Let's look into some of the assumptions behind this question. To your first question, regarding whether electric fields are positive or negative, we need to start with a little math.
A vector is a mathematical construct that has both magnitude and direction. Things like velocity, force, and electric fields (which we will get to in a second) are best described with vectors, since they have both a magnitude and a direction.
In contrast, quantities that are only defined by a number value (such as mass or charge or how much something costs in dollars) are called scalars. The magnitude component of a vector is, itself, a scalar.
For example, velocity is a vector: if we want to define the velocity of an object in space, the velocity vector will include both the magnitude (for examples, 10 meters per second) and direction (for example, in the direction of
x,
y,
z, or some combination of the three). In contrast, speed is a scalar: speed is the magnitude of the velocity vector. The speed of an object is simply a number, in meters per second. The velocity of an object is a vector, which includes the magnitude (speed), and the direction of movement.
Note that we generally define the magnitude of a vector as positive. The direction of the vector includes the information about which way it is "pointing". To use our velocity/speed example above, if you wanted to say that I were running "negative 10 meters per second in the north direction", I would instead just say I am running "10 meters per second in the south direction".
An electric field is a vector field - this means that at any instant in time, every point in space (
x,
y,
z) has a defined electric field vector. We will use
E to denote the electric field vector.
Electric fields interact with charges - basically, the electric force
F on a charge
q is defined by:
F =
E*
q
Note that the force
F is also a vector - it also has magnitude, and direction. Charge is a scalar - it a number with a value which can be positive or negative. For a given electric field
E, a negative charge "-
q" would experience a force in the opposite direction (but with the same magnitude) as a positive charge "+
q".
What I am getting at here is that electric fields cannot be "just positive" or "just negative" since they are more than just a single number - they have a direction and a magnitude.
Charges *do* have polarity - they can be positive or negative, and the sign on a charge will determine whether it is "pushed" or "pulled" along the direction of the electric field vector.
I hope this helps clarify the first part of your question - I recommend you check out these pages to help you get a better understanding of vectors and electric fields:
http://en.wikipedia.org/wiki/Euclidean_vector
http://en.wikipedia.org/wiki/Electric_field
I hope this helps clear things up about electric fields. To your question about Tesla coils - a tesla coil operates by using alternating current through a transformer to generate really high voltages. The electric field is flipping back and forth in direction quickly - but again, the field is not "positive" or "negative". It is a vector that varies (in direction and magnitude) with time.
To answer the second part of your question, yes - an electric field will exhibit a force on all positive charges in the plasma in one direction (along the direction of
E) and will exhibit a force in the opposite direction on all negative charges in the plasma.
However, this is only one part of the puzzle. Electric fields don't just push/pull on charges, they are also *generated* by charges (they are also generated by changing magnetic fields, but that's a story for another day). A positive charge will generate electric field vectors pointing away from it (thus repelling other positive charges and attracting negative charges), and a negative charge will generate electric field vectors pointing towards it (again, repelling negative charges and attracting positive charges). Since like charges attract and opposite charges repel, most everyday stuff we interact with is net neutral as far as charge goes - if charges are free to move around, the positives and the negatives will try to get together and cancel each other out. Electric fields sum together, so all the charges in space will contribute to the *net* electric field at any given point in space.
Plasmas are generally really good conductors, since the positive and negative ions have been kicked to such high energy levels that they are ripped apart from their usual electrostatic attraction to their respective atoms, and are free to move around. For example, air in our atmosphere is a very good insulator and will inhibit current flow, but if sufficiently "pushed on" by an electric field, it "breaks down" and gets ionized - and changes phase from gas (air) to plasma (lightning bolt). Lightning bolts (a type of plasma) offer a good path for electric current to flow through the atmosphere.
Generally speaking, a static electric field will not exist inside the bulk of a conductor - because the charges in a conductor are free to move around, they will reorient themselves under the influence of any external electric field. In the case of a plasma that has a static external electric field applied to it, free charges in the plasma will form a thin, charged "sheath" on the outside of the plasma bulk; the electric fields created by charges in the sheath will cancel out external electric fields, leaving no static electric field inside the plasma bulk itself. The way that this "sheath" works is a little more complicated than how I am describing due to the unique nature of a plasma (the positive ions and negative ions can coexist at different temperatures, resulting in interesting behavior), so while a metal (another type of good conductor) will also "block" electric fields, the properties of a plasma sheath are unique and differ from how a metal behaves.
I recommend checking out the following articles for more detail:
http://en.wikipedia.org/wiki/Plasma_(physics)
http://en.wikipedia.org/wiki/Debye_sheath
I hope this helped answer your question - I have simplified the explanations of some of the concepts above, so I highly recommend digging deeper and using the resources available online or in your school to deepen your understanding of electromagnetics. Feel free to respond if you have more questions or need some more direction!