Topicm578872b9c35da6fc_1528449000663_0Topic

External photoelectric effectphotoelectric effectphotoelectric effect and its application

Levelm578872b9c35da6fc_1528449084556_0Level

Third

Core curriculumm578872b9c35da6fc_1528449076687_0Core curriculum

X. Atomic physics. The student:

5) describes ionization, photoelectric and photochemical phenomena as induced only by radiation with a frequency above threshold.

Timingm578872b9c35da6fc_1528449068082_0Timing

45 minutes

General learning objectivesm578872b9c35da6fc_1528449523725_0General learning objectives

Explains the occurrence of the photoelectric effect.

Key competencesm578872b9c35da6fc_1528449552113_0Key competences

1. Specifies the conditions required for the occurrence of the photoelectric effect.

2. Describes the experimental setup needed to observe the photoelectric effect.

3. Performs simple calculations related to the photoelectric effect.

Operational (detailed) goalsm578872b9c35da6fc_1528450430307_0Operational (detailed) goals

The student:

- explains qualitatively the photoelectric effectphotoelectric effectphotoelectric effect,

- solves simple problems related to the photoelectric effect.

Methodsm578872b9c35da6fc_1528449534267_0Methods

1. Discussion.

2. Experiment.

Forms of workm578872b9c35da6fc_1528449514617_0Forms of work

1. Individual work.

2. Group work.

Lesson stages

Introductionm578872b9c35da6fc_1528450127855_0Introduction

Revision:

Explain what a photon is and how its energy is defined.

Procedurem578872b9c35da6fc_1528446435040_0Procedure

Photoelectric effect:

When light is incident on the metal surface, electrons are emitted from it. This phenomenon is called photoelectric effect. These electrons are called photoelectrons and the electric current produced by the photoelectrons is called photoelectric current.
The outermost shell of an atom is occupied by valence electrons. They are relatively loosely attached to the atom. A specific energy is needed to remove the valence electrons from the atom. This energy is provided by the incident light.m578872b9c35da6fc_1527752256679_0When light is incident on the metal surface, electrons are emitted from it. This phenomenon is called photoelectric effect. These electrons are called photoelectrons and the electric current produced by the photoelectrons is called photoelectric current.
The outermost shell of an atom is occupied by valence electrons. They are relatively loosely attached to the atom. A specific energy is needed to remove the valence electrons from the atom. This energy is provided by the incident light.

[Illustration 1]

The photoelectric effectphotoelectric effectphotoelectric effect occurs under certain conditions:

The frequency $\nu$ of the incident light must be above a minimal value called threshold frequencythreshold frequencythreshold frequency ${\nu }_0$, which is specific for any material.

High‑frequency light (light with short wavelengths) have energy greater then low‑frequency light (light with long wavelengths).

In order for the electron to be emitted from the metal surface, it must gain from an incident quantum of light or photon more energy than the work functionwork functionwork function - the quantity that characterizes a given metal. The kinetic energy of photoelectrons increases with increasing frequency of incident light (energy of incident photons).

Definitions:

Photoelectron is an electron emitted from a metal surfacesurfacesurface by interaction with an incident photon.

Work function is a minimum energy required to eject an electron from a metal surface.

Task 1

Look at the slideshow and learn what kind of an experimental setup is necessary to observe the photoelectric effect. Try to formulate some general conclusions.

[Slideshow]

Observations:

If the incidentincidentincident light frequency is less than the threshold frequencythreshold frequencythreshold frequency, increasing the light intensity does not help and the electrons will not be emitted from the metal surfacesurfacesurface.

The photoelectric current increases as the light intensity increases, given that the frequency of light is greater than the threshold frequency.

The relation between the energy of incident photon, work functionwork functionwork function and kinetic energy of emitted photon is described by the Einstein‑Millikan equation:

where:
h - the Plank constant 6.63 · 10Indeks górny -34^-34 J·s,
$\nu$ - the frequency of the incidentincidentincident light (Hz),
W - the work functionwork functionwork function (J),
$E_k$ - the maximum kinetic energy of the emitted electrons (J).

The relation between the work function at the threshold frequency ${\nu }_0$ is expressed by the formula:

Braking potential:

The photoelectric current in a typical experimental system is very small. The number of electrons reaching the anode can be measured using anode current. However, this measurement is not very precise. You can also apply reverse voltage to the anode. This reverse voltage repels electrons and prevents them from reaching the anode - photoelectric current decreases to zero. The highest required voltage is called the VIndeks dolny 0₀ braking potential. The energy corresponding to this potential is eVIndeks dolny 0₀ and is equal to the maximum kinetic energy EIndeks dolny k_k of the emitted electrons. In this way, we can measure output work with greater precision.

[Illustration 2]

Definition:

Electronvolt is the amount of energy a single electric charge (electron) gains while being accelerated in an electric field with the potential difference of 1 volt. Energy of photons is conveniently expressed in eV. For instance, visible light photons have energies in the range of 1,8 - 3,1 eV.m578872b9c35da6fc_1527752263647_0Electronvolt is the amount of energy a single electric charge (electron) gains while being accelerated in an electric field with the potential difference of 1 volt. Energy of photons is conveniently expressed in eV. For instance, visible light photons have energies in the range of 1,8 - 3,1 eV.

1 eV = 1,6 · 10Indeks górny -19^-19 J.

Task 2

If the wavelength of light is 250 nm, what is the maximum kinetic energy of the emitted photoelectrons from a metal for which a work functionwork functionwork function is equal to 4,5 eV?

Answer:

0,47 eV.

Lesson summarym578872b9c35da6fc_1528450119332_0Lesson summary

The photoelectric effectphotoelectric effectphotoelectric effect occurs only when the incident light frequency is greater than the threshold frequencythreshold frequencythreshold frequency characteristic for a given metal.

Selected words and expressions used in the lesson plan

currentcurrentcurrent

incidentincidentincident

outermost shelloutermost shelloutermost shell

photoelectric effectphotoelectric effectphotoelectric effect

potentialpotentialpotential

stopping potentialstopping potentialstopping potential

surfacesurfacesurface

threshold frequencythreshold frequencythreshold frequency

valence electronsvalence electronsvalence electrons

work functionwork functionwork function