification. 1) You have a cylindrical metal shell of

inner radius 6 cm and outer radius 9 cm. The shell has no net charge. Inside the shell is a line of charge of linear density of – 7 μC/m. Find the magnitude and direction of the electric field at (a) r = 3 cm, (b) r = 7 cm, and (c) r = 13 cm. Also, calculate the surface charge density of the shell on (d) the inner surface and (e) the outer surface.

2) You have a uniformly charged

sphere of radius 5 cm and volume charge density of – 7 mC/m3. It is surrounded by a metal spherical shell with inner radius of 10 cm and outer radius of 15 cm. The shell has a net charge 8 μC. (a) Calculate the total charge on the sphere. Find the magnitude and direction of the electric field at (b) r = 13 cm and (c) r = 18 cm. Also, calculate the surface charge density of the shell on (d) the inner surface and (e) the outer surface.

3) Two 2 cm thick infinite slabs of metal are positioned as

shown in the diagram. Slab B has no net charge but Slab A has an excess charge of 5 μC for each square meter. The infinite plane at the origin has a surface charge density of – 8 μC/m2. Find the magnitude and direction of the electric field at (a) x = 2 cm, and (b) x = 4 cm. Also, calculate the surface charge density on (c) the left edge of A, (d) the right edge of A, and (e) the left edge of B.

4) A positive charge of 16 nC is nailed down with a #6 brad.

Point M is located 7 mm away from the charge and point G is 18 mm away. (a) Calculate the electric potential at Point M. (b) If you put a proton at point M, what electric potential energy does it have? (c) You release the

proton from rest and it moves to Point G. Through what potential difference does it move? (d) Determine the velocity of the proton at point G.

5)

All the charges above are multiples of “q” where

q = 1μC. The horizontal and vertical distances between the charges are 25 cm. Find the magnitude of the net electric potential at point P.

6) Use the same charge distribution as in problem #5 but

change all odd-multiple charges to the opposite sign. Find the magnitude of the net electric potential at point P.

7) A parallel plate setup has a distance

between the plates of 5 cm. An electron is place very near the negative plate and released from rest. By the time it reaches the positive plate it has a velocity of 8 x 106 m/s. (a) As the electron moves between the plates what is the net work done on the charge? (b) What is the potential difference that the electron moves through? (c) What is the magnitude and direction of the electric field in between the plates?

3 cm 5 cm 8 cm 0 10 cm

A B

– 8q

– 4q

+9q

+9q

– 5q

+6q +6q

+2q

P

+ M

G

–

8)

A uniform line of charge with density, λ, and length, L is positioned so that its left end is at the origin. See diagram above. (a) Determine an equation (using integration) for the magnitude of the total electric potential at point P a distance, d, away from the origin. (b) Calculate the magnitude of the electric potential at P if d = 2 m, L = 1 m, and λ = – 5 μC/m. c) Using the equation you derived in part a), calculate the equation for the electric field at point P. It should agree with the result we got in Lecture Example #19.

9) You have a thin spherical shell

of radius 10 cm with a uni- form surface charge density of 11 μC/m2. Centered inside the sphere is a point charge of – 4 μC. Using integration, find the magnitude of the total electric potential at: (a) r = 16 cm and (b) r = 7 cm.

10) You have a uniformly

charged sphere of radius 5 cm and volume charge density of 6 mC/m3. It is surrounded by a metal spherical shell with inner radius of 10 cm and outer radius of 15 cm. The shell has no net charge. Find the magnitude of the electric potential at (a) r = 20 cm, (b) r = 12 cm, and (c) r = 8 cm.

11) Use the same physical situation with the exception

of changing the inner sphere to a solid metal with a surface charge density of 9 μC/m2 and giving the shell a net charge of – 3 μC. Find magnitude of the electric potential at (a) r = 20 cm, (b) r = 12 cm, (c) r = 8 cm, and (d) r = 2 cm.