Franck-Hertz Experiment

1. Nov 10, 2015

ghaleb hamdan

I have a question that might be very basic.
The Franck-Hertz experiment shows that as the voltage (KE of the electrons) increase, the current/energy of the electron increases up to 4.9v at which point it drops due to the non elastic collision. this happens at intervals of 4.9v. assume that the current at 6v was measured to be X
Assume there is no increase in voltage and the voltage was set at 6v from the get go. Would the current be the X (the same as increasing it slowly)? At 6v it would be an elastic collision, and since there wasn't any non elastic collision, i would suspect the energy would be higher than X

2. Nov 11, 2015

Staff: Mentor

Yes. I don't know how the Franck-Hertz experiment was done originally, but you can take the result at any voltage to be independent of the result at any other voltage. These kind of experiments are often done not continuously, but performed for discrete values of the adjustable parameters.

3. Nov 11, 2015

ghaleb hamdan

Hello,

If the energy is lost in a non elastic collision when the voltage is 4.9voltage, then i see how the energy at 6v would be reduced once the voltage is increases. But how will the energy be lost if we never pass through the 4.9v? is it cause the KE will have to increase and pass through the 4.9v equivalent energy prior to getting to the 6v?

4. Nov 11, 2015

Staff: Mentor

I'm sorry, but I don't understand what you mean. The electric potential is there to accelerate the electrons. As you said so yourself in the OP, what is relevant here is the kinetic energy of the electrons. For a given value of the KE, you get a certain current, and the greater the KE, the greater the current.

What the Franck-Hertz experiment shows is that the electrons will lose only a fixed amount of energy when they collide with mercury atoms.

5. Nov 11, 2015

ghaleb hamdan

Sorry let me rephrase my question in a better manner
if the electrons are accelerated (maybe in a vacuum?) to a KE are between the first and second "threshold" (KE required for the electrons to lose energy in the collision with the mercury atoms) prior to allowing them to enter the mercury vapor environment, will they lose energy in the collision with the atoms or do they need to be firstly accelerated to the second threshold?

6. Nov 12, 2015

Staff: Mentor

As soon as the potential reaches 4.9 V, the electrons gain sufficient KE to be able to excite a Hg atom when colliding with it. Between the first and second threshold, electrons have sufficient KE to excite the Hg and have KE left after the collision.

7. Nov 12, 2015

ghaleb hamdan

That's exactly what I needed to know
Thanks!