Homework Section 1 - University of Texas at Dallas



Homework Section 1

1) Analytically prove the following. (Do this for arbitrary dimension)

a) The commutative law: A + B = B + A

b) The Associative Law: A + (B + C) = (A+B) + C

c) A + B = C if and only if B = C - A

d) A + 0 = A and A - A = 0

e) Scalar product is commutative [A•B=B•A] and

f) Scalar product is distributive [A•(B+C)=A•B+A•C].

2) Prove that the area of a parallelogram with sides A and B is |A x B|. Note that the surface area has a direction associated with it.

3) Prove that the volume of a parallelepiped with side A, B and C is A•(B x C).

4) Find the magnitude of the following vectors.

g) (1, 4)

h) (4, 3, 0)

i) (0, -1, 1)

j) (6, 1, 0, -1, 2)

5) Which of the following vectors are unit vectors?

k) (1, 0)

l) (1, 1/2)

m) (1, -1)

n) (1/√2, -1/√2)

o) (1/2, 0, √3/2)

p) (1, 0, 0, 0)

6) Express the vector A = (2, 7) as a linear combination of the vectors:

q) B1 = (2, 4), B2 = (-1, 3).

r) C1 = (4, 4), C2 = (5, 5).

Express the vector A = (2, -1, 3) as a linear combination of the vectors:

s) B1 = (2, 4, 1), B2 = (3, 7, 1).

t) C1 = (1, 0, 0), C2 = (0, 1, 0), C3 = (0, 0, 1).

u) D1 = (2, 4, 1), D2 = (3, 7, 1), D3 = (-1, 2, 2).

7) Show that in a 3-dimensional space, a set of three vectors A, B and C are linearly independent if and only if

[pic];

(Linear independence requires that: a A + b B + c C = 0 if and only if a, b, c = 0).

8) Determine if the vector R1 = (2, 4, 1), R2 = (0, 3, -1) and R3 = (2, 1, 1) are linearly independent.

9) Consider a system of n electric charges, e1 through en. Let ri be the position vector of charge ei. The dipole moment of the system of charges is defines as

[pic]

and the center of the charge of the system is

[pic]

where

[pic]

The system is called neutral is

[pic]

v) Show that the dipole moment of a neutral system is independent of the origin.

w) Express this moment in terms of the centers of the systems of negative and positive charges making up the original system.

10) Find the scalar product of the following vectors.

x) (2, 3)•(1, -1)

y) (4, 1)•(6, -5)

z) (1, 2, -3)•(-1, 1, 2)

aa) (2, 4)•(1, 5, 3)

ab) (0, sinα, 1, 3)•(2, 4, -2, 1)

ac) (sin(ωt), cos(ωt))•( sin(ωt), cos(ωt))

11) Find the angles that the vector (2, 4, -5) makes with the coordinate axes.

12) Find the projection of the vector (2, 5, 1) on the vector (1, 1, 3).

13)

ad) Using the dot product, prove the law of cosines.

ae) Let U1 and U2 be two vectors in the x-y plane with angles α and β between U1 and x and U2 and x. Using the dot product show that cos(β-α) = cos(β)cos(α)+sin(β)sin(α)

14) Determine the value of α such that A and B are perpendicular and C and D are perpendicular.

af) A = (2, 3α, 1), B = (4, 2, 4α)

ag) C = (2α, 4, 3, 1), D = (α, 2, -1, 2)

15) Let A = (-1, 2, 4), B = (3, 2, 7). Find the unit vector perpendicular to the plane determined by A and B.

16) The force F = (2, 3, 1) is applied to an object which move along a vector r = (1, 4, 1). What is the work done?

17) Determine the magnitude, phase angle, real and imaginary parts of the following

1) 3+i

2) 3

3) 3ei2π/3

4) 2(cos (π/6) +i sin (π/6))

5) 3∠45°

18) A Force F = 2i - 3j + k acts at the point (1, 5, 2). Find the torque due to F about

6) The origin

7) The y axis

8) The line x/2=y/1=z/(-2)

Del operator questions

19) Find the gradient of w=x2y3z at (1, 2, -1)

20) Find the gradient of

Compute the divergence and the curl of each of the following Vector Fields.

21) [pic]

22) [pic]

Calculate the Laplacian [pic] of each of the following

23) [pic]

24) [pic]

25) For [pic], Prove

[pic]

26) For [pic], evaluate [pic].

Divergence, Stokes and Green’s Theorem Problems

27) Evaluate the integral [pic] along each of the following paths from (0,0) to (1,2)

ah) y = 2x2

ai) x = t2, y = 2t

aj) y = 0, for x = 0 -> 1 and then x = 1 for y = 0 -> 2.

28) Evaluate the integral [pic] along each of the following paths – each path.

ak) a) (0,0) to (1,2)

al) b) (0, 0) to (3, 0) to (1,2).

29) Determine if the following force fields are conservative. Then determine a scalar potential for each field.

am) [pic]

an) [pic]

30) Which, if either, of the following force fields is conservative? Calculate the work done moving a particle around a circle of x = cos t, y = sin t in the x-y plane.

ao) [pic]

ap) [pic]

Explain why you have gotten these answers.

31) In spherical coordinates, show that the electric field E of a point charge is conservative. Determine and write the electric potential [pic] in rectangular (cartesian) and cylindrical coordinates. Find [pic] using both cartesian and cylindrical coordinates and show that the results are the same as in spherical coordinates.

32) Derive [pic] using methods similar to that used in class.

33) Evaluate [pic], around the curve (1, 0) to (4, 0) to (4, .5) along [pic] to (1, 1).

34) For a simple closed curve C in a plane show by Green’s theorem that the area enclosed is [pic].

35) Find the area inside the curve [pic].

Evaluate the following three problems using either a surface or a volume integral, whichever is easier.

36) [pic], over the volume bounded by x2 + y2 ≤ 4, 0 ≤ z ≤ 5. (Remember the top and bottom!)

37) [pic], over the surface of the cone with base [pic] and vertex at (0,0,3).

38) [pic], over the unit cube in the first octant.

Using either Stoke’s Theorem or the Divergence Theorem evaluate each of the following.

39) [pic], where σ is a tin can defined by x2 + y2 ≤ 9, 0 ≤ z ≤ 5. (Remember the top and bottom!).

40) [pic], where σ is any surface with a bounding curve entirely in the x-y plane.

41) [pic], where σ is the closed surface of the ellipsoid 1 = x2/4 + y2/9 + z2/16.

Electro and Magneto static point source

42) Determine the electric potential of a point charge from the electric field

[pic]

43) Determine the magnetic potential of a current element.

[pic]

44) Need to add A cross B = magnitude times sin of angle

Homework Section 2

Static electric fields using Coulomb’s Law – notice that symmetry is lacking in most of these problems.

Line charges

(These might approximate what you would find on a line if it were exposed to an external charge. Also note that these represent – as best I can tell – all of the possible problems of this form that one can solve analytically.)

1) Use cylindrical coordinates to calculate the electric field in the x-y plane for a line charge where the charge density is

[pic]

2) Use cylindrical coordinates to calculate the electric field in the x-y plane for a line charge where the charge density is

[pic]

3) Use cylindrical coordinates to calculate the electric field in the x-y plane for a line charge where the charge density is

[pic]

4) Use cylindrical coordinates to calculate the electric field in the x-y plane for a line charge where the charge density is

[pic]

5) Use cylindrical coordinates to calculate the electric field in the x-y plane for a line charge where the charge density is

[pic]

6) Use cylindrical coordinates to calculate the electric field in the x-y plane for a line charge where the charge density is

[pic]

7) Use cylindrical coordinates to calculate the electric field in the x-y plane for a line charge where the charge density is

[pic]

Surface charges

(These might approximate what you would find on a surface if it were exposed to an external charge. Again note that these represent – as best I can tell – all of the possible problems of this form that one can solve analytically.)

8) Use cylindrical coordinates to calculate the electric field on the z-axis for a surface of charges on the x-y plane where the charge density is

[pic]

9) Use cylindrical coordinates to calculate the electric field on the z-axis for a surface of charges on the x-y plane where the charge density is

[pic]

10) Use cylindrical coordinates to calculate the electric field on the z-axis for a surface of charges on the x-y plane where the charge density is

[pic]

11) Use cylindrical coordinates to calculate the electric field on the z-axis for a surface of charges on the x-y plane where the charge density is

[pic]

12) Use cylindrical coordinates to calculate the electric field on the z-axis for a surface of charges on the x-y plane where the charge density is

[pic]

13) Use cylindrical coordinates to calculate the electric field on the z-axis for a surface of charges on the x-y plane where the charge density is

[pic]

14) Use cylindrical coordinates to calculate the electric field on the z-axis for a surface of charges on the x-y plane where the charge density is

[pic]

Static electric fields using Gauss’ Law – notice the symmetry in these problems.

15) Use cylindrical coordinates to calculate the electric field in the x-y plane for a line charge where the charge density is

[pic]

16) Use cylindrical coordinates to calculate the electric field on the z-axis for a surface of charges on the x-y plane where the charge density is

[pic]

Cylindrical Volume charges

(These might approximate what you would find in a volume of a material. Under some conditions it might be an insulator with charges distributed around the volume, in others it might be a wire (or two) with charge carriers near surfaces.)

17) Calculate the electric field EVERYWHERE for a volume of charges where the charge density is

[pic]

18) Calculate the electric field EVERYWHERE for a volume of charges where the charge density is

[pic]

19) Calculate the electric field EVERYWHERE for a volume of charges where the charge density is

[pic]

20) Calculate the electric field EVERYWHERE for a volume of charges where the charge density is

[pic]

21) Calculate the electric field EVERYWHERE for a volume of charges where the charge density is

[pic]

22) Calculate the electric field EVERYWHERE for a volume of charges where the charge density is

[pic]

23) Calculate the electric field EVERYWHERE for a volume of charges where the charge density is

[pic]

24) Calculate the electric field EVERYWHERE for a volume of charges where the charge density is

[pic]

25) Calculate the electric field EVERYWHERE for a volume of charges where the charge density is

[pic]

26) Calculate the electric field EVERYWHERE for a volume of charges where the charge density is

[pic]

27) Calculate the electric field EVERYWHERE for a volume of charges where the charge density is

[pic]

28) Calculate the electric field EVERYWHERE for a volume of charges where the charge density is

[pic]

Spherical Volume charges

(These might approximate what you would find in a volume of a material. Under some conditions it might be an insulator with charges distributed around the volume.)

29) Calculate the electric field EVERYWHERE for a volume of charges where the charge density is

[pic]

30) Calculate the electric field EVERYWHERE for a volume of charges where the charge density is

[pic]

31) Calculate the electric field EVERYWHERE for a volume of charges where the charge density is

[pic]

32) Calculate the electric field EVERYWHERE for a volume of charges where the charge density is

[pic]

33) Calculate the electric field EVERYWHERE for a volume of charges where the charge density is

[pic]

34) Calculate the electric field EVERYWHERE for a volume of charges where the charge density is

[pic]

35) Calculate the electric field EVERYWHERE for a volume of charges where the charge density is

[pic]

36) Calculate the electric field EVERYWHERE for a volume of charges where the charge density is

[pic]

37) Calculate the electric field EVERYWHERE for a volume of charges where the charge density is

[pic]

38) Calculate the electric field EVERYWHERE for a volume of charges where the charge density is

[pic]

39) Calculate the electric field EVERYWHERE for a volume of charges where the charge density is

[pic]

40) Calculate the electric field EVERYWHERE for a volume of charges where the charge density is

[pic]

Physically realistic systems

Need to add problems

Capacitors etc

Static Magnetic fields using Biot-Savart Law – notice that symmetry is lacking in most of these problems.

Line currents

(These might approximate what you would find on a few loops of wire. There perhaps a few more problems that can be solve analytically, but not many. )

41) Use Cartisean coordinates to calculate the magnetic field on the z-axis for a line current where the charge density is

[pic]

42) Use cylindrical coordinates to calculate the magnetic field on the x-y plane for a line current where the charge density is

[pic]

43) Use cylindrical coordinates to calculate the magnetic field on the z-axis for a line current where the charge density is

[pic]

44) Use cylindrical coordinates to calculate the magnetic field on the z-axis for a line current where the charge density is

[pic]

45) The magnetic field around a strip cable (2 parallel wires) can be examined if one considers where the current is flowing. Assume that for the specific strip cable that the wires are filamentary strands of metal (typically Cu) and that the net flow on one wire is balanced by an opposite net flow on the other wire. First show that the surface current density on the wires should be given by:

[pic]

Then calculate the magnetic field EVERYWHERE.

Static magnetic fields using Ampere’s Law – notice the symmetry in these problems.

Cylindrical current densities

(These might approximate what you would find in a volume of a material carrying a current.)

46) Calculate the magnetic field EVERYWHERE where the current density is

[pic]

47) Calculate the magnetic field EVERYWHERE where the current density is

[pic]

48) Calculate the magnetic field EVERYWHERE where the current density is

[pic]

49) Calculate the magnetic field EVERYWHERE where the current density is

[pic]

50) Calculate the magnetic field EVERYWHERE where the current density is

[pic]

51) Calculate the magnetic field EVERYWHERE where the current density is

[pic]

52) Calculate the magnetic field EVERYWHERE where the current density is

[pic]

53) Calculate the magnetic field EVERYWHERE where the current density is

[pic]

54) Calculate the magnetic field EVERYWHERE where the current density is

[pic]

Physically realistic systems

These problems are intended to approximate physically real situations. Use Ampere’s Law to solve for the magnetic fields everywhere.

Need to add problems

Parallel wires, coaxial wires, electromagnets, speaker coils, (Long coils vs. short coils) etc

55) The magnetic field around a strip cable (2 parallel wires) can be examined if one considers where the current is flowing. Assume that for the specific strip cable that the wires are filamentary strands of metal (typically Cu) and that the net flow on one wire is balanced by an opposite net flow on the other wire. First show that the surface current density on the wires should be given by:

[pic]

Then calculate the magnetic field EVERYWHERE.

56) The magnetic field around a coaxial cable can be examined if one considers where the current is flowing. Assume that for the specific coaxial cable that the wires are made from single strands of metal (typically Cu) and that the net flow on one wire is balanced by an opposite net flow on the other wire. First show that the surface current density on the wires should be given by:

[pic]

Then calculate the magnetic field EVERYWHERE.

57) Speakers/microphone coils, transformers and inductors rely on the magnetic field produced by or induced from coils. The simplest geometry is that of an infinitely long coil. (In fact this is one of only a few geometries that can be solved analytically.) Assume the coil has a radius of ‘a’ and N loops per length ‘l’. Calculate the magnetic field EVERYWHERE.

Material issues

For more information look at:













|Material |er () |µr () |σ (S//m)) |

|Al | |1.000021 |3.5E7 |

|Al2O3 |4.5-8.4 |0.999963 |1e-16 |

|Au (gold) | |0.99996 |4.1E7 |

|Ag (silver) | |0.9999976 |6.1E7 |

|Ag2O | |0.999866 | |

|Cu | |0.99999 |5.7E7 |

|CuO | |1.000250 | |

|W | |1.00008 |1.8E7 |

|WO2 | |1.000057 | |

|Teflon |2.1 |~1 |1E-20 |

|Wax (paraffin) |2.0-2.5 |0.99999942 |1E-17 |

|Sea water |70 |0.9999901 |4 |

|Distilled H2O |81 |0.999987 |1E-4 |

|Mica |5.4 | |1E-15 |

|Air |1.006 |~1 | |

|Mineral Oil | | | ................
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