Using PhET simulations in a large lecture class – The Photoelectric Effect

Students should be able to...

• describe the results of the photoelectric effect experiment
• explain why a photon model of light is necessary to explain the results
• practice hypothetical reasoning and explain the difference between predictions for a wave model and a particle model of light
• explain how changing the wavelength of light, the intensity of light, the work function of the target, or the potential difference between the plates does and does not affect the behavior of the electrons
• calculate the wavelength of light, the work function of the target, or the stopping potential, if given the other two
• draw and interpret graphs of current vs. voltage for a range of experimental set-ups
• draw and interpret graphs of initial kinetic energy vs. frequency for a range of experimental set-ups

This activity was originally designed to be used in a large-lecture modern physics course for engineering majors. It includes PowerPoint slides for two to three 50-minute lectures using Peer Instruction with clickers, and one homework assignment suitable for an online homework system. The homework was designed for the students to work together on it in small groups, and is probably most effective if they do so. Students in a modern physics course for engineering majors reported that they spent an average of 2 hours and 16 minutes on the homework and about half of them worked with other students on it.

The activity could be adapted for any course that covers the photoelectric effect, including an introductory undergraduate or high school physics course. In a course that does not use machine grading, some of the multiple choice questions on the homework could be changed to open-ended questions.

The activity is mostly conceptual, with some mathematical problems involving basic algebra. The content assumes a basic knowledge of electric fields, potentials, and circuits in order to interpret the experimental apparatus. It also assumes a basic knowledge of the wave nature of light. Slides 26-29 of the lecture assume a familiarity with potential energy functions. In a course that does not address this more advanced topic, these slides could be eliminated without losing the flow of the argument.

This activity includes PowerPoint slides for two to three 50-minute lectures using Peer Instruction with clickers, and one homework assignment suitable for an online homework system. Both the lecture slides and the homework can be downloaded from the PhET activities database .

The photoelectric effect proves to be a much harder topic for students than it first appears. The two research papers listed in the References and Resources section (McKagan et al., 2009 and Steinberg et al., 1996) include detailed description of student difficulties and how to address them. Some common problems for students include:

• thinking voltage rather than light takes electrons off plate
• thinking current increases with speed of electrons
• difficulty explaining basic function of experiment
• difficulty explaining classical model of light
• difficulty explaining why PE experiment leads to photon model of light
• difficulty with thinking hypothetically and separating what was expected classically from what really happens

The simulation can help students resolve all of these difficulties except the last one.

A research paper by McKagan et al. describes the assessment of this activity by analyzing student responses to two exam questions. The first exam question, designed by Steinberg et al., was given to assess the learning goal of correctly predicting the results of experiments of the photoelectric effect. Students in classes using this curriculum performed well on all aspects of this question. The second exam question was given to assess the learning goal of describing how the results of the photoelectric effect experiment lead to the photon model of light. Most students were able to correctly describe both observations in the experiment and inferences about the photon model of light, but they could not necessarily reason about the connection between the two or distinguish between them. Both exam questions are described in detail in McKagan et al. and could be copied or adapted for assessment by other instructors.