Obtain the continuity equation (Eq. 12.126) directly from Maxwell’s equations (Eq. 12.127).

Short Answer

Expert verified

The continuity equation is obtained as .δJμδXμ=0

Step by step solution

01

Expression for Maxwell’s equation:

Using equation 12.127, write the expression for Maxwell’s equation.

δFμνδXν=μ0Jμ …… (1)

It is known that:

δδXv=δv

Substitute δvfor δδXvin equation (1).

δvFμν=μ0Jμ…… (2)

02

Determine the continuity equation from Maxwell’s equation:

Differentiate the equation (2).

δvδμFμν=μ0δμJμ

From the above equation, observe the symmetric and anti-symmetric combination.

δμδv=δνδμ(symmetric)Fμν=-Fμν(Anti-symmetric)

Since it is known that:

δμδvFμν=0

As the above indices are summed from 0 to 3, the term and v can be pronounced as the same. Hence,

δμδvFμν=δvδμFμν=δμδv(-Fμν)=-δμδvFμν

Now, as the above quantity is equal to minus itself, it must be zero. Hence,

δJμδXμ=0

Therefore, the continuity equation is obtained as .δJμδXμ=0

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Most popular questions from this chapter

(a) Construct a tensor Dμυ(analogous to Fμυ) out of Dand H. Use it to express Maxwell's equations inside matter in terms of the free current density Jfμ.

(b) Construct the dual tensor Hμυ(analogous to Gμυ)

(c) Minkowski proposed the relativistic constitutive relations for linear media:

Dμυηυ=c2εFμυηυ andHμυηυ=1μGμυηυ

Where εis the proper permittivity, μis the proper permeability, andηυ is the 4-velocity of the material. Show that Minkowski's formulas reproduce Eqs. 4.32 and 6.31, when the material is at rest.

(d) Work out the formulas relating D and H to E and B for a medium moving with (ordinary) velocity u.

(a) In Ex. 12.6 we found how velocities in thex direction transform when you go from Sto S. Derive the analogous formulas for velocities in the y and z directions.

(b) A spotlight is mounted on a boat so that its beam makes an angleθ with the deck (Fig. 12.20). If this boat is then set in motion at speedv, what angleθ does an individual photon trajectory make with the deck, according to an observer on the dock? What angle does the beam (illuminated, say, by a light fog) make? Compare Prob. 12.10.

Obtain the continuity equation (Eq. 12.126) directly from Maxwell’s equations (Eq. 12.127).

(a) Write out the matrix that describes a Galilean transformation (Eq. 12.12).

(b) Write out the matrix describing a Lorentz transformation along the yaxis.

(c) Find the matrix describing a Lorentz transformation with velocity v along the x axis followed by a Lorentz transformation with velocity valong they axis. Does it matter in what order the transformations are carried out?

An ideal magnetic dipole moment m is located at the origin of an inertial system S¯ that moves with speed v in the x direction with respect to inertial system S. InS¯ the vector potential is

A¯=μ04πm¯×r^¯r¯2

(Eq. 5.85), and the scalar potentialV¯ is zero.

(a) Find the scalar potential V in S.

(b) In the nonrelativistic limit, show that the scalar potential in S is that of an ideal electric dipole of magnitude

p=v×mc2

located atO¯ .

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