UIUC Physics 436 EM Fields & Sources II Fall Semester, 2015 Lect. Notes 2 Prof. Steven Errede

LECTURE NOTES 2

CONSERVATION LAWS (continued)

 Conservation of Linear Momentum p

Newton’s 3rd Law in Electrodynamics – “Every action has an equal and opposite reaction”.

 Consider a point charge +q traveling along the zˆ -axis with constant speed v vvz ˆ . Because the electric charge is moving (relative to the laboratory frame of reference), its electric field is not given perfectly/mathematically precisely as described by Coulomb’s Law,   1 q i.e. Ert,  2 rˆ . 4 o r   Nevertheless, Ert, does still point radially outward from its instantaneous position,  – the location of the electric charge qrt, . {n.b. when we get to Griffiths Ch. 10 (relativistic   electrodynamics, we will learn what the fully-correct form of Ert, is for a moving charge..}

  E B

q  q  v v zˆ zˆ

 n.b. The E -field lines of a moving electric charge are compressed in the transverse direction!

Technically speaking, a moving single point electric charge does not constitute a steady/DC electrical current (as we have previously discussed in P435 Lecture Notes 13 & 14). Thus, the magnetic field associated with a moving point charge is not precisely mathematically   correctly/properly given as described by the Biot-Savart Law. Nevertheless, Brt, still points in the azimuthal (i.e.ˆ -) direction. (Again, we will discuss this further when we get to Griffiths Ch. 10 on relativistic electrodynamics…)

© Professor Steven Errede, Department of Physics, University of Illinois at Urbana-Champaign, Illinois 1 2005-2015. All Rights Reserved. UIUC Physics 436 EM Fields & Sources II Fall Semester, 2015 Lect. Notes 2 Prof. Steven Errede

Let us now consider what happens when a point electric charge q traveling with constant  1 velocity vvz ˆ encounters a second point electric charge q , e.g. traveling with constant 11 2 velocity vvx22 ˆ as shown in the figure below:   elect  xˆ   Frt12 2 , B12rt,  mag   q Frt2 , 2 12  mag    Frt21 1, v2  v q 1 1 zˆ     elect  Frt , B rt, 21 1 yˆ 21

The two like electric charges obviously will repel each other (i.e. at the microscopic level, they will scatter off of each other – via exchange of one (or more) virtual photons).

As time progresses, the electromagnetic forces acting between them will drive them off of their initial axes as they repel / scatter off of each other. For simplicity’s sake (here) let us assume that (by magic) the electric charges are mounted on straight tracks that prevent the electric charges from deviating from their initial directions.

Obviously, the electric force between the two electric charges (which acts on the line joining them together – see above figure) is repulsive and also manifestly obey’s Newton’s 3rd law:  elect elect  FrtFrt1221,, 21  Is the magnetic force acting between the two charges also repulsive??

By the right-hand rule:   The magnetic field of q1 at the position-location of q2 points into the page: B12rt,   yˆ   The magnetic field of q2 at the position-location of q1 points out of the page: B21rt,   yˆ  mag    ˆ Thus, the magnetic force FrtqvrtBrtz12 222212,,,    due to the effect of charge q1’s     B -field B12rt, at charge q2’s position r2 points in the zˆ direction (i.e. to the right in the above figure).  mag    ˆ However the magnetic force FrtqvrtBrtx21 111121,,,    due to the effect of charge     q2’s B -field B21rt, at charge q1’s position r1 points in the xˆ direction (i.e. upward in the  mag mag ˆ  ˆ above figure). Thus we see that Frtz1221,  Frtx 21  ,  !!!

The electromagnetic force of q1 acting on q2 is equal in magnitude to, but is not opposite to the rd electromagnetic force of q2 acting on q1, in {apparent} violation of Newton’s 3 Law of Motion! Specifically, it is the magnetic interaction between two charges with relative motion between them that is responsible for this {apparent} violation of Newton’s 3rd Law of Motion:

2 © Professor Steven Errede, Department of Physics, University of Illinois at Urbana-Champaign, Illinois 2005-2015. All Rights Reserved. UIUC Physics 436 EM Fields & Sources II Fall Semester, 2015 Lect. Notes 2 Prof. Steven Errede

  elect elect mag mag   FrtFrt21,,  but: FrtFrt21,,  12 21 12 21 elect elect mag mag   But: FrtFrt1221,, 21  and: FrtFrt1221,, 21 

  EMtot elect mag EMtot elect mag   Then: FrtFrtFrt222,,,   FrtFrtFrt111,,,   12 12 12 21 21 21 EMtot EMtot EMtot EMtot    FrtFrt1221,, 21  but: FrtFrt1221,, 21  n.b. In electrostatics and in magnetostatics, Newton’s 3rd Law of Motion always holds.

In electrodynamics, Newton’s 3rd Law of Motion does not hold for the apparent relative motion of two electric charges! (n.b. Isaac Newton could not have forseen this {from an apple falling on his head} because gravito-magnetic forces associated with a falling apple are so vastly much feebler than the gravito-electric force (the “normal” gravity we know & love about in the “everyday” world).

Since Newton’s 3rd Law is intimately connected/related to conservation of linear momentum, on the surface of this issue, it would seem that electrodynamical phenomena would then also seem to violate conservation of linear momentum – eeeeeEEEEKKKK!!!

If one considers only the relative motion of the (visible) electric charges (here q1 and q2) then, yes, it would indeed appear that linear momentum is not conserved!

However, the correct picture / correct reality is that the EM field(s) accompanying the  (moving) charged particles also carry linear momentum p (as well as energy, E)!!!

Thus, in electrodynamics, the electric charges and/or electric currents plus the electromagnetic  fields accompanying the electric charges/currents together conserve total linear momentum p . Thus, Newton’s 3rd Law is not violated after all, when this broader / larger perspective on the nature of electrodynamics is properly/fully understood!!!

Microscopically, the virtual (and/or real) photons associated with the macroscopic / “mean field”   electric and magnetic fields Ert12,, Ert 21 , and B12rt,, B 21  rt, do indeed carry / have associated with them linear momentum (and {kinetic} energy) {as well as angular momentum..}!!!

In the above example of two like-charged particles scattering off of each other, whatever momentum “lost” by the charged particles is gained by the EM field(s) associated with them!

Thus, Newton’s 3rd Law is obeyed - total linear momentum is conserved when we consider the true total linear momentum of this system:

  p p pppppmech mech EM EM tot mechanical EM 1212 EM EM mv11 mv 2 2  p 1 p 2

Non-relativistic case (here)!!!

© Professor Steven Errede, Department of Physics, University of Illinois at Urbana-Champaign, Illinois 3 2005-2015. All Rights Reserved. UIUC Physics 436 EM Fields & Sources II Fall Semester, 2015 Lect. Notes 2 Prof. Steven Errede

Note that in the above example of two moving electrically-charged particles there is an interesting, special/limiting case when the two charged particles are moving parallel to each  other, e.g. with equal constant velocities vv12 vzˆ relative to a fixed origin in the lab frame:

 elect    ˆ Fr12 2  x xˆ   B2  ˆ q2 vvzvz22ˆˆ   mag   Fr   xˆ r 12 2 2 mag  Fr   xˆ q 21 1  1  r1 vvzvz11ˆˆ   elect  zˆ  Frx   ˆ  21 1 B1  ˆ yˆ

We can easily see here that the electric force on the two electrically-charged particles acts on the line joining the two charged particles is repulsive (due to the like-charges, here) and is such  elect elect  that Fr21 Fr   xˆ . Similarly, because the two like-charged particles are also 12 21  mag  traveling parallel to each other, the magnetic Lorentz force FqvBext on the two charged particles also acts along this same line joining the two charged particles and, by the right-hand  mag mag   ˆ rule is such that Fr1221 Fr 21   x.

Thus, we also see that:  EMtot elect mag Fr  FrFr 12222 12  12   EMtot elect mag Fr  FrFr   21111 21  21 

Thus, here, for this special case, Newton’s 3rd law is obeyed simply by the mechanical linear momentum associated with this system – the linear momentum carried by the EM field(s) in this special-case situation is zero.

Note that this special-case situation is related to the case of parallel electric currents attracting each other – e.g. two parallel conducting wires carrying steady currents I1 and I2 . It must be remembered that current-carrying wires remain overall electrically neutral, because real currents in real conducting wires are carried by negatively-charged “free” conduction electrons that are embedded in a three-dimensional lattice of positive-charged atoms. The positively-charged atoms screen out / cancel the electric fields associated with the “free” conduction electrons, thus only the (attractive) magnetic Lorentz force remains!

Yet another interesting aspect of this special-case situation is to go into the rest frame of the  two electric charges, where for identical lab velocities vv12 vzˆ , in the rest frame of the charges, the magnetic field(s) both vanish – i.e. an observer in the rest frame of the two charges sees no magnetic Lorentz force(s) acting on the like-charged particles!

4 © Professor Steven Errede, Department of Physics, University of Illinois at Urbana-Champaign, Illinois 2005-2015. All Rights Reserved. UIUC Physics 436 EM Fields & Sources II Fall Semester, 2015 Lect. Notes 2 Prof. Steven Errede  Maxwell’s Stress Tensor T  EM Let us use the Lorentz force law to calculate the total electromagnetic force FtTot  due to the totality of the electric charges contained within a (source) volumev :

  FEM t f EM  rtd,,,, Ert  vrt  Brt   rtd, Totvv Tot    EM  3 where: fTot rt, = EM force per unit volume (aka force “density”) (SI units: N/m ),   and: Jrt,,,   rtvrt electric volume current density (SI units: A/m2).

   Thus: FEM  t  rt,, Ert J rt , Brt  , d Tot v     EM     fTot rt, = EM force per unit volume =  rt,, Ert J rt , Brt  ,  EM  n.b. If we talk about fTot rt, in isolation (i.e. we do not have to do the integral), then, for  transparency’s sake of these lecture notes, we will (temporarily) suppress the rt, arguments – however it is very important for the reader to keep this in mind (at all times) that these arguments are there in order to actually (properly/correctly) do any calculation!!!   EM Thus: fTot EJB  Maxwell’s equations (in differential form) can now be used to eliminate  and J :   Coulomb’s Law: E   o  o E

Ampere’s Law: (with Maxwell’s displacement current correction term):     E 1 E BJ ooo   JB o t o t       EM 1 E Thus: fTotEJB o EE   B  o  B o t  EB    Now: EB  B E (by the chain rule of differentiation) tt  t  EB    BEBE   tt   t     B B  E  Faraday’s Law: E   E  B EB  E  E t t tt     1      Thus: f EM EE  E E B  B E  B Tot o    o  o t

© Professor Steven Errede, Department of Physics, University of Illinois at Urbana-Champaign, Illinois 5 2005-2015. All Rights Reserved. UIUC Physics 436 EM Fields & Sources II Fall Semester, 2015 Lect. Notes 2 Prof. Steven Errede  Without changing the physics in any way, we can add the term B B to the above expression  since B 0 (i.e. there exist no isolated N/S magnetic charges/magnetic monopoles in nature).   EM Then fTot becomes more symmetric between the EB and fields (which is aesthetically pleasing):

    1       f EM EE  E E BB  B B E  B Tot o      o  o t  Now: EEEEE2 22   Using Griffiths “Product Rule #4”     1 2 Or: EE 2  EEE   {n.b. is also applied similarly for B } Thus:

  111      f EM EE  E E   BB  B B E22 B  E  B Tot o   oo oo2 t  We now introduce Maxwell’s stress tensor T (a 33 matrix), the nine elements of which are defined as:

11221 TEEEijoijij22 BBB ijij  o where: i, j = 1, 2, 3 and: i, j = 1 = x i, j = 2 = y i, j = 3 = z i.e. the i, j indices of Maxwell’s stress tensor physically correspond to the x, y, z components of the E & B-fields. and: δij = Kroenecker δ-function: δij = 0 for i ≠ j, δij = 1 for i = j   2222 2222 and: EEEEEE x yz B BB Bx Byz B

Tij Column index  T11 T12 T13 Txx Txy Txz T = T21 T22 T23 = Tyx Tyy Tyz Row index T31 T32 T33 Tzx Tzy Tzz

  Note that from the above definition of the elements of T , we see that T is symmetric under the interchange of the indices ij (with indices i, j = 1, 2, 3 = spatial x, y, z) , i.e. one of the symmetry properties of Maxwell’s stress tensor is: TTij ji  We say that T is a symmetric rank-2 tensor (i.e. symmetric 33 matrix) because: TTij ji .  Thus, Maxwell’s stress tensor T actually has only six (6) independent components, not nine!!!  ˆˆ Note also that the 9 elements of T are actually “double vectors”, e.g. Tijij , Txyxy ˆˆ , Tzzzz ˆˆ, etc.

6 © Professor Steven Errede, Department of Physics, University of Illinois at Urbana-Champaign, Illinois 2005-2015. All Rights Reserved. UIUC Physics 436 EM Fields & Sources II Fall Semester, 2015 Lect. Notes 2 Prof. Steven Errede

The six independent elements of the symmetric Maxwell’s stress tensor are:

11222 222 TT11 xxoxyz  EEE   BBB xyz  22o

11222 222 TT22 yy o EEE y  z x  BBB y  z x generate by cyclic permutation 22o

11222 222 T33  Tzz o EEE z  x y  BBB z  x y generate by cyclic permutation 22o

1 TTTT12 21 xyyxoxy  EE  BB xy o 1 TTTT13 31 xzzxoxz   EE  BB xz generate by cyclic permutation o 1 TTTT23 32 yz zy o EE y z  BB y z generate by cyclic permutation o   Note also that T contains no E  B (etc.) type cross-terms!!  Because T is a rank-2 tensor, it is represented by a 2-dimensional, 3×3 matrix. kl {Higher rank tensors: e.g. Tijk (= rank-3 / 3-D matrix), Aij (= rank-4 / 4-D matrix), etc.}

     We can take the dot product of a vector a with a tensor T to obtain (another) vector b : baT 

  3 baTaTaTjiijiij j  i1

explicit summation over implicit summation over i-index; only index j remains. i-index is implied, only index j remains. n.b. this summation convention is very important: 3 baTaT jiijiij  i1 implicit sum over i explicit sum over i  3 Note also that another vector can be formed, e.g.: cTa  where (here): cTaTaiijjijj j1 Compare these two types of vectors side-by-side:

    3  baT  : baTaTaTjiijiij n.b. summation is over index i (i.e. rows in T )!! j  i1    3  cTa  : cTaiijjijj TaTa  n.b. summation is over index j (i.e. columns in T )!! i  j1

© Professor Steven Errede, Department of Physics, University of Illinois at Urbana-Champaign, Illinois 7 2005-2015. All Rights Reserved. UIUC Physics 436 EM Fields & Sources II Fall Semester, 2015 Lect. Notes 2 Prof. Steven Errede n.b. Additional info is given on tensors/tensor properties at the end of these lecture notes…

     Now if the vector a in the dot product baT  “happens” to be a , then if:

11122 TEEEij o i j ij BBB i j  ij 22o  1122 1 TEEEEEBBBBB oj  jj   j  jj j     22o

Thus, we see that the total electromagnetic force per unit volume (aka force “density”) can be written (much) more compactly (and elegantly) as:     EM S 3 fTotTNm  o o , t

11122 Maxwell’s stress tensor: TEEEij o i j ij BBB i j  ij 22o  1  Poynting’s vector: SEB o

The total EM force acting on the charges contained within the (source) volume v is given by:    S FfdTEM EM    d  Totvv Tot o o t

Explicit reminder of the (suppressed) arguments:

    Srt, FtEM f EM  rtd,,  Trt   d   Totvv Tot o o t

Using the divergence theorem on the LHS term of the integrand:        EM  S EM  Srt, FTdadTot  o o i.e. FtTot Trtda ,   o o d  Svt Svt

      EM  d EM d  Finally: FTot T da o o Sd i.e. FtTot Trtda ,,   o o Srtd  Svdt Svdt

8 © Professor Steven Errede, Department of Physics, University of Illinois at Urbana-Champaign, Illinois 2005-2015. All Rights Reserved. UIUC Physics 436 EM Fields & Sources II Fall Semester, 2015 Lect. Notes 2 Prof. Steven Errede

      EM  d EM d  FTot T da o o Sd i.e. FtTot Trtda ,,   o o Srtd  Svdt Svdt

Note the following important aspects/points about the physical nature of this result:

  1.) In the static case (i.e. whenever Trt,, Srt , fcnst ), the second term on the RHS in the above equation vanishes – then the total EM force acting on the charge configuration contained within the (source) volume v can be expressed entirely in terms of Maxwell’s stress tensor at the boundary of the volume v , i.e. on the enclosing surface S :

   F EM Tr da fcnt  Tot S   2.) Physically, Trt,  da = net force (SI units: Newtons) acting on the enclosing surface S . S  2 Then {here } T is the net force per unit area (SI units: Newtons/m ) acting on the surface S – i.e. T corresponds to an {electromagnetically-induced} pressure (!!!) or a stress acting on the enclosing surface S .

2 th 3.) More precisely: physically, Tij represents the force per unit area (Newtons/m ) in the i direction acting on an element of the enclosing surface S that is oriented in the jth direction.  Thus, the diagonal elements of T(i = j): Tii = Txx, Tyy, Tzz physically represent pressures. The off-diagonal elements of T (i ≠ j): Tij = Txy, Tyz, Tzx physically represent shears.  Griffiths Example 8.2: Use / Application of Maxwell’s Stress TensorT

Determine the net / total EM force acting on the upper (“northern”) hemisphere of a uniformly electrically-charged solid non-conducting sphere (i.e. uniform/constant electric charge volume 4 3 density   QR3  ) of radius R and net electric charge Q using Maxwell’s Stress Tensor T (c.f. with Griffiths Problem 2.43, p. 107). zˆ

nrˆˆsphere  Sphere of radius R

R

 yˆ nzˆ ˆ disk Disk of radius R xˆ lying in x-y plane

 n.b. Please work through the details of this problem on your own, as an exercise!!! 

  First, note that this problem is static/time-independent, thus (here): FTrdaEM   Tot S

  1 2 Note also that (here) Br 0 and thus (here): TEEEij o i j ij {only}. 2

© Professor Steven Errede, Department of Physics, University of Illinois at Urbana-Champaign, Illinois 9 2005-2015. All Rights Reserved. UIUC Physics 436 EM Fields & Sources II Fall Semester, 2015 Lect. Notes 2 Prof. Steven Errede

The boundary surface S of the “northern” hemisphere consists of two parts – a hemispherical bowl of radius R and a circular disk lying in the x-y plane (i.e.    2 ), also of radius R.  a.) For the hemispherical bowl portion of S , note that: daRd22cos drˆˆ R sin ddr  1 Q The electric field at/on the surface of the charged sphere is: Er  rˆ rR 2 4 o R In Cartesian coordinates: rxyzˆˆˆsin cos sin sin cos ˆ

Thus:  1 Q Er  E r xˆˆ E r y  E r zˆ r ˆ rR xyzrRrR rR 4 R2 o 1 Q 2  sin cosxyzˆˆ sin  sin cos ˆ 4 o R

2 Q Thus: TTzxrR xzrR o EE z x  o2 sin cos cos 4 o R 2 Q TTzy r R yz r R o EE z y  o 2 sin cos sin 4 o R 2 11222Q 2 2 TEEEzz r R o z x y o 2 cos  sin 224 o R

The net / total force (due to the symmetry associated with this problem) is obviously only in the zˆ -direction, thus we “only” need to calculate:

  T dar R T zx da x T zy da y T zz da z r R z 

 Now: da R2 sin d d rˆ and rxyzˆˆˆsin cos sin sin cos ˆ

2 Then: dax   Rsin d d sin  cos  xˆ 2  day   Rsin d d sin  sin  yˆ since da dax xˆˆ dayz y da zˆ 2 daz   Rsin d d cos  zˆ 2   Q Then: Tda   sin cos cos  R2 sindd sin  cos   o 2  z rR 4 R  o 2 Q   sin cos sin  R2 sindd sin  sin  o 2  4 R  o 2 1 Q   cos22 sin R 2sindd cos  o 2  2 4 R  o

10 © Professor Steven Errede, Department of Physics, University of Illinois at Urbana-Champaign, Illinois 2005-2015. All Rights Reserved. UIUC Physics 436 EM Fields & Sources II Fall Semester, 2015 Lect. Notes 2 Prof. Steven Errede

  Tda = 1 z rR 2 Q 2222221 o sin cos cos  sin  cos sin  cos  sin  cos  sin dd 42 o R  2 Q 1  sin2 cos cos22 sin cos  sindd o   4 o R 2 2 Q 1122  o sin cos cos  cos sin dd 42 o R  2 2 = 1 Q 1  o cos sin dd 42 o R  2 1 Q   o cos sin dd 24 o R

2  2    11QQ   FTdaEM  2cossin2 dzzˆˆ   bowl o  2 r R bowl z rR  0 24ooRR 4   8 1 11u1  udu u2  0 22u0  1 Q2 FzEM  ˆ bowl rR 2 48 o R b.) For the equatorial disk (i.e. the underside) portion of the “northern” hemisphere:

 dadisk  rdrd  zˆ  n.b. outward unit normal for equatorial disk (lying in x-y plane) rdrd zˆ points in zˆ direction on this portion of the bounding surface S

daz zˆ And since we are now inside the charged sphere (on the x-y plane/at    2 ):   11QQr  ErDisk 32  2 44RRR  oo 22 1 Q  ˆˆ 3 rxryrzsin cos sin  sin cos ˆ 4 R  o 110  2 1 Q 3 rxcos ˆˆ sin y 4 o R

Then for the equatorial, flat disk lying in the x-y plane:  2  11222Q 2 Tda  Tdazz z where: TEEEzz o z x y o r z 3 224 o R

© Professor Steven Errede, Department of Physics, University of Illinois at Urbana-Champaign, Illinois 11 2005-2015. All Rights Reserved. UIUC Physics 436 EM Fields & Sources II Fall Semester, 2015 Lect. Notes 2 Prof. Steven Errede

22   11QQ  T da T da r23 zzˆˆ rdrdz ˆ r drdz ˆ Thus: zz z o33   o   z 24RR 24  oo 

The EM force acting on the disk portion of the “northern” hemisphere is therefore:

2 2    11QQR FTdaEM  2 rdrzz3 ˆˆ  disk o 32 disk z  2 0 24ooR 416  R  2 EM 1 Q Fzdisk  2 ˆ 416 o R

The total EM force acting on the upper / “northern” hemisphere is:

 22 2 EM EM EM 11QQ   13  Q  FFFTOT bowl disk 22 zˆˆˆ    z   2  z 48ooRRR 416416     o  

   EM d  Note that in applying FTrtdaSrtdTOT,   o o , that any volume v that Svdt encloses all of the electric charge will suffice. Thus in above problem, we could equally-well have instead chosen to use e.g. the whole half-region z > 0 – i.e. the “disk” consisting of the  entire x-y plane and the upper hemisphere (at r = ∞), but note that since E  0 at r = ∞, this latter surface would contribute nothing to the total EM force!!! 2 o Q 1 Then for the outer portion of the whole x-y plane (i.e. r > R): Tzz  4 24 o r  2  o Q 1 Then for this outer portion of the x-y plane (r > R): T da Tzz da z drd z 3 24 o r

The corresponding EM force on this outer portion of the x-y plane (for r > R) is:

2 2   EM 111QQ FrRdisk o 2 drz ˆˆ  z R 32 24oorR 4   8

  22 2 EM EM EM 1113QQ   Q Thus: FFrRFrRTOT disk disk  22 zˆˆ  z   2z ˆ 41648oooRR   416   R

n.b. this is precisely the same result as obtained above via first method!!!

Even though the uniformly charged sphere was a solid object – not a hollow sphere – the use of Maxwell’s Stress Tensor allowed us to calculate the net EM force acting on the “northern” hemisphere via a surface integral over the bounding surface S enclosing the volume v 4 3 containing the uniform electric charge volume density   QR3  . That’s pretty amazing!!

12 © Professor Steven Errede, Department of Physics, University of Illinois at Urbana-Champaign, Illinois 2005-2015. All Rights Reserved. UIUC Physics 436 EM Fields & Sources II Fall Semester, 2015 Lect. Notes 2 Prof. Steven Errede  Further Discussion of the Conservation of Linear Momentum p

We started out this lecture talking ~ somewhat qualitatively about conservation of linear momentum in electrodynamics; we are now in a position to quantitatively discuss this subject.  nd    Newton’s 2 Law of Motion F t ma  t m dv  t dt  d mv t dt  dpmech  t dt  The total {instantaneous} force Ftacting on an object = {instantaneous} time rate of change of    dp t its mechanical linear momentum dp t dt i.e. Ft mech . But from above, we know that: dt

  dp t  d   FtEM mech Trtda,,   Srtd ** TotdtSv o o dt

 where pmech t = total {instantaneous} mechanical linear momentum of the particles contained in the (source) volume v . (SI units: kg-m/sec)

 1 2 We define: pEMt o o S  rtd,,  S rtd  {since c 1   in free space} vvc2 oo  where pEM t = total {instantaneous} linear momentum carried by / stored in the (macroscopic)  electromagnetic fields ( EB and ) (SI units: kg-m/sec). At the microscopic level – linear momentum  is carried by the virtual {and/or real} photons associated with the macroscopic E and B fields!

We can also define an {instantaneous} EM field linear momentum density (SI Units: kg/m2-sec):  1  rt,,, S  rt S rt = instantaneous EM field linear momentum per unit volume EM o o c2

 Thus, we see that the total {instantaneous} EM field linear momentum p trtd ,   EMv EM

  Note that the surface integral in ** above, Trtda,  physically corresponds to the total S {instantaneous} EM field linear momentum per unit time flowing inwards through the surface S .

Thus, any instantaneous increase in the total linear momentum (mechanical and EM field) = the linear momentum brought in by the EM fields themselves through the bounding surface S .

  dpmech t dp EM  t  1 Thus:   Trtda,  where: pEMt o o S  rtd,,  2 S rtd  dt dt S vvc  dp t dp t   Or: mech EM Trtda,  dt dt S

  But: pTottp mech  tpt EM    dp t dp t dp t   Expresses conservation of linear  Tot mech EM Trtda,   dt dt dt S momentum in electrodynamics

© Professor Steven Errede, Department of Physics, University of Illinois at Urbana-Champaign, Illinois 13 2005-2015. All Rights Reserved. UIUC Physics 436 EM Fields & Sources II Fall Semester, 2015 Lect. Notes 2 Prof. Steven Errede

Note that the integral formula expressing conservation of linear momentum in electrodynamics is similar to that of the integral form of Poynting’s theorem, expressing conservation of energy in electrodynamics and also to that of the integral form of the Continuity Equation, expressing conservation of electric charge in electrodynamics:

dUTot t dU mech t dU EM  t Poynting’s Theorem: Pt  Tot dt dt dt Energy Conservation

d   urturtdmech , EM  , Srtda , Srtd  ,  dt vSv   free rt,   Continuity Equation: Electric dJrtd   ,  vvt free Charge Conservation:

Note further that if the volume v = all space, then no linear momentum can flow into / out of v through the bounding surface S. Thus, in this situation:

  pTottp mech  tpt EM  = constant    dp t dp t dp t   dp t dp t and: Tot mech EM Trtda,0   mech EM dt dt dt S dt dt

We can also express conservation of linear momentum via a differential equation, just as we have done in the cases for electric charge and energy conservation. Define:   2 mech rt,  {instantaneous} mechanical linear momentum density (SI Units: kg/m -sec)   2 EM rt,  {instantaneous} EM field linear momentum density (SI Units: kg/m -sec)  1  rt,,, S  rt S rt EM o o c2   2 Tot rt,  {instantaneous} total linear momentum density (SI Units: kg/m -sec)   Totrt,,, mech  rt EM  rt Then:   p trtd ,   and p trtd ,   , thus: mechv mech EMv EM    p t  rtd,,,    rt   rt d  p tp  tpt  Totvv Tot mech EM Tot mech EM

 dp t dp t dp t   Then: TOT mech EM Trtda,  dt dt dt S

Using the divergence theorem on the RHS of this relation, this can also be written as:

dd d   Totrtd,,,, mech rtd EM  rtd T  rtd dtvv dt dt  vv

 rt,,   rt    Thus: mech EM Trt,0 d v  tt

14 © Professor Steven Errede, Department of Physics, University of Illinois at Urbana-Champaign, Illinois 2005-2015. All Rights Reserved. UIUC Physics 436 EM Fields & Sources II Fall Semester, 2015 Lect. Notes 2 Prof. Steven Errede  This relation must hold for any volume v , and for all points & times rt, within the volume v :

 rt,,   rt    Differential form of conservation  mech EM Trt,  tt of linear momentum

 rt,,   rt    mech EM Trt, (Linear momentum conservation) tt    Continuity Equation:  rt,, J  rt (Electric charge conservation) t     Poynting’s Theorem: urturtmech,, EM   Srt , (Energy conservation) t

  Thus, we see that the negative of Maxwell’s stress tensor, Trt, physically represents linear   momentum flux density, analogous to electric current density Jrt, which physically represents the electric charge flux density in the continuity equation, and analogous to Poynting’s vector   Srt, which physically represents the energy flux density in Poynting’s theorem.  rt,,   rt    Note also that relation mech EM Trt, , while mathematically tt describing the “how” of linear momentum conservation, says nothing about the nature/origin of why linear momentum is conserved, just as in case(s) of the Continuity Equation (electric charge conservation) and Poynting’s Theorem (energy conservation).

  Since Trt, physically represents the instantaneous linear momentum flux density (aka  momentum flow = momentum current density) at the space-time point rt, , the 9 elements of

{the negative of} Maxwell’s Stress Tensor, Tij are physically interpreted as the instantaneous EM field linear momentum flow in the ith direction through a surface oriented in the jth direction.

Note further that because TTij ji , this is also equal to the instantaneous EM field linear momentum flow in the jth direction through a surface oriented in the ith direction!  rt,,   rt    From the equation mech EM Trt, , noting that the del-operator tt     xˆˆ yz  ˆ has SI units of m1, we see that the 9 elements of Trt, x yz {= linear momentum flux densities = linear momentum flows} have SI units of:

length  linear momentum density/unit time = linear momentum density  length/unit time = linear momentum density  (length/unit time) = linear momentum density  velocity = {(kg-m/s)/m3}  (m/s) = kg/m-s2

  Earlier (p. 9 of these lect. notes), we also said that the 9 elements of Trt, have SI units of pressure, p (Pascals = Newtons/m2 = (kg-m/s2)/m2 = kg/m-s2). Note further that the SI units of pressure are also that of energy density, u (Joules/m3 = (Newton-m)/m3 = Newtons/m2 = kg/m-s2)!

© Professor Steven Errede, Department of Physics, University of Illinois at Urbana-Champaign, Illinois 15 2005-2015. All Rights Reserved. UIUC Physics 436 EM Fields & Sources II Fall Semester, 2015 Lect. Notes 2 Prof. Steven Errede

Griffiths Example 8.3 EM Field Momentum:

A long coaxial cable of length  consists of a hollow inner conductor (radius a) and hollow outer conductor (radius b). The coax cable is connected to a battery at one end and a resistor at the other end, as shown in figure below:

I  0 b outer a I zˆ

 0 V inner R V

ab,

The hollow inner conductor carries uniform charge / unit length  Q  and a steady  inner inner DC current I  Izˆ (i.e. flowing to the right). The hollow outer conductor has the opposite charge and current. Calculate the EM momentum carried by the EM fields associated with this system.

Note that this problem has no time dependence associated with it, i.e. it is a static problem.  The static EM fields associated with this long coaxial cable are:

 1  E   ˆ  x22y in cylindrical coordinates 2o   I   B  o ˆ n.b. E  and B only non-zero for ab 2 

   E  B S 

     a   b   a   b   a   b

zˆ  1  II ˆˆ ˆ Poynting’s vector is: SEB  22 22 z (for ab ) ooo44 

 Even though this is a static problem (i.e. no explicit time dependence), EM energy contained / stored in the EM fields {n.b. only within the region ab } is flowing down the coax cable in the zˆ -direction, from battery to resistor!

16 © Professor Steven Errede, Department of Physics, University of Illinois at Urbana-Champaign, Illinois 2005-2015. All Rights Reserved. UIUC Physics 436 EM Fields & Sources II Fall Semester, 2015 Lect. Notes 2 Prof. Steven Errede

The instantaneous EM power (= constant  fcn t ) transported down the coax cable is obtained  by integrating Poynting’s vector Sz   ˆ (= energy flux density) over a perpendicular / cross- 22 sectional area of A  ba, with corresponding infinitesimal area element  da  2 d zˆ :

  I  b 1 Ib PS da2ln d   22 a  42oo   a

 b But: V ln   EM Power PVI  = EM Power dissipated in the resistor! 2 o a  I Inside the coax cable (i.e. ab ), Poynting’s vector is: Sz  22ˆ 4 o

The linear momentum associated with / carried by / stored in the EM field(s) ( ab ) is:

  b   oI 1 pEM EM dSdz  o o    ˆ 2  d vv422  a

 o I b pEM  ln zˆ 2 a

 b 2 1 Using V ln  and c  we can rewrite this expression as: 2 o a  oo

  bVI  pEM o o ln Izˆˆ2 z 2 o ac    1 The EM field(s) E and B (via SEB ) in the region ab are responsible for o  transporting EM power / energy as well as linear momentum pEM down the coaxial cable! Microscopically, EM energy and linear momentum are transported down the coax cable by the  {ensemble of} virtual photons associated with the EB and fields in the region ab .

Transport of non-zero linear momentum down the coax cable might seem bizarre at first encounter, because macroscopically, this is a static problem – we have a coax cable (at rest in the lab  frame), a battery producing a static E -field and static electric charge distribution, as well as a steady  / DC current I and static B -field. How can there be any net macroscopic linear momentum?

The answer is: There isn’t, because there exists a “hidden” mechanical momentum:  Microscopically, virtual photons associated with macroscopic E and B fields are emitted (and absorbed) by electric charges (e.g. conduction / “free” electrons flowing as macroscopic current I down / back along coax cable and as static charges on conducting surfaces of coax cable (with potential difference V across coax cable). As stated before, virtual photons carry both kinetic  energy K.E. and momentum p .

© Professor Steven Errede, Department of Physics, University of Illinois at Urbana-Champaign, Illinois 17 2005-2015. All Rights Reserved. UIUC Physics 436 EM Fields & Sources II Fall Semester, 2015 Lect. Notes 2 Prof. Steven Errede

In the emission (and absorption) process – e.g. an electron emitting a virtual photon, again, energy and momentum are conserved (microscopically) – the electron “recoils” emitting a virtual photon, analogous to a rifle firing a bullet:

e virtual photon electron recoil

n.b. emission / absorption of virtual photons e.g. by an isolated electron is responsible for a phenomenon known as zitterbewegung – “trembling motion” of the electron…

The sum total of all electric charges emitting virtual photons gives a net macroscopic “mechanical” linear momentum that is equal / but in the opposite direction to the net EM field momentum. At the microscopic level, individually recoiling electrons (rapidly) interact with the surrounding matter (atoms) making up the coax cable – scattering off of them – thus this (net) recoil momentum (initially associated only with virtual photon-emitting electric charges) very rapidly winds up being transferred (via subsequent scattering interactions with atoms) to the whole/entire macroscopic physical system (here, the coax cable):

 hidden  pmech pEM

 phidden  p coax cable mech EM Total linear momentum is conserved in a closed system of volume v (enclosing coax cable):

 hidden   pptotEMmechEMEM p  p p 0

Now imagine that the resistance R of the load resistor “magically” increases linearly with time {e.g. imagine the resistor to be a linear potentiometer (a linear, knob-variable resistor), so we can simply slowly rotate the knob on the linear potentiometer CW with time} which causes the current I flowing in the circuit to (slowly) decrease linearly with time.

Then, the slowly linearly-decreasing current will correspondingly have associated with it a slowly linearly-decreasing magnetic field; thus the linearly changing magnetic field will induce   dB  d  an electric field - via Faraday’s Law - using either E  or Ed Bda  : dt CSdt  ind o dI t Et,ln Kzˆ {where K = a constant of integration} 2 dt    ind ind ind This induced E -field exerts a net force FtFab a   atF,,  b  bt  on the ±λ  ind ind charges residing on the inner/outer cylinders of the coax cable {where FQEiii , i = a,b} of:

 dI t dI t dI t b {n.b. points in Ftind oooln aKz ˆˆˆ ln bKz    ln z ab    ˆ 222dt dt dt a the z -direction}

18 © Professor Steven Errede, Department of Physics, University of Illinois at Urbana-Champaign, Illinois 2005-2015. All Rights Reserved. UIUC Physics 436 EM Fields & Sources II Fall Semester, 2015 Lect. Notes 2 Prof. Steven Errede

The total/net mechanical linear momentum imparted to the coax cable as the current slowly   decreases from I tI0 to It tfinal 0 , using dpmech  t Ftdt  is:

 pttmech final final  mech mech  mech  pdptpttpt    0 ptt  mechptmech 00  mech final final final final final final     0  ttfinal  tt final dI t b Find  t dt o  dt ln zˆ tt00ab  2 dt a

It()0 t oofinal bbI  VI  dI t ln zˆ  ln zzˆˆ 2 22It(0) I aa  c

 o I bVI  which is precisely equal to the original EM field momentumt  0 : pEM ln zzˆˆ2 . 2 ac Note that the coax cable will not recoil, because the equal, but opposite impulse is delivered to the coax cable by the “hidden” momentum, microscopically (and macroscopically), in just the same way as described above.

Note further that energy and momentum are able to be transported down the coax cable   1 because there exists a non-zero Poynting’s vector SEB 0 and a non-zero linear  o    2 momentum density EM  Srt, cdue to the {radial} electric field E in the region ab , arising from the presence of static electric charges on the surfaces of the inner & outer cylinders,  in conjunction with the {azimuthal} magnetic field B associated with the steady current I    flowing down the coax cable. If either E or B were zero, or their cross-product E  B were zero, there would be no transport of EM energy & linear momentum down the cable.

Recall that the capacitance C of an electrical device is associated with the ability to store  energy in the electric field E of that device, and that the {self-} inductance L of an electrical  device is associated with the ability to store energy in the magnetic field B of that device. We thus realize that:

 The capacitance of the coax cable CQV2ln o  ba QCV is responsible

for the presence of the surface charges   QAinner and   QAouter on the inner & outer conductors of the coax cable when a potential difference V is imposed between the inner/outer conductors, which also gives rise to the existence of the transverse/radial electric   field E V in the region ab . The energy stored in the electric field E of the coax cable is UCV1122 ln ba (Joules). E 24o 

   The {self-} inductance of the coax cable LI  o  ln  ba LIBda   is M 4 M S  associated with the azimuthal magnetic field B for the in the region ab resulting from the flow of electrical current I down the inner conductor. The energy stored in the magnetic  1 22o field B of the coax cable is ULIIbaM 28 ln  (Joules).

© Professor Steven Errede, Department of Physics, University of Illinois at Urbana-Champaign, Illinois 19 2005-2015. All Rights Reserved. UIUC Physics 436 EM Fields & Sources II Fall Semester, 2015 Lect. Notes 2 Prof. Steven Errede

 The total EM energy stored in the coax cable {using the principle of linear superposition} is the sum of these two electric-only and magnetic-only energies:

2 U U U 111 C V22 LI  2ln ba o I 2 ln ba  11 2  Ic  ln  ba Tot E M 224oo 8  42  

 EM power transport in a electrical device occurs via the electromagnetic field and necessarily     1 requires SEB 0 (i.e. both E and B must be non-zero, and must be such that they o  also have non-zero cross-product E  B ).

 EM power transport in a electrical device necessarily requires the utilization of both the capacitance C and the {self-}inductance L of the device in order to do so!

 The EM power transported from the battery down the coax cable to the resistor (where it is ultimately dissipated as heat/thermal energy) is: PIbaVI1  ln * (Watts = J/sec) 2o  {n.b. the EM power is proportional to the product of the charge {per unit length} and the   current I }. But VQC and I  L ; from Gauss’ Law Eda  Q  M EencloS we see that PIbaVIQCLCL1 ln * , 2o  MoEM i.e. EM power transport in/through an electrical device manifestly involves:

a.) the product of the electric and magnetic fluxes: EM and b.) the product of the device’s capacitance and its inductance: CL !!!

20 © Professor Steven Errede, Department of Physics, University of Illinois at Urbana-Champaign, Illinois 2005-2015. All Rights Reserved. UIUC Physics 436 EM Fields & Sources II Fall Semester, 2015 Lect. Notes 2 Prof. Steven Errede A (Brief) Review of Tensors/Tensor Properties:

TTT11 12 13  TTTxxxyxz   Maxwell’s stress tensor is a rank-2 3×3 tensor: TT TTT  TTT ij21 22 23  yx yy yz  i = row index TTT31 32 33  TTTzx zy zz j = column index  Note that since T is a “double vector” the above expression is actually “short-hand” notation for:   TTT11 12 13 TTTxx xy xz TxxTxyTxz xxˆˆ xy ˆˆ xz ˆˆ      TT TTT  TTT  TyxTyyTyzˆˆ ˆˆ ˆˆ ij21 22 23  yx yy yz yx yy yz TTT TTT TzxTzyTzzˆˆˆˆˆˆ 31 32 33 zx zy zz zx zy zz

Dot-product multiplication of a tensor with a vector – there exist two types:

    3  1.) baT  : baTaTaTjiijiij n.b. summation is over index i (i.e. rows in T )!! j  i1

 The matrix representation of vector aaaa 123 = (1×3) row vector, and also vector    bbbb 123= (1×3) row vector. Thus,baT  can thus be written in matrix form as:

TTT11 12 13  bbb aa a T T T 1 2  3 1 2   3 21 22 23 13 13 TTT 31 32 33 33 aT  aT  aT aT  aT  aT aT  aT  aT 1 11 2 21 3 31 1 12 2 22 3 32 1 13 2 23 3 33 13    3  2.) cTa  : cTaiijjijj TaTa  n.b. summation is over index j (i.e. columns in T )!! i  j1

a1   Here, the matrix representation of vector aa = (3×1) column vector, and also vector 2  a3

c1    cc = (3×1) column vector. Thus, cTa  can thus be written in matrix form as: 2  c3

cTTTaaTaTaT11112131111212313      cTTTaaTaTaT 22122232121222323   cTTTaaTaTaT   33132333131232333    3133 31  31

© Professor Steven Errede, Department of Physics, University of Illinois at Urbana-Champaign, Illinois 21 2005-2015. All Rights Reserved. UIUC Physics 436 EM Fields & Sources II Fall Semester, 2015 Lect. Notes 2 Prof. Steven Errede

  Note that if T is a symmetric tensor, i.e. TTij ji , then cTa  can be equivalently written in matrix form as:

c1 TTT 1112131  a aTaTaT 111212313111221331  aTaTaT     c TTT a aTaTaTaTaTaT 2 21 22 23  2 1 21 2 22 3 23  1 12 2 22 3 32 cTTTaaTaTaTaT  aT aT 3 31 32 33  3  1 31 2 32 3 33  1 13 223 333 3133 31  31 31

 But caT  written in matrix form is:

TTT11 12 13  ccc aaaT T T 1 2  3 1 2   3 21 22 23 13 13 TTT 31 32 33 33 aT  aT  aT aT  aT  aT aT  aT  aT 1 11 2 21 3 31 1 12 2 22 3 32 1 13 2 23 3 33 13

   Thus, we see/learn that if T is a symmetric tensor, then: caTTa.

  If T is not a symmetric tensor, then: aT T a.

In general, matrix multiplication AB BA because matrices A and B do not in general commute.

Note further, from the above results, we also see/learn that:    The vector baT  is not in general parallel to the vector a .    Similarly, the vector cTa  is (also) not in general parallel to the vector a .   The divergence of a tensor T is a vector, and has the same mathematical structure as that of   baT  :   3 T  T ij n.b. summation is over index i (i.e. rows in T )!! j  i1 ri

TTT11 12 13  TTTxx xy xz      TTTTTTT21 22 23  yx yy yz  xyz  xyz    13 TTT31 32 33 TTTzx zy zz 13  13  33 33

TTxxTTyx zx  xy TTyy zyTxz T yz T     zz xyz  xy zxyz   13   TTT   xyz 13

22 © Professor Steven Errede, Department of Physics, University of Illinois at Urbana-Champaign, Illinois 2005-2015. All Rights Reserved.