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Moving Charges and Magnetism — Physics STD 12 Science — Question

Bihar BoardEnglish MediumSTD 12 SciencePhysicsMoving Charges and Magnetism1 Mark
Question
Two statements are given-one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below.
  1. Both A and R are true and R is the correct explanation of A.
  2. Both A and R are true but R is NOT the correct explanation of A.
  3. A is true but R is false.
  4. A is false and R is also false.
Assertion (A): A charged particle moving in a uniform magnetic field penetrates a layer of lead and there by loses half of its kinetic energy. The radius of curvature of its path is now reduced to half of its initial value.
Reason (R): Kinetic energy is inversely proportional to radius of curvature.
✓

Answer

  1. Both A and Rare true but R is NOT the correct explanation of A.

Explanation:

The radius of curvature of a charged particle in a magnetic re rs given y,

$\text{r}=\frac{\text{mv}}{\text{qB}}=\frac{\sqrt{2\text{mK.E.}}}{\text{qB}}\text{i.e. r}\propto\sqrt{\text{K.E}}.$

When kinetic energy is halved, the radius is reduced to $\Big(\frac{1}{\sqrt{2}}\Big)$ times its initial value.

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