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Moving Charges and Magnetism — Physics STD 12 Science — Question

Gujarat BoardEnglish MediumSTD 12 SciencePhysicsMoving Charges and Magnetism2 Marks
Question
An electron is not deflected while passing through any field. Is it possible that there is no magnetic field present at that place? Explain.
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Answer

Exerting force in magnetic field on a moving electron with velocity $\vec{v}$
$\overrightarrow{ F }=-e(\vec{v} \times \overrightarrow{ B })$
or $\overrightarrow{ F }=-e v B \sin \theta$
If $\theta=0^{\circ}$ or $180^{\circ}$, the magnitude of force F will be zero. Hence, an electron will not be deflected if it moves parallel or opposite to the direction. So, we cannot say that magnetic field is absent.

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