Electric Field

  • Thread starter Cyrus
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  • #26
Tide
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cyrusabdollahi said:
But a capacitor is NOT a complete path, there is a break in it! So even if BOTH ends are hooked up, there is no complete circuit is there?

The light bulb problem isn't a complete path either! If you want, connect a wire to the second terminal on the battery and bring it close to but not touching the other "connector" on the light bulb. It still won't light up even though you can consider the air between the connector to be a dielectric effectively making the system a capacitor!

You need potential difference to drive charge onto the capacitor plates.
 
  • #27
reilly
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First of all, equipotential surfaces are, for the most part, convenient mathematical fictions. Certainly for any static finite charge distribution, continuous or discrete, there are an infinity of equipotential surfaces -- obviously spheres with a single point charge at the center are equipotential surfaces. There are a huge variety of surfaces that can be equipotentials -- see, for example, Smythe's Static and Dynamic Electricity, chap. V.

But there is Earnshaw's Thrm, which says there is no stable equilibrium for charged particles subject to electrical forces. when we say that a conductor's surface is an equipotential, we necessarily assume that there are non-electrical forces at play to keep the charges from flying off the conductor. QM goes a long way toward explaining these containing forces -- in QM can have electrical forces involved, which lead to stationary states. In classical physics, standard classical E&M, these containing forces are simply assumed to exist, as necessary -- see older E&M books for more on this topic. For the most part, classical E&M does not deal with the granularity of matter.

Remember that classically a hydrogen atom cannot exist, except briefly as the electron spirals into the proton.


Be careful about the conductivity of a capacitor. Capacitors do conduct alternating current, and if you make the gap between the terminal and the wire (cf #26) small enough, the current will flow -- maybe it will even arc.

Regards,
Reilly Atkinson
 

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