Negative maximum kinetic energy in Millikan's data?

In summary, the conversation discusses the linear relationship between maximum kinetic energy and frequency in Millikan's data from the photoelectric effect. It is noted that the graph should not be extrapolated below the threshold frequency, as it is not physically meaningful to have negative maximum kinetic energy. The work function and threshold frequency are also mentioned as important factors in this relationship.
  • #1
aeromat
114
0
What am I finding to be confusing..
The graphs of Millikan's data were straight lines with equal slopes. The graph was plotted maimum kinetic energy (eV) versus the frequency of the wave. This was the graph that produced the linear relationship from the photoelectric effect.

What does it mean on the graph when it is below the x-axis? The y-axis is the maximum kinetic energy, so if the y-values become negative, what is happening to the maximum kinetic energy?

I understand that the y-intercept is for the work function of the metal, and that the x-intercept accounts for the threshold frequency, but I am confused as to what is the meaning behind "negative maximum kinetic energy" in the linear relationship.

Equation:
Ek(max) = hf - W
 
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  • #2
It's not physically meaningful. The work function is just the amount of energy required to liberate the electron from its parent atom. At f = W/h, the energy of the photon will be JUST enough to liberate the electron from its atom, and it will be a free electron, but with ZERO surplus kinetic energy.

At f below W/h, the photon will NOT have enough energy to liberate the electron, and so it's not meaningful to talk about how much "extra kinetic energy' it gets freed with (since it doesn't get freed at all).

The plot should not be extrapolated below f = W/h.
 

Related to Negative maximum kinetic energy in Millikan's data?

1. What is negative maximum kinetic energy in Millikan's data?

Negative maximum kinetic energy in Millikan's data refers to a phenomenon observed by Robert Millikan during his famous oil drop experiment. It occurs when the electric field applied to the oil droplets is strong enough to overcome the downward force of gravity, causing the droplets to accelerate upwards. At this point, the kinetic energy of the droplet becomes negative, since its velocity is directed opposite to its acceleration.

2. How does negative maximum kinetic energy relate to the charge of an electron?

During the oil drop experiment, Millikan measured the negative maximum kinetic energy of the droplets and used this information to determine the charge of an electron. By comparing the electric field strength required to balance the force of gravity on the droplet with the known charge of an electron, he was able to calculate the charge of the droplet and therefore the charge of the electron.

3. Why is negative maximum kinetic energy important in Millikan's experiment?

Negative maximum kinetic energy is important in Millikan's experiment because it allows for the determination of the charge of an electron. By measuring the electric field strength required to balance the force of gravity on the droplet, the charge of the droplet can be calculated. Since the droplet's charge is a multiple of the charge of an electron, this allows for the accurate determination of the charge of an electron.

4. What factors can affect the measurement of negative maximum kinetic energy in Millikan's experiment?

There are several factors that can affect the measurement of negative maximum kinetic energy in Millikan's experiment. These include variations in the electric field strength, air currents in the chamber, and variations in the density and viscosity of the oil used. To obtain accurate results, Millikan had to carefully control these factors and repeat the experiment multiple times.

5. How does the observation of negative maximum kinetic energy support the idea of quantized charge?

The observation of negative maximum kinetic energy in Millikan's experiment supports the idea of quantized charge, which states that electric charge can only exist in discrete, quantized units. The fact that the charge of the droplet was always a multiple of the charge of an electron supports this idea, as it suggests that the charge of an electron is the smallest unit of electric charge that can exist.

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