What is the Physical Explanation Behind the Lit Bulb Under an Outside Conductor?

[brianparks]

Consider the following electrodynamics problem:

A 765 kV rms, 3 phase, 60 Hz transmission line has conductors spaced 16m apart. Their height is 12m above ground. Each conductor is a bundle of smaller conductors with a diameter of .6m. A fluorescent lamp bulb held 2m above ground at point P under an outside conductor lights to full brilliance. No wires are connected to the bulb. What is the magnitude of the rms electric field at P?

I know how to solve the problem mathematically, but its physical implications do not make sense to me.

If the bulb is lit, then current must be flowing across its terminals. But its terminals are not electrically connected; there are no wires across them. How, then, can there be current flow? Does current flow through the air or through some other medium that might connects the terminals?

Thanks for any help,
--Brian

 

[chroot]

Energy is being transported through the changing electric field. The moving electrons in the transmission line create a changing electric field. That changing electric field prompts electrons in the fluorescent tube to move about as well, producing light. It's AC, so the same electrons just move back and forth through the tube 60 times per second.

Transformers and capacitors operate on the same principle -- no electrons actually go from one side of a transformer or capacitor to the other, but both can carry energy from one side to the other through an electric field.

This really isn't as bizarre as you might think at first -- imagine standing under a heat lamp -- no electrons are moving between you and the lamp, yet the motion of the electrons in the lamp sets up a changing electric field that causes electrons in your body to move about, heating you up.

In the same way, a radio transmission tower puts out radio waves which cause the electrons in your car's antenna to move about, allowing you to receive the signal.

- Warren

 

[pervect]

Consider the following electrodynamics problem:

I know how to solve the problem mathematically, but its physical implications do not make sense to me.

If the bulb is lit, then current must be flowing across its terminals. But its terminals are not electrically connected; there are no wires across them. How, then, can there be current flow? Does current flow through the air or through some other medium that might connects the terminals?

--Brian

There is some capacitance from the end of the bulb to the wires. So the current flows through the capacitor.

How does current flow "through" a capacitor? That's the displacement current turm in Maxwell's equations. There is nothing physically moving, but there is a rate of change of electric field, \frac{\partial E}{\partial t}

 

[brianparks]

Thanks for the responses.

Would it be correct to say, then, that if the power line were constant DC, that the same "lighting" effect would not occur?

 

[chroot]

That's correct. Of course, then you could make the electric field change through your fluorescent tube by running around with it! (I'm being half serious -- you'd get some movement of electrons, but not enough to noticably light the tube.)

- Warren

 

[Metallicbeing]

Try this experiment. It will illustrate what Chroot is talking about. Get a small compact fluorescent lamp. Put it in your microwave oven. Start the oven only long enough to light the lamp (otherwise, you'll damage the oven).

The electrons inside the tube were excited by the microwave energy. When the electrons collide with the mercury in the tube, it emits ultraviolet light which makes the phosphorous coating inside the tube glow. Notice there were no electrical connections.