Question

A certain metal plate is completely illuminated by a monochromatic light source. Which of the following would increase the number of electrons ejected from the surface of the metal? 1\. Increasing the intensity of the light source 2\. Increasing the frequency of the light source 3\. Increasing the surface area of the metal plate 1\. I only 2\. I and II only 3\. I and III only 4\. II and III only

Short answer

Based on the analysis of the photoelectric effect, increasing the intensity of the light source (option 1) and increasing the frequency of the light source (option 2) would both increase the number of electrons ejected from the metal plate. However, increasing the surface area of the metal plate (option 3) would only increase the overall total number of electrons ejected, but not per unit area. Therefore, the correct answer is 2. I and II only.

Step-by-step solution

Understanding the Photoelectric Effect

The photoelectric effect is a phenomenon in which electrons are ejected from a metal surface when it is exposed to light of a certain frequency. This effect can be described by the following equation: \[KE = h(f - f_0)\] Where KE is the kinetic energy of the ejected electrons, h is the Planck's constant, f is the frequency of the incident light, and \(f_0\) is the threshold frequency of the metal. According to this equation, the number of ejected electrons is related to the frequency of the light source and the threshold frequency of the metal.

1. Increasing the intensity of the light source

According to the photoelectric effect equation, increasing the intensity of the light source won't directly impact the number of electrons ejected. However, increasing the intensity can indirectly affect the number of electrons ejected by providing more photons per unit area. The more photons reaching the metal surface, the higher the probability of photoemission events. So, option 1 will increase the number of electrons ejected.

2. Increasing the frequency of the light source

The photoelectric effect equation shows that the kinetic energy of ejected electrons is directly proportional to the frequency of the incident light. If we increase the frequency of the light beyond the threshold frequency, this will result in a higher kinetic energy for the ejected electrons and an increased number of photoemission events. Therefore, option 2 will also increase the number of electrons ejected.

3. Increasing the surface area of the metal plate

Increasing the surface area of the metal plate will not impact the number of electrons ejected per unit area. However, a larger surface area would have more surface atoms that can be subjected to photoemission events. So, increasing the surface area would increase the overall total number of electrons ejected. Thus, option 3 will also increase the number of electrons ejected. Based on our analysis, the correct answer is: 2. I and II only

Key concepts

LSAT Physics Questions

Physics questions featured on standardized tests like the LSAT (Law School Admission Test) are designed to evaluate critical thinking and problem-solving skills in the context of scientific concepts. For instance, a common topic is the photoelectric effect, where candidates must understand the underlying principles to determine how changes in light properties affect electron emission. An LSAT physics question tests the ability to apply concepts rather than just recall facts. It requires a deep understanding of the theory and the capability to draw conclusions from given scenarios.

It's important to consider the logic behind the phenomena when dealing with these types of questions. For the photoelectric effect question, one must grasp how intensity, frequency, and surface area can influence electron ejection to answer confidently. The problem-solving steps provided give a structured approach to reasoning through to the correct choice, which is a critical skill both in physics and the LSAT.

Electron Ejection

Electron ejection, essential in the photoelectric effect, involves electrons escaping from the surface of a material when it is exposed to light. But not just any light will do; it must have a frequency that meets or exceeds the material's threshold frequency. This threshold is a characteristic of the material itself, related to the work function – the minimum energy required to remove an electron from the metal surface.

Factors Influencing Electron Ejection

Intensity and frequency are critical when examining electron ejection. While increasing light intensity can lead to more electrons being ejected due to the higher number of incoming photons, it is the frequency (or equivalently, the photon's energy) which determines if the electrons will be ejected at all. When the frequency exceeds the threshold, each photon has enough energy to overcome the work function, resulting in the release of electrons. Teaching students about these nuances is key for a full understanding of the concept.

Monochromatic Light

Monochromatic light plays a pivotal role in the discussion of the photoelectric effect. The term 'monochromatic' literally means 'one color', but in the physics world, it refers to light that has a single wavelength or frequency. When monochromatic light shines upon a metal surface, it provides a consistent energy pattern which simplifies the analysis of electron ejection.

Why Monochromatic Light Matters

By using monochromatic light, the experiment ensures that only photons of a single energy are involved, making it easier to observe and quantify the effects on electron ejection. It is because of this controlled simplicity that experiments using monochromatic light can provide clear evidence of how light frequency affects the ejection of electrons, directly linking to the concept of threshold frequency in the photoelectric effect. Educators should emphasize the importance of monochromatic light in conducting precise scientific experiments in this domain.

Kinetic Energy Equation

The kinetic energy equation for the photoelectric effect is an instrumental and fundamental expression in understanding how electrons behave when subjected to light. Given by the equation \[KE = h(f - f_0)\], where KE is the kinetic energy of the ejected electrons, 'h' the Planck's constant, 'f' is the frequency of the incident light, and \(f_0\) represents the threshold frequency.

Breaking Down the Equation

The equation shows that the kinetic energy of ejected electrons is directly proportional to the frequency of the light minus the threshold frequency. This relationship is the core principle behind electron ejection and is crucial to any physics-related questions on standardized tests like the LSAT. When providing educational content, it's important to dissect this equation thoroughly, explaining each component and their physical significance. Understanding the equation allows students to predict how changes in light frequency affect the ejection of electrons, reinforcing their comprehension of the photoelectric effect.