A) \[B\bot v\]
B) \[B||v\].
C) it obeys inverse cube law.
D) it is along the line joining the electron and point of observation.
Correct Answer: A
Solution :
| Option [a] is correct. |
| Explanation: In Biot-Savart's law, magnetic field \[\left. B \right\|idl\times r\] and idl due to flow of electron is in opposite direction of v and by direction of cross product of two vectors, B. |
| \[\operatorname{dB}=\frac{\operatorname{I}.dl\ \sin \theta }{{{\operatorname{r}}^{2}}}\] |
| Or \[\operatorname{dB}=\frac{\operatorname{I}\times dl\ }{\operatorname{r}}\] |
| According to Biot-Savart law, if magnetic field is not perpendicular to the motion of charge, then it will not move in helical path, which is not possible for motion of a charge in magnetic field. So, the magnetic field is perpendicular to the direction of flow of charge verifies answer 'A'. |
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