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11.4.2. Effect of potential on photoelectric current
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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?
I think the electrons will be attracted to the plate, which should increase the current.
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?
I think saturation current is when all the emitted electrons get collected, so no more increase in current happens.
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.
It sounds like the current won’t increase anymore because all the electrons are collected.
Right! Each increase beyond saturation does not change the flow of current. This is an important concept.
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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?
I guess it would push the electrons away, which would decrease the current.
Exactly! When we increase the negative potential, it repels the electrons more strongly. Could someone explain why we observe a cutoff, or stopping potential?
So, there’s a point at which even the most energetic electrons can't reach the collector anymore, right?
Spot on! That critical point is the stopping potential. If we relate it back, what equation shows this relationship with kinetic energy?
?
Correct! This illustrates that the stopping potential is directly linked to the energy of the emitted electrons.
So the stopping potential doesn't change with intensity, only with the frequency of light?
Exactly! More on that in our next session.
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Create a free accountNow, let's dive deeper into the concepts of frequency and intensity. How do they interact with the photoelectric effect?
I recall the saturation current is affected by the intensity but the stopping potential isn’t.
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?
That would be the frequency of the light. Higher frequency means higher energy for the emitted electrons.
"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.
Overview
Short Summary
This section discusses how varying the potential of the collector plate affects the photoelectric current in the photoelectric effect experiment.
Medium Summary
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 Summary
Detailed Explanation of the Effect of Potential on Photoelectric Current
In the photoelectric effect experiment, a photosensitive plate (emitter) is illuminated, resulting in the emission of electrons. By observing how the current changes with varying the potential of the collector plate, we can understand critical aspects of the photoelectric phenomenon:
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Impact of Positive Potential: When the collector plate (A) is maintained at a positive potential relative to the emitter plate (C), the emitted electrons are attracted towards the collector, leading to an increase in photoelectric current. As the positive potential is gradually increased, the current rises until it reaches a maximum value known as the saturation current, which indicates that all emitted electrons are being collected.
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Saturation Current: Beyond a certain positive potential, further increases do not result in higher currents. This is because all the electrons emitted from the emitter have been collected, illustrating the limit of current flow based on the number of emitted electrons.
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Effect of Negative Potential: Applying a negative potential to the collector plate causes a decrease in photocurrent. As the negative potential is increased, even the more energetic electrons are unable to reach the collector plate, reducing the current until it eventually drops to zero at a critical value known as the stopping potential.
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Understanding Stopping Potential: The stopping potential indicates the threshold value at which no photoelectrons, no matter their energy, can overcome the repelling force of the negative potential. The relationship between the stopping potential and the maximum kinetic energy of the emitted electrons is defined by the equation:
where is the maximum kinetic energy of photoelectrons and is the charge of the electron.
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Observation of Frequency Dependence: As experiments are conducted with different light intensities but at a constant frequency, the saturation current will vary, indicating a direct relation with the number of photoelectrons emitted. However, the stopping potential remains constant, reinforcing the idea that it is determined by the frequency of the incident radiation rather than its intensity.
Through these observations, this section emphasizes how varying potentials in the photoelectric effect relates directly to the behavior of electrons and elucidates the fundamental principles governing their emission from metals upon illumination.
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Create a free accountWe 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).
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Create a free accountIt 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).
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Create a free accountIf 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).
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Create a free accountThe 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.
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Create a free accountFor 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).
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Key Concepts
Core takeaways and short definitions to help you quickly recall the key ideas from this section.
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
Step-by-step examples to apply the section's ideas and test your understanding.
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.
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Glossary
Photoelectric Current
The flow of electric current resulting from the emission of electrons when light hits a photosensitive material.
Saturation Current
The maximum current reached when all emitted electrons are collected by the collector plate.
Stopping Potential
The negative potential at which the photocurrent drops to zero, indicating no emitted electrons can reach the collector due to repulsion.
Threshold Frequency
The minimum frequency of incident light required to cause photoelectric emission.
Intensity
The power per unit area received by the photosensitive material, which affects the number of emitted electrons.