1. http://www.colorado.edu/physics/phys2170/ Physics 2170 – Spring 20091 Photoelectric effect First midterm is 7:30pm on 2/17/09 in this room. The exam will have a formula sheet (which will be posted by Friday) and you will not be allowed to bring in your own formula sheet. Next weeks homework will be available by 4pm and will cover material on the exam and will be worth only 40 points (instead of 50). Announcements:

2. http://www.colorado.edu/physics/phys2170/ Physics 2170 – Spring 20092 Understanding the blackbody spectrum In 1900, Max Planck proposed a new theory which matched the blackbody observations perfectly. The new theory required a minimum energy in the emitted light which was proportional to the frequency of light. The proportionality constant is now called Planck’s constant h. This is the first example of a quantum effect. The energy coming out of the blackbody is quantized as a multiple of hf. Planck did not think that light itself was quantized. He just found that when he required the atoms in the blackbody to emit quantum amounts of energy in the form of light that everything worked. Energy emitted is where n is an integer.

3. http://www.colorado.edu/physics/phys2170/ Physics 2170 – Spring 20093 The photoelectric effect had been observed 20 years before Einstein finally figured out the explanation in 1905. Plan: I. Understand the photoelectric effect experiment and what results you would expect if light were a classical wave (like physicists at the time expected the experiment should give). II. What experimental results it actually did give. III. The implications/interpretation of the results. The Photoelectric Effect

4. http://www.colorado.edu/physics/phys2170/ Physics 2170 – Spring 20094 Electrons Test metal Two metal plates in vacuum, adjustable voltage between them, shine light on one plate. Measure current between plates. Photoelectric effect experiment apparatus. 10 V A B Potential difference between A and B = +10 V. Measure of energy an electron gains going from A to B. + -

5. http://www.colorado.edu/physics/phys2170/ Physics 2170 – Spring 20095 Uniform E-field between plates 10Volts0V 10V 0V Constant force on electron  constant acceleration E F ++++++++++ Electron feels electric field, accelerates to + plate, and picks up kinetic energy = qV = 1 electron charge x 10 V = 10 eV A B What happens if drop an electron next to plate A?

6. http://www.colorado.edu/physics/phys2170/ Physics 2170 – Spring 20096 10 V A B + - What is the current from A to B? A. 0 amps B. 5 amps, C. 0.2 amps 2 ohms With the light off, there aren’t any electrons to move which means no current. The light is turned off and a resistor is put into the circuit as shown. No DC current across a capacitor (open circuit). Clicker question 1 Set frequency to DA

7. http://www.colorado.edu/physics/phys2170/ Physics 2170 – Spring 20097 If no water slops over side of pool, no flow no matter what size pump you have. Similarly, if electrons are stuck inside metal plate, no current will flow no matter what voltage you have. Pool party!: put a bunch of energy into water, splash some out, get flow through pump. Put energy into metal by heating it, gives electrons energy, some “splash” out giving current. pump Swimming pool analogy

8. http://www.colorado.edu/physics/phys2170/ Physics 2170 – Spring 20098 Hot plate model. A few electrons get enough energy to just barely “splash” out. What is the current vs battery voltage? A Clicker question 2 D B C E. None of these I II I V V V V

9. http://www.colorado.edu/physics/phys2170/ Physics 2170 – Spring 20099 Higher voltage means higher KE when they reach other side but doesn’t change the number. So current stays constant. 0 Battery Voltage Current When voltage is reversed, only a few electrons that come off with relatively high KE make it to the other side. So low current. reverse V, no electrons flow. Vacuum tube diode. Early electronic device. Once the voltage is >0, all electrons that come off are accelerated to the other side. So high current. Current versus voltage for the “hot plate” model

10. http://www.colorado.edu/physics/phys2170/ Physics 2170 – Spring 200910 Electrons Test metal Classical solution to photoelectric effect So if light is classical wave, predict that it just puts energy into plate, heats up, get diode current voltage curve. 0 Voltage Current Also takes time to heat up. Light on longer, heat more, more electrons and electrons have higher KE. Color light does not matter, only intensity.

11. http://www.colorado.edu/physics/phys2170/ Physics 2170 – Spring 200911 I. How apparatus works. II. What would expect to see from classical wave view: previous experiments like double slit interference, heating barrels, etc. Current vs voltage step at zero then flat. Light COLOR does not matter, only intensity. Takes time to heat up ⇒ current increases with time. Increasing intensity should increase the current. III. Do actual experiment, see if agrees with prediction. I vs V (current vs. voltage) How does this depends on intensity and color of light? 0 http://phet.colorado.edu Recap so far (what we know):

12. http://www.colorado.edu/physics/phys2170/ Physics 2170 – Spring 200912 I e’s 1) First experiment- I vs. V high intensity, low intensity 2) Second experiment- I vs. V two different colors write down what happens

13. http://www.colorado.edu/physics/phys2170/ Physics 2170 – Spring 200913 0 Batt. V Which graph represents low and high intensity curves? A B C D Clicker question 3 Set frequency to DA I I I I E. None of them

14. http://www.colorado.edu/physics/phys2170/ Physics 2170 – Spring 200914 I e’s HIGH intensity LOW intensity 0 Battery Voltage Fewer electrons pop off metal So current decreases. Find that current is proportional to light intensity. I

15. http://www.colorado.edu/physics/phys2170/ Physics 2170 – Spring 200915 Predict what happens to the initial KE of the electrons as the frequency of light changes? (Light intensity is constant) Predict shape of the graph I e’s 0 Frequency of light Initial KE

16. http://www.colorado.edu/physics/phys2170/ Physics 2170 – Spring 200916 0 Frequency Initial KE 0 Frequency Initial KE 0 Frequency Initial KE 0 Frequency Initial KE E. something different A. C. B. D. Clicker question 4 Set frequency to DA Which graph is correct?

17. http://www.colorado.edu/physics/phys2170/ Physics 2170 – Spring 200917 I e’s 0 Frequency of light Initial KE As the frequency of light increases (shorter  ), the KE of electrons being popped off increases. (it is a linear relationship) There is a minimum frequency below which the light cannot kick out electrons… no matter what intensity!