Px 3A4 Plasma Physics Exam 2026
Questions and Answers Graded A+
What is a plasma? - Correct answer-A gas where particle motion is dominated by
collective E and B fields rather than binary collisions. Charges interact with the
mean field of all other particles, giving rise to collective phenomena absent in
neutral gases.
What does quasi-neutrality mean physically? - Correct answer-On scales L >>
λ_D, the plasma screens any charge imbalance so net charge density ≈ 0. It does
not mean no electric field exists — finite fields can exist — just no net charge
accumulation on large scales.
What is the Debye length and what does it represent? - Correct answer-λ_D =
sqrt(ε₀k_BT/ne²). The scale over which a charge imbalance is screened. Beyond
λ_D the plasma looks neutral; within it individual charges are felt. Larger T or
smaller n → larger λ_D.
Why must N_D >> 1 for a plasma? - Correct answer-N_D is the number of
particles in a Debye sphere. Too few particles and the plasma cannot statistically
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,maintain quasi-neutrality — there are not enough charges to collectively screen a
perturbation.
What is the plasma frequency ω_pe and what does it represent? When does quasi-
neutrality break down? - Correct answer-ω_pe = sqrt(ne²/m_eε₀). The natural
oscillation frequency of displaced electrons. Sets the fastest timescale on which the
plasma can respond — on timescales shorter than 1/ω_pe, quasi-neutrality breaks
down.
Why can EM waves only propagate when ω > ω_pe? - Correct answer-At ω <
ω_pe, electrons respond fast enough to cancel the wave — it is reflected. At ω >
ω_pe, electrons cannot keep up with the rapidly oscillating field and the wave
propagates through.
Why does HF radio reflect off the ionosphere but VHF does not? - Correct answer-
HF frequencies fall below the ionosphere's ω_pe → reflected back to Earth.
VHF/UHF exceed ω_pe → propagate through to space. Re-entering satellites raise
local n → raise ω_pe above communication frequency → blackout.
Why do gyrating particles move in circles perpendicular to B? - Correct answer-
The Lorentz force qv×B is always ⊥ to v → does no work → speed is constant.
The force acts as centripetal acceleration, producing circular motion at the
cyclotron frequency ω_c = |q|B/m.
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,Why does the E×B drift not depend on charge or mass? - Correct answer-It arises
from a frame transformation — in a frame moving at v_E = E×B/B², the electric
field vanishes. This transformation is identical for all particles, so all species drift
together → no relative motion → no current.
Why does the gravitational drift produce a current but E×B (E field drift) does not?
- Correct answer-Gravity drift v_g = mg×B/(qB²) contains q in the denominator →
ions and electrons drift opposite ways → net current. E×B drift is q-independent →
both species drift identically → no current.
What is the physical origin of the grad-B drift? - Correct answer-Non-uniform B
means the Larmor radius is larger on the weak-B side of the orbit than the strong-B
side. This asymmetry means the particle doesn't complete a perfect circle — it
drifts sideways, perpendicular to both B and ∇B.
What is the physical origin of the curvature drift? - Correct answer-A particle
following a curved field line experiences a centrifugal force directed away from
the centre of curvature. This drives a drift perpendicular to both B and the
curvature vector, analogous to the gravitational drift with g replaced by centrifugal
acceleration.
What is the physical origin of the polarisation drift? - Correct answer-When E
changes in time, the guiding centre must continuously adjust its E×B drift velocity.
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, This acceleration requires a force from a small additional velocity component
perpendicular to B — the polarisation drift. It is the inertial response to a changing
E field.
Why is the polarisation drift essential for quasi-neutrality? - Correct answer-When
another drift causes charge separation, an electric field builds up. The polarisation
drift of ions compensates the charge-separating drift to maintain n_i ≈ n_e. Without
it, momentum conservation would be violated.
What is the magnetic moment μ and how is it conserved? - Correct answer-μ =
mv_⊥²/2B. It is conserved (adiabatic invariant) when B changes slowly compared
to the gyration period. As B increases, the mirror force does work on the parallel
motion that exactly compensates the change in perpendicular energy — keeping μ
constant.
What is the mirror force and what causes it? Motion of particle and behaviour of v.
- Correct answer-F_∥ = -μ∇_∥B. As a particle moves into stronger B, conservation
of μ forces v_⊥ to increase → v_∥ must decrease by energy conservation. This
deceleration along B is the mirror force — a magnetic pressure pushing the particle
back toward weaker B.
Explain magnetic mirroring in terms of conservation laws. - Correct answer-Two
conserved quantities: total KE (W = W_∥ + W_⊥) and μ = W_⊥/B. As B increases,
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Questions and Answers Graded A+
What is a plasma? - Correct answer-A gas where particle motion is dominated by
collective E and B fields rather than binary collisions. Charges interact with the
mean field of all other particles, giving rise to collective phenomena absent in
neutral gases.
What does quasi-neutrality mean physically? - Correct answer-On scales L >>
λ_D, the plasma screens any charge imbalance so net charge density ≈ 0. It does
not mean no electric field exists — finite fields can exist — just no net charge
accumulation on large scales.
What is the Debye length and what does it represent? - Correct answer-λ_D =
sqrt(ε₀k_BT/ne²). The scale over which a charge imbalance is screened. Beyond
λ_D the plasma looks neutral; within it individual charges are felt. Larger T or
smaller n → larger λ_D.
Why must N_D >> 1 for a plasma? - Correct answer-N_D is the number of
particles in a Debye sphere. Too few particles and the plasma cannot statistically
©COPYRIGHT 2025,ALL RIGHTS RESERVED 1
,maintain quasi-neutrality — there are not enough charges to collectively screen a
perturbation.
What is the plasma frequency ω_pe and what does it represent? When does quasi-
neutrality break down? - Correct answer-ω_pe = sqrt(ne²/m_eε₀). The natural
oscillation frequency of displaced electrons. Sets the fastest timescale on which the
plasma can respond — on timescales shorter than 1/ω_pe, quasi-neutrality breaks
down.
Why can EM waves only propagate when ω > ω_pe? - Correct answer-At ω <
ω_pe, electrons respond fast enough to cancel the wave — it is reflected. At ω >
ω_pe, electrons cannot keep up with the rapidly oscillating field and the wave
propagates through.
Why does HF radio reflect off the ionosphere but VHF does not? - Correct answer-
HF frequencies fall below the ionosphere's ω_pe → reflected back to Earth.
VHF/UHF exceed ω_pe → propagate through to space. Re-entering satellites raise
local n → raise ω_pe above communication frequency → blackout.
Why do gyrating particles move in circles perpendicular to B? - Correct answer-
The Lorentz force qv×B is always ⊥ to v → does no work → speed is constant.
The force acts as centripetal acceleration, producing circular motion at the
cyclotron frequency ω_c = |q|B/m.
©COPYRIGHT 2025,ALL RIGHTS RESERVED 2
,Why does the E×B drift not depend on charge or mass? - Correct answer-It arises
from a frame transformation — in a frame moving at v_E = E×B/B², the electric
field vanishes. This transformation is identical for all particles, so all species drift
together → no relative motion → no current.
Why does the gravitational drift produce a current but E×B (E field drift) does not?
- Correct answer-Gravity drift v_g = mg×B/(qB²) contains q in the denominator →
ions and electrons drift opposite ways → net current. E×B drift is q-independent →
both species drift identically → no current.
What is the physical origin of the grad-B drift? - Correct answer-Non-uniform B
means the Larmor radius is larger on the weak-B side of the orbit than the strong-B
side. This asymmetry means the particle doesn't complete a perfect circle — it
drifts sideways, perpendicular to both B and ∇B.
What is the physical origin of the curvature drift? - Correct answer-A particle
following a curved field line experiences a centrifugal force directed away from
the centre of curvature. This drives a drift perpendicular to both B and the
curvature vector, analogous to the gravitational drift with g replaced by centrifugal
acceleration.
What is the physical origin of the polarisation drift? - Correct answer-When E
changes in time, the guiding centre must continuously adjust its E×B drift velocity.
©COPYRIGHT 2025,ALL RIGHTS RESERVED 3
, This acceleration requires a force from a small additional velocity component
perpendicular to B — the polarisation drift. It is the inertial response to a changing
E field.
Why is the polarisation drift essential for quasi-neutrality? - Correct answer-When
another drift causes charge separation, an electric field builds up. The polarisation
drift of ions compensates the charge-separating drift to maintain n_i ≈ n_e. Without
it, momentum conservation would be violated.
What is the magnetic moment μ and how is it conserved? - Correct answer-μ =
mv_⊥²/2B. It is conserved (adiabatic invariant) when B changes slowly compared
to the gyration period. As B increases, the mirror force does work on the parallel
motion that exactly compensates the change in perpendicular energy — keeping μ
constant.
What is the mirror force and what causes it? Motion of particle and behaviour of v.
- Correct answer-F_∥ = -μ∇_∥B. As a particle moves into stronger B, conservation
of μ forces v_⊥ to increase → v_∥ must decrease by energy conservation. This
deceleration along B is the mirror force — a magnetic pressure pushing the particle
back toward weaker B.
Explain magnetic mirroring in terms of conservation laws. - Correct answer-Two
conserved quantities: total KE (W = W_∥ + W_⊥) and μ = W_⊥/B. As B increases,
©COPYRIGHT 2025,ALL RIGHTS RESERVED 4
