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Current Electricity — Physics STD 12 Science — Question

Gujarat BoardEnglish MediumSTD 12 SciencePhysicsCurrent Electricity1 Mark
Question
For 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: If the length of the conductor is doubled, the drift velocity will become half of the original value (keeping potential difference unchanged).
Reason: At constant potential difference, drift velocity is inversely proportional to the length of the conductor.
✓

Answer

  1. Both A and R are true, and R is the correct explanation of A.
Explanation:

Drift velocity of free electrons is given by, $\text{v}_\text{d}=\frac{\text{eE}}{\text{m}}\tau$

Where, $\text{E}=\frac{\text{Potential difference }}{\text{ength }}=\frac{\text{V}}{\text{l}}$

$\therefore\text{v}_\text{d}=\frac{\text{eE}}{\text{ml}}\tau$ r.e., $\text{v}_\text{d}\propto\frac{\text{1}}{\text{l}}$ where, $\frac{\text{eV}\tau}{\text{m}}$ is constant.

It mean if l is doubled, the drift velocity will become half of the original value.

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