Question

Answer between T(true) or F(false).

(a) Ion responds more promptly than electron to the given electric field perturbation.

(b) Longitudinal wave means wavevector k is parallel to magnetic field B.

(c) Electrostatic wave is always longitudinal wave.

(d) Plasma approximation is invalid for electrostatic ion waves.

(e) When magnetic field is exactly perpendicular to wavevector of longitudinal wave, electron motion can be described by Boltzmann’s relation.

Answer 1

A) False

This statement is false because ions are massive and heavier than electrons and that's why electrons respond more promptly to electric field perturbation than ions.

B) False

Longitudinal waves in plasma means the wave vector k is parallel to the electric field E and not the magnetic field B.

C) True

As we have the relation

k× E= omegaB

Now when wave vector k is parallel to electric field E , the cross product vanishes and hence magnetic field B becomes zero. This kind of wave is called electrostatic wave. And hence electrostatic wave is always longitudinal.

D) False

Plasma approximation is valid for all except for the shortest wavelength waves.

E) False

We assume the magnetic field B almost perpendicular to wave vector k and then for longitudinal waves we can use the boltzmann relation for describing electron motion. If we take magnetic field exactly perpendicular to wave vector k then the dot product k.B= 0 and it leads to a different mathematical relation that doesn't describe what usually happens in real plasmas.