Displacement Current



" The Stone Age didn't end because the World ran out of stones"
Niels Bohr
Gauss' Law:   elecgausseqn3

Ampere's Law:  magampereeqn1

Faraday's Law:  magfaradayeqn6

Nobody's Law:  magmonopolefig5

"A changing magnetic field ( magdisplacementeqn1 ) gives rise to an electric field ( magdisplacementeqn2 )"

For a more complete correspondence between E and B we would expect a term added to the right hand side of Ampere's Law which indicates,

 "A changing electric field ( magdisplacementeqn3)  gives rise to a magnetic field ( magdisplacementeqn4 )"

  • Maxwell proposed this addition, the existence of which is now physically verified.  Furthermore, Maxwell showed that this additional term, known as the displacement current, is essential for the description of electromagnetic waves.

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Inconsistency in Ampere's Law

  • Ampere's Law can be written
magdisplacementeqn15
magdisplacementfig3where we have made use of the general relationship between current and current density,
eleccurrenteqn3

Note that the surface over which the flux of J is evaluated, S, can be any open surface bounded by the closed Amperian loop C.

In the figure at right, showing a schematic of a parallel plate capacitor being charged, 4 possible surfaces are shown bounded by a single Amperian loop.  For each of these surfaces the flux of J must give the same current.  As indicated, surfaces 1, 2 and 4 are "pierced" by the current I.  However, as can be seen, no current passes through surface 3. 

What to do about this inconsistency ?


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Fixing Ampere's Law

  • The existence of a Displacement Current "flowing" between the plates of the capacitor, passing through surface 3, is the solution.  The displacement current through surface 3 must be equal to the "normal" (conduction) current passing through surface 1.
  • The Displacement Current through surface 1 can be written,
magdisplacementeqn6

Therefore, if we define
magdisplacementeqn7
as the displacement current, by writing
magdisplacementeqn8
we can ensure this inconsistency in Ampere's Law is removed.
  • Therefore, including the displacement current, Ampere's Law becomes,



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Parallel Plate Capacitor

  • magdisplacementfig2For a parallel plate capacitor we have,
magdisplacementeqn10
where,
magdisplacementeqn11
therefore,
magdisplacementeqn12
So that the displacement current id is equal to the conduction current bringing charge to the plates, i.

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  • exclamation The displacement current can be considered to be distributed throughout the volume between the plates.  The associated magnetic field can be calculated in a similar manner to that due to a thick wire carrying (conduction) current.
  • exclamation The direction of the magnetic field created by the displacement current can be found by applying the right-hand rule in the usual way - thumb points in the direction of the (displacement) current, fingers wrap around in the direction of the magnetic field.  The direction of the displacement current itself is in the same direction as E if E is increasing with time (dE/dt > 0).  When E is decreasing with time the displacement current is opposite to E (dE/dt < 0).
  •   So why didn't Faraday and others detect the displacement current confused smiley 

The presence of the displacement current would be signaled by detection of its associated magnetic field.  Assuming only a displacement current we can write Ampere's Law,

magdisplacementeqn13

so that,
magdisplacementeqn14

Thus, unless dE/dt and A/l are very large, B will be very small.  In Faraday's time it was technologically impossible to vary the E field fast enough to obtain a measurable B.

Famous Physicists at a party : Everyone gravitated toward Newton, but he just kept moving around at a constant velocity and showed no reaction.

 

Dr. C. L. Davis
Physics Department
University of Louisville
email: c.l.davis@louisville.edu