ARRL Extra Class Q&A #2 - Energy Stored in Electro-Magnetic Field?

[Tom WA3KLR]

Here is another question & answer from the ARRL Extra class 2002 question pool that I say has a picky technical semantic error in the question, not critical though and maybe some philosophical thought here too.  So I am interested in your expert comments.

“E5F11 What is the term for energy that is stored in an electromagnetic or electrostatic field?

A.   Ampere-Joules
B.   Potential energy
C.   Joules-coulombs
D.   Kinetic energy

B. Stored energy is potential energy as long as it is stored.  Don’t be confused.  In the metric system, energy is measured in joules, but that is not a given choice.”

I have no argument with the correct answer.  In the question, the statement of energy that is stored in an “electromagnetic or electrostatic field” is not correct.  It doesn’t affect the answer though.  “Electro-magnetic field” refers to co-existing electrostatic AND magnetic fields, so I think the question should say either “What is the term for energy that is stored in a magnetic or electrostatic field?” OR “What is the term for energy that is stored in an electro-magnetic field?”  End of my correction.

Now, perhaps getting philosophical here, it is common to say and teach (in colleges) that an inductor stores energy in a magnetic field.  To do that you have to maintain a steady current in the inductor.  Can we store a magnetic field?  Is this what a permanent magnet is doing?

We can think of an ideal inductor with no resistance and a battery in series with a resistor connected to the inductor.  Then there is an ideal switch connected across the series resistor and battery.  When the current stabilizes, we throw the switch on and short the inductor leads, and take away the battery and resistor.  The current loop runs forever?  Since no resistance in the loop, no voltage potential, no current flow?

I guess I have trouble with the definition of the word store too.  Can you really store energy in a magnetic field, or just loan it out?  Any Physics gurus out there with comments?

All fluxed up near Phillie.

[N8LGU]

Tom, I think you are right. No resistance=No Voltage=No Current Flow. But wait a minute- What about an inductor submersed in liquid Nitrogen? I've heard that the current would flow indefinitly would it not be for induced losses to the surrounding environment. These examples are, of course theoretical and not achievable. In reality, You can't have one (E) without the other (R)... I think...

[Rob K2CU]

Tom,

Interesting question that has been raised many times in physics lecture halls. If you limit the discussion to magnetic fileds produced by electric induction, that is the flow of electricity, then the first thing to be understood, is that electrostatic fields, by their very name mean stationary fields. We all know this as the example of a charged up capacitor sitting on the bench and not connected to anything. If there is no leakage, it will stay charged up forever with potential energy. In the case of an electrically induced magnetic field, a soleniod for example, a static field means no change in the current flow. In your example, you could have just forgotten about the resitor and used a current source, but that's another issue. When the switch is closed, the current in the ideal (lossless) inductor will continue to flow. With the assumption that the switch and circuitry also have zero ohms resistance, there would be no IxR loss and the current would flow forever.  Remember, for an inductor, the operative word is "change" in the current flow.  Even for an ideal inductor any change in current invloves a voltage. The voltage is found from:

V = L dI/dT,

which you probably already know.  dI is the change in current, and dT is the amount of time for the change.  So basically, no voltage is required to keep the current flowing as there is no change in the current in the inductor. IF you had even a small resitance in the external circuit, the resulting voltage would end up applying a voltage on the inductor such as to cause a negative change in the current.  Since that resistive voltage becomes less as the current is reduced, you get the familiar exponential decay.

As to the test question, and symantics, they refer to "electromagnetic or electrostatic filed." The point is that both are in the singular. Not "electromagnetic fields."  I would interpret the signular to mean a static electrically induced magnetic field, and the plural to mean the fileds associated with wave propagation.  But it's just an opinion.

[WA1GFZ]

Hey Rob, if Max can reduce ground losses does it mean he releases stored energy?
or is it recycled energy or maybe reconfigured energy???

[AB9IL]

Hi Tom,

Rob pretty well nailed the answer I was pondering here.  The question pool has ways of phrasing things that will throw one off.  Many of those choices are deliberate destractors, but sometimes pooly chosen, as in the PLL question.

In this case, the generic "potental energy" works - but it isn't stored in the conventional sense.  Instead of storage in a spring, rubber band, bomb, or battery, it is in a field...when the field reaches a distant antenna, the energy is going to make a current flow.  It'll even exert an incredibly small force on charged particles in the field.  So storage here means more "vessel of transport" than "vessel to hold in reserve."  My old physics instructors would ramble about how energy is transferred and stored in fields, but that some fields are easier to isolate and concentrate than others...