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Alternating Current — Physics STD 12 Science — Question

Bihar BoardEnglish MediumSTD 12 SciencePhysicsAlternating Current1 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: An electric heater is heated first by direct and then by alternating currents. For both the currents, the potential difference across the ends of the heater is the same. The rate of production of heat will be different in two cases.
Reason: The resistance of a coil in altemating current will be more than the resistance of a coil in direct current, hence heat produced in case of direct current will be low.
✓

Answer

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

Explanation:

The element of the heater is in the form of a coil so that it has inductance L also besides the resistance R. So, for the alternating current, the effective resistance of the heater would be $\sqrt{\text{R}^2+(\omega\text{L})^2}$ which is larger than the resistance R for the direct current. Hence on healing by alternating current, heat produced per second would be less.

$\big(\text{H}=\frac{\text{V}^2}{\text{R}}$ and V is same in both the cases$\big).$

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