The Photoelectric Effect
BlackBoard Notes
- Measure stopping potential (Vstop) of capacitor (~ energy of ejected electrons)
- Determine value of Planck's constant, h
- Find the work function, φ
- Photon energy: E = hf, absorbed by electron
- Electron ejected when φ (part of the photon energy) is greater than the binding energy; remainder of photon energy goes to ejected electron's kinetic energy:
kmax = hf – φ
- Ejected electrons charge capacitor until kmax = eVstop
- So,
Measuring Vstop:
- Check alignment of photo head (!!!!)
- Press "Zero" button on photo head
- Record Vstop; lather, rinse, repeat.
Note: Use yellow and green filters for yellow and green lines only!!
Instructor's Note: Why can you see the UV line at 365 nm?
- From Pasco's Instruction Manual: "The white reflective mask on the h/e apparatus is made of a special fluorescent material. This allows you to see the ultraviolet line as a blue line, and it also makes the violet line appear more blue. You can see the actual colors of the light if you hold a piece of white non-fluorescent material in front of the mask. (The palm of your hand works in a pinch, although it fluoresces enough that the UV line will still be visible.)"
- For demonstration, use a piece of cardboard (e.g. from the pad of graph paper) or a manilla folder!
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Revised: 23 Apr 09
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Canton, NY 13617
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