- #1
rmatei
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Hello,
I am trying to fully wrap my head around signal shielding so as to take it from the trial-and-error domain to the theoretical design domain. The problem is that I am stuck on one of the very basic and fundamental principles of shielding - the floating conductor as an electrostatic shield.
I am reading a textbook "Grounding and Shielding Techniques in Instrumentation" by Ralph Morrison. According to Morrison, any fully-closed conducting surface is enough to keep external shields out and internal fields in. The reason a shield should be tied to reference has to do with feedback gain elements. Colleagues in my field always say that a shield should be grounded so that shield currents "have a place to drain to," but I don't believe this is correct according to Morrison's explanation.
Lets say we have a conductor (small sphere or point charge) with excess + charge surrounded by a floating conductor shell with no excess charge. How is there no field outside of the shell? Two theories tell me that there must be a field outside.
1: Gauss's law - The total charge inside the entire volume is not zero due to the excess charge of the inner conductor. Then, the surface integral cannot be zero, meaning that the electric field is emanating away from the system. A time-varying field in the inner conductor would still generate an EM wave outside of the system.
2: Circuit theory - (capacitors shown)
------||------||---------
a...b...c
if 'a' is the inner conductor, it will have a capacitance to the outer conductor, 'b', and that outer conductor will have a capacitance to another conductor, 'c', such as ground. Just because we added a capacitor in series doesn't nullify the capacitance between 'a' and 'c' at all.
Am I misunderstanding Morrison? I would think that a grounded or voltage-controlled shield would be mandatory, because the charges in the outer conductor would migrate into/out of ground or voltage controller, establish charges equal in magnitude and opposite in polarity on the outer conductor, and Gauss's law would yield a surface integral of 0. Then, external fields remain external, internal fields remain internal.
What am I missing about the floating/unreferenced shield? How is it effective? Does it just slightly reduce capacitance?
Thanks to all in advance,
Radu
I am trying to fully wrap my head around signal shielding so as to take it from the trial-and-error domain to the theoretical design domain. The problem is that I am stuck on one of the very basic and fundamental principles of shielding - the floating conductor as an electrostatic shield.
I am reading a textbook "Grounding and Shielding Techniques in Instrumentation" by Ralph Morrison. According to Morrison, any fully-closed conducting surface is enough to keep external shields out and internal fields in. The reason a shield should be tied to reference has to do with feedback gain elements. Colleagues in my field always say that a shield should be grounded so that shield currents "have a place to drain to," but I don't believe this is correct according to Morrison's explanation.
Lets say we have a conductor (small sphere or point charge) with excess + charge surrounded by a floating conductor shell with no excess charge. How is there no field outside of the shell? Two theories tell me that there must be a field outside.
1: Gauss's law - The total charge inside the entire volume is not zero due to the excess charge of the inner conductor. Then, the surface integral cannot be zero, meaning that the electric field is emanating away from the system. A time-varying field in the inner conductor would still generate an EM wave outside of the system.
2: Circuit theory - (capacitors shown)
------||------||---------
a...b...c
if 'a' is the inner conductor, it will have a capacitance to the outer conductor, 'b', and that outer conductor will have a capacitance to another conductor, 'c', such as ground. Just because we added a capacitor in series doesn't nullify the capacitance between 'a' and 'c' at all.
Am I misunderstanding Morrison? I would think that a grounded or voltage-controlled shield would be mandatory, because the charges in the outer conductor would migrate into/out of ground or voltage controller, establish charges equal in magnitude and opposite in polarity on the outer conductor, and Gauss's law would yield a surface integral of 0. Then, external fields remain external, internal fields remain internal.
What am I missing about the floating/unreferenced shield? How is it effective? Does it just slightly reduce capacitance?
Thanks to all in advance,
Radu