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11.4.2 - Effect of potential on photoelectric current

Interactive Audio Lesson

Listen to a student-teacher conversation explaining the topic in a relatable way.

Understanding Positive Potential

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Teacher
Teacher

Today, we're going to explore how the potential on the collector plate influences the photoelectric current. Can anyone tell me what happens when we have a positive potential on the collector?

Student 1
Student 1

I think the electrons will be attracted to the plate, which should increase the current.

Teacher
Teacher

Exactly! As we increase the positive potential, the current indeed increases. This continues until we reach saturation current. Can anyone explain what saturation current involves?

Student 2
Student 2

I think saturation current is when all the emitted electrons get collected, so no more increase in current happens.

Teacher
Teacher

Great! Thus, the saturation current represents the maximum flow of electrons. Let’s remember: **Saturation current = maximum collected electrons**. Now, let's discuss what happens when we increase potential beyond saturation.

Student 3
Student 3

It sounds like the current won’t increase anymore because all the electrons are collected.

Teacher
Teacher

Right! Each increase beyond saturation does not change the flow of current. This is an important concept.

Analyzing Negative Potential

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Teacher
Teacher

Now, let's shift our focus to negative potential applied to the plate A. What effect do you think this will have on our photoelectric current?

Student 4
Student 4

I guess it would push the electrons away, which would decrease the current.

Teacher
Teacher

Exactly! When we increase the negative potential, it repels the electrons more strongly. Could someone explain why we observe a cutoff, or stopping potential?

Student 1
Student 1

So, there’s a point at which even the most energetic electrons can't reach the collector anymore, right?

Teacher
Teacher

Spot on! That critical point is the stopping potential. If we relate it back, what equation shows this relationship with kinetic energy?

Student 2
Student 2

$K_{max} = eV_0$?

Teacher
Teacher

Correct! This illustrates that the stopping potential is directly linked to the energy of the emitted electrons.

Student 3
Student 3

So the stopping potential doesn't change with intensity, only with the frequency of light?

Teacher
Teacher

Exactly! More on that in our next session.

Exploring Frequency and Intensity

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Teacher
Teacher

Now, let's dive deeper into the concepts of frequency and intensity. How do they interact with the photoelectric effect?

Student 4
Student 4

I recall the saturation current is affected by the intensity but the stopping potential isn’t.

Teacher
Teacher

Absolutely! The intensity pertains to the number of electrons emitted per second, leading to higher saturation currents. However, what defines the energy of the emitted photoelectrons?

Student 1
Student 1

That would be the frequency of the light. Higher frequency means higher energy for the emitted electrons.

Teacher
Teacher

"Exactly! And just to reinforce this, we have a **mnemonic** you can use: **I-F for I-ncrease in F-requency** – intensity swells saturation while frequency influences energy.

Introduction & Overview

Read a summary of the section's main ideas. Choose from Basic, Medium, or Detailed.

Quick Overview

This section discusses how varying the potential of the collector plate affects the photoelectric current in the photoelectric effect experiment.

Standard

In the study of photoelectric current, the section explains how increasing positive potential enhances the current until saturation is reached, while applying negative potential decreases the current until it ceases at a critical point known as the stopping potential. The relationship between potential, current, and electron energies is emphasized, highlighting the significance of intensity and frequency of incident light.

Detailed

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Audio Book

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Introduction to Photoelectric Current and Potential

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We first keep the plate A at some positive potential with respect to the plate C and illuminate the plate C with light of fixed frequency n and fixed intensity I . We next vary the positive potential of plate A gradually and measure the resulting photocurrent each time.

Detailed Explanation

This initial setup involves two components: a photosensitive plate (C) and a collector plate (A), where plate A is set at a positive potential relative to plate C. By illuminating plate C with consistent light, we can systematically adjust the potential of plate A and observe how the photocurrent changes accordingly. This is key to understanding how electron emission varies with potential.

Examples & Analogies

Think of this like adjusting the water pressure in a pipe. When you gradually increase the pressure (similar to increasing the potential), you can see how much more water flows through the pipe (similar to how more photoelectrons are emitted).

Saturation Current

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It is found that the photoelectric current increases with increase in positive (accelerating) potential. At some stage, for a certain positive potential of plate A, all the emitted electrons are collected by the plate A and the photoelectric current becomes maximum or saturates.

Detailed Explanation

As the accelerating potential increases, more electrons emitted from plate C are directed towards plate A, thereby increasing the photoelectric current. At a certain potential, every emitted electron finds its way to plate A, resulting in a maximum current known as saturation current. Beyond this point, increasing the potential further does not lead to an increase in current because all electrons are already being collected.

Examples & Analogies

Imagine filling a glass with water. Initially, as you pour water (increase the potential), the glass fills up (more current). Once it reaches the brim (saturation), any additional water poured will overflow but won't increase the amount in the glass (the current can't increase).

Cut-off or Stopping Potential

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If we apply a negative (retarding) potential to the plate A with respect to the plate C and make it increasingly negative gradually, the photocurrent is found to decrease rapidly until it drops to zero at a certain sharply defined, critical value of the negative potential V0.

Detailed Explanation

By introducing a negative potential on plate A, the electric field repels the emitted electrons. Eventually, if the negative potential is strong enough, even the most energetic electrons cannot reach plate A, causing a downturn in the photocurrent until it completely ceases at the critical value called the stopping potential. This emphasizes how the energy of the emitted electrons is critical for their collection.

Examples & Analogies

Consider a bouncy ball (the photoelectrons) thrown in the air (towards plate A). If you hold a net (negative potential) at varying heights, initially the ball could reach it. However, if you hold the net too high (too negative), the ball can't get to the net and falls short (no photocurrent).

Maximum Kinetic Energy of Photoelectrons

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The photocurrent is zero when the stopping potential is sufficient to repel even the most energetic photoelectrons, with the maximum kinetic energy (Kmax), so that Kmax = eV0.

Detailed Explanation

At the stopping potential V0, the most energetic electrons, having kinetic energy Kmax, are just prevented from reaching the collector. This maximum energy is directly related to the electric potential applied. The relationship shows that as you alter the stopping potential, the kinetic energy of the electrons can also be determined.

Examples & Analogies

Think of a race where runners are stopped just short of the finish line (the stopping potential). The fastest runner (the most energetic electron) would only reach the line if the finish line (collector) is not too far away – they can only run as far as their energy allows.

Independent Stopping Potential

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For a particular frequency of incident radiation, the minimum negative (retarding) potential V0 given to the plate A for which the photocurrent stops or becomes zero is called the cut-off or stopping potential.

Detailed Explanation

The stopping potential is determined by the frequency of the incident light, not its intensity. This means that no matter how intense the light is, if the frequency is below a certain threshold, there won’t be any emitted photoelectrons. The stopping potential defines a critical boundary that showcases the energy dynamics of photoelectrons relative to the frequency of the light used.

Examples & Analogies

Think of a battery that needs a specific voltage to power a device. If the voltage is too low (frequency of light too low), it doesn’t matter how much energy you supply (intensity); the device will not work (no photoelectrons emitted).

Definitions & Key Concepts

Learn essential terms and foundational ideas that form the basis of the topic.

Key Concepts

  • Photoelectric Current: The electric current produced when electrons are emitted from a metal upon exposure to light.

  • Saturation Current: The maximum current achieved when all emitted electrons are collected.

  • Stopping Potential: The critical negative voltage at which no electrons can reach the collector plate.

  • Threshold Frequency: The minimum frequency of light necessary to eject electrons.

  • Intensity: The energy per unit area received by the photosensitive material affecting emitted electron quantity.

Examples & Real-Life Applications

See how the concepts apply in real-world scenarios to understand their practical implications.

Examples

  • If a zinc plate is exposed to UV light, emitted electrons create a photoelectric current measurable in an external circuit as long as the light frequency exceeds the threshold.

  • Increasing the potential of a collector plate in a photoelectric effect setup shows a rise in current until saturation occurs, indicating all electrons are captured.

Memory Aids

Use mnemonics, acronyms, or visual cues to help remember key information more easily.

🎵 Rhymes Time

  • Electrons will flow, when the voltage is high, but too much won't help, lest it reach the sky!

📖 Fascinating Stories

  • Imagine a ball rolling down a hill. If the hill is high (positive potential), the ball (electron) rolls faster until it levels off at a plateau (saturation current). Pushing the ball back uphill (negative potential) will stop it from moving forward!

🧠 Other Memory Gems

  • S-P-T: Saturation, Photoelectric, Threshold—these tell us all we need to know about current!

🎯 Super Acronyms

LIGHT

  • Luminance Intensity Governing How Tricky (current and photoelectrons) operate!

Flash Cards

Review key concepts with flashcards.

Glossary of Terms

Review the Definitions for terms.

  • Term: Photoelectric Current

    Definition:

    The flow of electric current resulting from the emission of electrons when light hits a photosensitive material.

  • Term: Saturation Current

    Definition:

    The maximum current reached when all emitted electrons are collected by the collector plate.

  • Term: Stopping Potential

    Definition:

    The negative potential at which the photocurrent drops to zero, indicating no emitted electrons can reach the collector due to repulsion.

  • Term: Threshold Frequency

    Definition:

    The minimum frequency of incident light required to cause photoelectric emission.

  • Term: Intensity

    Definition:

    The power per unit area received by the photosensitive material, which affects the number of emitted electrons.