Physics307L F08:People/Le/Notebook/070924

--Linh N Le 17:26, 24 September 2007 (EDT)

Photoelectric Effect

Using PASCO Scientific AP-9368 h/e apparatus

Keithley 131 Digital Multimeter

Purpose
We are using the photoelectric effect to measure Planck's Constant. The photoelectric effect is when packets of light collide with a metal, and "pop" electrons off its surface. In our setup we use a photo diode. In the photo diode, these electrons pop off an anode then travel to a cathode. The electrons create a current, which is then stored in a capacitor. This charged capacitor creates the "stopping potential" that will eventually stop further electrons from getting to the anode.

In the first part of the lab, we will measure the amount of time it takes for the light to go from 0V to 80% of stopping potential, at different wavelengths and intensities of light. This should help demonstrate the particle nature of light. (We use a % of the stopping potential since the real stopping potential is reached asymptotically)

Lastly, using the stopping voltage, the wavelength of the light $$\nu$$ and the equation $$K=h \nu - \phi$$ we will plot our data and by finding the x intercept find the workfunction $$\phi$$ and the slope will find Planck's Constant

Procedure
For the first part, we will flood the apparatus with different colors of light emitted from a lamp. Watching the voltmeter, we can find the maximum stopping potential of the photoelectroms emitted by certain colors of light. We will also use filters that decrease the intensity of the light and watch how long it takes the light to return from 0 to its maximum potential (since the max potetential is reached asymptotically, we decided to measure it at some fixed percenet of max).

Data
Note: Voltmeter has .04V as it's "zero"

Experiment Part A
Looking at particle nature of light

UV light
100% filter Stopping Potential: 1.86V measure at 90% about 1.70V

80% filter SP : 1.84V 90% of that 1.65V

60% filter SP : 1.81V 90% of that 1.63V

40% filter SP: 1.78V 90% of that: 1.60 V

20% filter SP:1.74 V 90% of that:1.57 V

Note: the neutral filter filters out some UV light

Blue -Violet
Stopping Potential: 1.75V Measure to 1.70V

Green
Stopping Potential = .89V Measure at : .87V

Yellow
Stopping Potential : .73V Measure at: .70V

--Linh N Le 16:13, 1 October 2007 (EDT)

Yellow Round 2

Last week's yellow data seems bad.

Stopping V =.75V Measure to:.74V

Experiment Part B
Measuring Stopping Potentials

Yellow
First Order=.76V

Second Order=.75V

Green
First Order=.89V

Second Order=.86V

Blue-Violet
First Order=1.76V

Second Order=1.76V

UV
(since we cant see UV, found the spot with highest Stopping Potential)

First Order=2.06V

Second Order=2.07V