Question

# The space between two parallel metal plates is filled with an element in a gaseous state. Electrons leave one plate at negligible speed and are accelerated toward the other by a potential difference $\Delta V$ applied between the plates. As $\Delta V$ is increased from 0, the electron current increases more or less linearly, but when $\Delta V$ reaches 4.9 V, the current drops precipitously. From nearly 0, it builds again roughly linearly as $\Delta V$ is increased beyond 4.9 V. (a) How can the presence of the gas explain these observations? (b) The gas emits a rather pure "light" when $\Delta V$ exceeds 4.9 V. What is its wavelength?

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Electrons are accelerated between two parallel metal plates, with an element in its gaseous state between them, by a voltage $\Delta V$. The electron current increases almost linearly with $\Delta V$. But, at a certain value $(\Delta V=4.9 \rm\; V)$, the current drops to $0$, and then builds up again with the increasing voltage.

For part (a) of this question, we are asked how the gas present between the plates can explain the observations. For part (b), we are told that, beyond $4.9 \rm\; V$, the gas emits a pure light. We need to calculate the wavelength $\lambda$ of this light.

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