The work function of an oxide-coated cathode is much lower than that of a pure tungsten cathode. True or False?
Electronics and Communication Engineering
Electronic Devices and Circuits
Difficulty: Easy
Choose an option
-
ATrue
-
BFalse
-
CTrue only at cryogenic temperatures
-
DFalse for all practical vacuum tubes
-
EStatement is meaningless
Answer
Correct Answer: True
Explanation
Introduction / Context:Thermionic emitters in vacuum devices rely on cathode materials with low work function to achieve adequate emission at manageable temperatures. Oxide-coated cathodes are widely used for this reason.
Given Data / Assumptions:
- Material A: Oxide-coated cathode (typically BaO, SrO on Ni).
- Material B: Pure tungsten.
- Property compared: Work function (energy needed to liberate an electron).
Concept / Approach:Lower work function means electrons require less thermal energy to escape the surface, enabling strong thermionic emission at lower temperatures compared with high-work-function metals like tungsten.
Step-by-Step Solution:
Step 1: Recognize that typical oxide-coated cathodes have work functions roughly around 1.1–2.0 eV (application dependent).Step 2: Tungsten has a substantially higher work function, often near the range of about 4–4.6 eV.Step 3: Since lower work function reduces required operating temperature, the statement is consistent with thermionic practice.Step 4: Therefore, the statement is True.Verification / Alternative check:
Practical vacuum tubes commonly use oxide-coated cathodes to reduce power consumption and extend life compared with pure tungsten filaments.Why Other Options Are Wrong:
False: Contradicts well-established data on work functions.Temperature caveat and vacuum tube caveat options are distractors; work function difference is intrinsic and not limited to special conditions.Common Pitfalls:
Confusing emission current with work function alone; actual emission also depends on temperature and surface condition.Final Answer:
True