# Moving electric field effect on dielectric/gas

• artis
In summary, the conversation discusses the potential for a moving electric field to produce a drag force on gas, using the example of two charged wires dragged along the length of a pipe. The idea is that the motion of the edge of the dielectric slab will determine the percentage of the field that sees a polarizable medium. The equation of state and momentum balance must also be considered, as well as the assumption that the gas is in thermal equilibrium. By solving for the density and susceptibility, the electrostatic energy and force can be calculated.
artis
So I was wondering, can a moving electric field produce a drag force on gas for example similar to that which would result from physical blades moving the gas.

Electric field applied to a dielectric if not as strong as to produce breakdown produces polarization. I wonder can this polarization create a drag on the dielectric medium if the electric field moves in time and space.

For a simple example think of a pipe filled with gas, take two wires charged to a high potential and located at the sides of the pipe, the pipe itself is of such material as to allow the E field lines to pass through, surely it would attenuate the field but for the sake of argument suppose it doesn't.
So we drag the two charged wires along the length of the pipe, what would be the effect on the gas within the pipe?

My own idea is that either the polarization from the field passes along the gas but in such a way that each next segment is simply being polarized without any drag on it or the previously polarized segment tends to be pulled along and so creates a push on the gas in front of it creating drag.?thanks.

There's a classic problem that asks you to calculate the force required to pull a dieletric out of a parallel plate capacitor. The key is the motion of the edge of the dielectric slab, since that determines what percent of the field sees a polarizable medium and what percent sees vacuum.

In the case of the gas in the pipe, you will also need to factor in the equation of state and momentum balance: $$\rho \frac{D\vec{u}}{Dt} = -\nabla p - (\vec{P} \cdot \nabla) \vec{E} = 0$$ $$k_B T \nabla n = -(\vec{P} \cdot \nabla) \vec{E}$$ where ##p## is the pressure, ##\vec{P}## is the polarization density, and ##\frac{D\vec{u}}{Dt}## is the material derivative of fluid velocity (zero in this case, because we're assuming steady state) where in the last bit I made the assumption that the wires are moving slowly enough that the gas is in thermal equilibrium. From this you can solve for the density, and thus the susceptibility everywhere in the gas. From there you can find the electrostatic energy, and then calculate the electrostatic energy, and lastly the force by taking the derivative of the electrostatic energy with respect to the wires' positions.

## 1. How does a moving electric field affect a dielectric material?

When a dielectric material is placed in a moving electric field, the molecules within the material will align themselves with the direction of the field. This alignment creates an induced dipole moment in the material, causing it to become polarized. The strength of the induced dipole moment depends on the strength of the electric field and the properties of the dielectric material.

## 2. What is the difference between a moving electric field and a stationary electric field?

A moving electric field is one that is constantly changing in magnitude and direction, while a stationary electric field remains constant. This difference is important because it affects how the dielectric material responds to the field. A stationary electric field will only cause a temporary polarization in the material, while a moving electric field can create a more sustained polarization.

## 3. How does a moving electric field affect gases?

A moving electric field can affect gases in a similar way to dielectric materials. The molecules in the gas will align themselves with the direction of the field, creating a polarization. However, gases are more easily affected by electric fields due to their low density and lack of a fixed structure. This can lead to the formation of plasma or breakdown of the gas if the electric field is strong enough.

## 4. What is the significance of the moving electric field effect on dielectric/gas?

The moving electric field effect on dielectric/gas has many practical applications. It is used in technologies such as capacitors, where the polarization of a dielectric material is used to store electric charge. It is also important in understanding the behavior of gases in high voltage systems, and in plasma physics research.

## 5. Can a moving electric field be used to manipulate the properties of a dielectric/gas?

Yes, a moving electric field can be used to manipulate the properties of a dielectric/gas. By controlling the strength and direction of the field, the polarization of the material can be controlled, which in turn affects its electrical and optical properties. This is the basis for technologies such as liquid crystal displays (LCDs) and electroluminescent displays.

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