A positive particle initially has parallel velocity component v∥ along +B. A uniform electric field E is applied antiparallel to B. At what time does the helical pitch momentarily become zero?
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A positive particle initially has parallel velocity component v∥ along +B. A uniform electric field E is applied antiparallel to B. At what time does the helical pitch momentarily become zero?

G1 Physics · Accepted Answer

Pitch is proportional to the instantaneous axial velocity. With acceleration −qE/m, v∥(t)=v∥−qEt/m. Setting this to zero gives t=mv∥/(qE).