Question

Suppose that the collector is held at a small negative voltage with respect to the grid. Will the accelerated electrons reach the collecting plate?

Suppose that the collector is held at a small negative voltage with respect to the grid. Will the accelerated electrons reach the collecting plate?

Yes, but only those electrons with energy less than the potential difference established between the grid and the collector will reach the collector.

Yes, but only those electrons with energy greater than the potential difference established between the grid and the collector will reach the collector.

Yes; all the accelerated electrons will reach the collector.

No, none of the electrons will reach the collector.

Answer 1

Fig. 1

Setup for Franck-Hertz experiment.

Given: As evident from the figure, the potentiometer supplies a greater voltage as compared to the retarding voltage which supplies volts to the collector. Therefore the collector is at a small negative voltage as compared to the grid (anode).

Solution:

The electrons emitted by the cathode by thermionic emission will have different energies as they have to first overcome the Coulombic forces in the nucleus and they get emitted. The electrons that have energy greater than a threshold value as determined by the values of the applied voltages, will be able to cross the grid and reach the collector. Hence option (b) is the correct option.

Since, option (a) is just the opposite of option (b) and we have shown it is not possible, option (a) is inccorect.

Since all electrons do not have the same energy with which they ar emitted, some may and some may not reach the collector. Hence options (c) and (d) are wrong.

Answer: Option (b),Yes, but only those electrons with greater energy than the potential difference established between the grid and the collector will reach the collector.

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