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Can capacitors receive EM energy?

  1. Nov 7, 2016 #1
    Capacitor as Receiver.png
    Dynamos and transformers have inductor coils reacting with changing magnetic fields and importing energy into the circuit in form of induced current
    1. What about the counterpart of above principle in case of capacitors?
    2. Will capacitors interact with electric,magnetic, or EM fields creating emf in the circuit connected ?
    3. If so then why we don't have such generators?
  2. jcsd
  3. Nov 7, 2016 #2


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    Capacitors are mostly self-shielding, so the answer is no AFAIK. Do you see where the E-field is in a capacitor?
  4. Nov 7, 2016 #3


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    If the capacitor terminals form a large closed conductive loop then a changing magnetic field will cause a small AC voltage on the capacitor as an induced AC current flows.
    There will be no DC energy received by the capacitor unless something weird happens like the dielectric of the capacitor behaved like a polarised diode material.
  5. Nov 8, 2016 #4
    hmmm so unlike inductors, the capacitors can never react with EM fields right?

    but I wish for more theoretical explanation in this thread
  6. Nov 8, 2016 #5


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    The only possible example I can think of is the varicap/varactor diode. The width of the depletion region (and hence the capacitance) changes depending on the applied reverse bias DC Voltage/Electric field. They are used to tune radio receivers.

  7. Nov 8, 2016 #6


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    You can't say never.

    The capacitance is proportional to the area and inversely proportional to separation of the plates. So they are frequently made of layers of thin foil and thin separators rolled up so as much area as possible fits into the can. The can is sometimes metal. Look up Faraday cage.

    Some radios are tuned by air separated variable capacitors. These are less well screened...


    Some capacitors have wire leads and these can have a parasitic or self inductance. Can be an issue at very high frequencies.
  8. Nov 8, 2016 #7


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    The magnetic field is allowed to move and induce AC current in an inductor.
    A capacitor has a fringing field so it can pick up the local electric field, which if repeatedly reversed, would represent an AC electric field across the capacitor.

    Both the positive reactance inductor and the negative reactance capacitor will be effected by EM fields that enter into their space. They are scatterers or antennas like everything else. They both receive AC energy.

    Without a rectifier, neither will produce DC. The component must be in a circuit.

    I think this is a silly thread, probably because it can go nowhere.
  9. Nov 8, 2016 #8

    jim hardy

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    May i try to answer a question with a question ?

    What happens if we bring a point charge into proximity of your parallel plate capacitor?

    Looks to me like charge will move around in the circuit .

    Does location of circuit common (my green symbol) make a difference ? On opposite side from the charge as shown, or on same side(move it to the left side) ?

    ............... on a practical note...

    Many parallel plate film capacitors are made by applying a strip of metal foil to both sides of a film dielectric and rolling it up into a tube.
    That makes a cylindrical capacitor with one foil on the outside of the cylinder.

    Like this.


    a real one...


    The stripe on left indicates which end is the outside foil.
    I believe in placing such capacitors with the outside foil at point in the circuit that's at lowest voltage with respect to circuit common , or chassis ground if you can.
    That's so if something rubs through the insulating overwrap it's less of a transient to the circuit.
    I believe if you analyze my question above you'll conclude the left hand plate can shield the right hand plate provided it's on the 'grounded' side of the circuit.
    Move green ground to left . That holds left plate at zero volts referenced to circuit common so nothing else in the circuit gets perturbed .

    ------------------------------------Oh No, Not Another Boring Anecdote ! ------------------
    We had an instrument in the plant with one of those rolled foil capacitors touching the metal instrument cover. After a couple decades of vibration (the turbine shakes everything) it wore a hole through the outer insulation that exposed a tiny piece of the outside foil , smaller than the head of a pin.
    It made contact one night shorting the circuit to earth ground and tripped the plant. Luckily it stayed grounded long enough for us to find it with a voltmeter. It was barely visible to the naked eye .

    The other(the inner) foil was connected to chassis ground.
    Had the capacitor been soldered in facing the opposite direction nothing would have happened when it shorted because contact would have been to the already grounded outer foil.
    --------------- Okay i'm done. Thanks for reading.------------

    Improvements to my simplistic explanation are welcome. Is there an electrostatics person in the house?

    old jim
    Last edited: Nov 8, 2016
  10. Nov 8, 2016 #9


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    Usually, inductors are closed to confine their magnetic fields and so avoid interference with nearby components. But an inductor designed as part of a generator is made to be open so that a changing magnetic field can cut the inductive windings and so generate an EMF.
    Usually capacitors are designed to be closed, that is not to radiate fringing fileds or pick up stray fields. But if the capacitor is open it can extract power from EM waves.

    A T-antenna has a flat ground mat with a flat hat or horizontal wire above. That makes two plates of a capacitor. A vertical wire runs from the middle of the ground mat to the middle of the hat. That wire looks like a stem. That stem is a conductor that connects the capacitor plates, but it is a conductor with length and inductance. The antenna is resonant when the reactance of the positive inductive stem cancels the negative reactance of the capacitance between the plates. At that frequency the capacitor voltage will resonate if subjected to EM radiation having energy in that part of the spectrum. A light globe connected in the stem wire will glow when sufficient EM energy is available.
  11. Nov 8, 2016 #10


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    In Audio and low-level instrumentation amplifier circuits, the outside foil is connected in the circuit with the lowest impedance to ground. That reduces pickup of hum and extraneous noise.
  12. Nov 9, 2016 #11


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    I'm not so sceptical about this idea as previous respondents, though I must admit, I don't understand some of the responses.
    As far as I can see all antennae have both inductive and capacitative elements and produce or respond to both electric and magnetic components of em fields.
    Presumably most are designed on the basis of producing magnetic fields, because all the analysis software seems to work on summing the effects of current elements.

    There are people who claim to design capacitative antennae and something called a cross field antenna (described as, " to synthesise directly radiated Poynting vectors from separate E and H field sources." ), but these are not widely accepted (to put it mildly!), though G(M)3HAT did get a CFA design published in Wireless World about 27 years ago.

    What strikes me as nearer to the OP concept, are the slot and patch antennas. Just as with the above types I can't claim any real understanding of these, but some explanations make them sound like E-field radiators.
    (In this link Slot Antennas the video (a very informal description) shows the slot having a large E-field and any current effects more or less cancelling.
    Microstrip (patch) antennas seems to concentrate solely on the E-field, as does their video.)

    If indeed the basic concept of E-field radiation is ok, then I wonder if it's a matter of practicality that we use current mode antennae?
    When λ was 10m - 1000m+ , wires would be much more convenient than sheets (or even grids) of conductor, especially when it needed to be elevated.

    And there is the same factor that affects component values in tuned circuits. For efficiency and Q the inductive impedance needs to be high compared with the Ohmic resistance. Which means capacitance values need to be kept small. If you built an antenna with a large capacitative element and commensurately low inductance, then impedances are low and currents and Ohmic losses high.

    When I look at making a capacitor to use as an antenna, I keep coming up against the problem of currents. To charge a capacitor requires current and currents cause magnetic fields. However I try to balance these to cancel the net field, the current must go in opposite sense to the two plates. So it could always be argued that the radiation comes from this. (Capacitative radiators are likely to be less efficient due to the effect I mentioned above, so radiation could be attributed to even a small inductive path.) Perhaps accurate measurement of the radiation pattern (and maybe polarisation) could discriminate between the sources of radiation.

    It is certainly an intriguing question, but whether it is interesting may depend on finding some practical advantage of this approach. For G3HAT's CFA and DL7JV's capacitive antennas I think the intent was a much reduced size. But I think with so much focus now on UHF and SHF, interest is more in gain, directivity and efficiency.
  13. Nov 9, 2016 #12


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    Yes, there are capacitive antennas types. The Dielectric resonator is an example. These can be used in cases where EMP or HPM immunity is necessary.
  14. Nov 9, 2016 #13


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    The OP began with a magnet moving near an open inductor, then tried to compare it with an EM wave and a closed capacitor. To be fair, a wide open capacitor should have been influenced by a changing electric field.

    Every point on an antenna has a particular impedance that relates the ratio of voltage to current at that point. A transmit antenna with a high impedance has big voltages that tend to burn the insulation with corona discharge, one with low impedance has big currents that heat the conductors. Standing waves on a half-wave dipole burns the tips of the dipole while heating the conductor near the centre. The addition of a small circular capacity hat at the tips of a dipole reduces the impedance and voltage breakdown. That begins to approach the T-antenna I referred to earlier as a capacitor.

    The short external whip antenna used on a car for MW reception has a high impedance. You could say that it picks up the E-field, but like Yin and Yang there will always be the Electric and Magnetic.

    A dipole and a slot in a sheet with the same dimensions, are complementary antennas. The impedance in ohms of one is the reciprocal of the other multiplied by 188.4. Self-complementary antennas are symmetrical and have an impedance of 188.4 ohms = Zo/2.

    You can model antennas by either the currents or the voltages. The currents are easier to model because there is little magnetic material around antennas, but there are often electrical insulators present. Electrical insulation such as layers of oxide or paint distort the E field and make computation of surface impedance much more difficult. Traditionally the surface currents have been modelled, it is easier to keep it that way.

    Patch antennas are used at UHF and above, where a low profile or phased array is required, such as on the external surface of an aircraft. They operate on the same EM principle as any other antenna.
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