Assertion :The rate of thermionic emission is less in an evacuated bulb as compared to one which is filled with inert gas at low pressure. Reason: An inert gas at low pressure ionises where as vacuum is the poorest conductor of electricity.

Both Assertion and Reason are correct and Reason is the correct explanation for Assertion

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Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion

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Assertion is correct but Reason is incorrect

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Both Assertion and Reason are incorrect

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Solution

The correct option is A Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
Vacuum is the poorest conductor of electricity . Thus few electrons can migrate from cathode to anode. However if the bulb is filled with inert gas at low pressure, the rate of thermionic emission increases because inert gases ionises even at low pressure.
Hence option A is correct.

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