 Scientific Models

Models “Scienti fi c models are created to explain observations. Good models provide clear explanations for all known data and make predictions for new observations. New observations either support and strengthen the model or refute it. Models that fail to explain an observation are wrong and must be modi fi ed or replaced with better models—the old model might still be useful in a limited way but ultimately it has failed and must be replaced.” Modeling in Sci 10: Black boxes, jumping shampoo, and through Alice and Bob videos.

 Photoelectric Effect Background Info

PEE The photoelectric effect is an important focus in the Grade 12 Modern Unit. This unit is worth 12% of the total grade. More importantly, the photoelectric effect helped us realize the wave model of light does not account for all the behavior of light.

PEE – Where Does it Fit? I start the Modern Unit with the investigation we’ll do today. From this investigation, students realize a) The wave model of light cannot account for all the behavior of light b) Light can be considered as a “packet of energy” We then look at the atomic spectra from the discharge tubes. This reinforces that light is a packet of energy. Δ E = hf Modern Outline

PEE – Where Does it Fit? Modern Outline Since light can be modeled as a packet of energy, we might be able to model it as a particle. The Compton Effect shows that light scatters off of electrons and the electrons have momentum after the interaction with light. So light must have momentum. Using the an investigation into special relativity, we see that E = mc 2. Knowing that “waves” can act like “particles” might mean that “particles” can act like “waves”. This is the de Broglie wavelengths. This leads to the conclusion of wave-particle duality.

PEE The data for the photoelectric effect uses some of the relationships discussed in the Electromagnetism unit. Namely, that a charged object will experience a force when placed in an electric field. The electric field is uniform when it exists between two parallel plates.

PEE The electric field exists because the plates are charged. This also means that the plates are at different potentials. The potential difference between the plates, Δ V, is the difference across the battery. When a negatively charged object travels from the negative plate to the positive plate, it loses potential energy and gains kinetic energy.

Using the Conservation of Energy The electric field exists because the plates are charged. This also means that the plates are at different potentials. The potential difference between the plates, Δ V, is the difference across the battery. Or, if the negatively charged object is traveling through a space in the positive plate and moves toward the negative plate, it will slow down. This is just like a ball being thrown into the air. It goes from a point of low potential to high potential and thus will lose kinetic energy.

PEE We will use this transferring of energy from one from to another in the collection of data for the photoelectric effect. The potential energy of a negative charge at the negative plate is: E Q = Δ V*q The charge on the object (C) The charge on an electron is 1.6 x C Now we’re ready to start!

Investigating the PEE Answers 1a) List three properties of waves. Examples: 1) Interfere with other waves as they pass through each other. 2) Diffract around objects. 3) Can cause resonance. 4) Can create standing waves 5) Reflect/Refract 1b) Light is how nature transfers energy without transferring mass. List three experiments (or observations) that demonstrate light is a wave. 1) Young’s double slit 2) Thin film interference 3) Microwaves ovens using microwaves to resonate the water molecules in food. Standing waves are set up in the microwave.

Investigating the PEE Answers 3) The light is incident on a metal plate. Even though there is not a completed circuit, current flows. Make a hypothesis about what is happening. Some things to consider are: what is electricity? This effect does not happen when light hits an insulator, like wood. What happens to the light after it hits the metal? Discuss these questions with a partner. Electricity is the flow of moving charges. The charges in this case are probably electrons since the protons in the metal are more or less stationary. The metal has a “sea of electrons”. The light could hit the metal and the electrons are ejected.

Investigating the PEE Answers

7) How does this amount of potential energy compare to the electron’s initial kinetic energy? This potential energy would be the initial kinetic energy of the electrons. 8) Predict what will happen to the stopping potential when we increase the intensity of the wave. Remember, the intensity of a wave is proportional to the energy of the wave. An increase in intensity means the wave is more energetic. Therefore more energy is transferred to the electrons. The electrons have more energy and so a higher stopping potential is required to stop them.

Investigating the PEE Answers 9) How does the stopping potential depend on the intensity? It doesn’t depend on the intensity. 10) What can we conclude about the kinetic energy of the photoelectrons? Justify with reference to your answers in 6-9. The kinetic energy of the electrons can be measured by using the stopping potential. This kinetic energy is not dependent on the intensity. 11) In this section, we are trying to discover why increasing the intensity of the light leads to a higher current reading. In question 5, you hypothesized about the increased current being due more energetic electrons. If the increased current is not due to more energetic electrons, what is causes it? Increasing the intensity does not increase the energy of the electrons. It simply means more electrons must be emitted from the metal, which explains the increased current.