Photoelectric Effect Learning Outcomes

Transcription

1 1 Photoelectric Effect Learning Outcomes Describe the photoelectric effect. Demonstrate the photoelectric effect. Discuss the nature of the photon. Discuss the effect of intensity and frequency on the photoelectric effect. Solve problems about the photoelectric effect. HL: Discuss the concept of threshold frequency. HL: Solve problems using Einstein s photoelectric law. Describe the structure and operation of a photocell. Demonstrate the use of a photocell. Give uses of photocells.

2 2 Photoelectric Effect Light is known to liberate electrons from atoms. To be liberated, electrons need to be given a minimum amount of energy, which frees them from their bond to the atom. Any energy less than this will not free the electrons. The photoelectric effect is the emission of electrons from the surface of a metal due to incident light of a suitable frequency.

3 3 Demonstrate the Photoelectric Effect 1. Place a zinc plate on an electroscope. 2. Charge the plate negatively by induction. 3. Shine a white light lamp onto the plate. 4. Note no effect. 5. Replace the white light lamp with a UV lamp. 6. Note that the leaves of the electroscope collapse. 7. The UV light causes electrons to be emitted from the zinc.

4 4 Demonstrate the Photoelectric Effect

5 Einstein got his Nobel prize for this by Andrew Dunn CC-BY-SA Photon At the time, scientists couldn t understand why frequency mattered instead of intensity. Imagine a large wave being unable to capsize a ship, but frequent small waves doing the job. Einstein later proposed that light existed not as a continuous wave, but as separate packets of energy he called quanta.

6 Higher Level 6 Photon Energy Today, these quanta are called photons. The energy of a photon is given by E = hf E = energy, h = planck s constant, f = frequency The workfunction of a metal, Φ is the minimum energy required to liberate an electron. The workfunction is the energy of a photon with a threshold frequency, Φ = hf 0 The intensity of the light will increase the number of electrons released, provided the photon energy exceeds the workfunction.

7 Higher Level 7 Einstein s Photoelectric Law Any photon energy above the workfunction adds to the kinetic energy of the liberated electrons. HL Formula: hf = Φ + 1 mv 2 2 max e = C m e = kg h = Js e.g. Calculate the energy of a photon of frequency Hz. e.g. The threshold frequency of a certain metal is Hz. If light of frequency Hz is incident on the metal, what is the kinetic energy of the liberated electrons? e.g. When light of frequency Hz falls on a metal, the photocurrent is 0.22 ma. How many photons strike the metal per second if every photon causes photoemission? How much light energy is falling on the metal per second?

8 8 Photocell Photocells are electronic devices designed to take advantage of the photoelectric effect. They use a cylindrical photocathode which will emit electrons when light of a suitable frequency falls on it. The electrons flow towards the anode, creating a current which is proportional to the intensity.

9 9 Demonstrate a Photocell 1. Connect a photocell and galvanometer in series. 2. Shine a light source at the photocell and note the current. 3. Move the light source closer to the photocell and note the current. 4. Since intensity increases as the light source gets closer, the photocell produces a larger current.

10 by cmacd123 public domain 10 Uses of Photocells Photocells will turn on when light shines on them and turn off when in darkness. This allows them to be used in: Burglar alarms, Automatic doors, Controlling burners in central heating, Sound tracks in films.