Physics SS 3 Week 7
Topic: PHOTOELECTRIC EFFECT
INTRODUCTION
In 1887, photoelectric effect was invented by the scientist H. Hertz. When we passing a light into a material, the material should emit an electrons. This effect is called as Photoelectric effect. Some of the rays produced in Photoelectric Effect are x-rays, ultraviolet rays and γ-rays.
Einstein’s says that every photon or a quantum of light has an energy value hν. Electrons are located inside the surface of the metal. There may be some amount of work W should be required for an electron to come out of the metal. If the transferred energy is high comparing the internal attractions then the electrons gets liberated. In this the one photon can release a one electron. This is nothing but the Photoelectric Effect. Let us study more about the Photoelectric effect in this section.
Photoelectric Effect Definition
It is the phenomenon of emission of electrons from the surface of metals when the radiations of suitable frequency and suitable wavelength fall on the surface of the metal.
The following parameters are related to Photoelectric effect:
Photoelectric Current
Stopping Potential
Threshold Energy
Work Function
The emitted electrons are called photo electrons and the current so produced is called as the Photoelectric Current. The intensities of photo electrons vary with material.
The Stopping Potential is the potential difference applied to stop the electrons from being ejected from the surface when the light falls on it.
The Threshold Frequency is defined as the minimum frequency of incident light required for the photo electric emission. It is denoted by ν.
Work Function is the minimum amount of energy necessary for the photo electric emission to start. It is denoted by ϕ0.
Photoelectric Effect Equation
The Einstein equation describes the phenomena of photoelectric effect but the validity of this equation is only up to two regions, one is visible light and second is for ultraviolet light.
The energy of photon is equal to energy required to remove electron + kinetic energy of the emitted electron.
E = W + K.E.
where E = Energy of photon
W = Work function
K.E = Kinetic Energy.
Energy of photon, E = h ν …………………….. (a)
Where h = Planck’s constant
ν = frequency of photon incident light or
So h ν = W + K.E …………………………… (b)
The work function W is defined as the minimum energy needed to remove an electron from the surface of a given metal. It is equal to h ν0.
K.E= This is the kinetic energy (maximum) of emitted electrons.
K.E = 1/2 mv2.
Here ν0 is the threshold frequency.
m = rest mass of the emitted electron
v = The speed or velocity of the emitted electron.
Work function or threshold energy ( ϕ ):
The minimum energy of incident photon below which no ejection of photon electron from a metal surface will take place is known as work function of threshold energy for that metal.
ϕ = h ν0 = hC/λ0
Work function is the characteristic of a given metal. If E is the Energy of incident photon then
1. If E < ϕ, no photo electric effect will take place.
2. If E = ϕ, photoelectric effect will take place but kinetic effect of ejected photo electron is zero.
3. If E > ϕ, photo electric effect will take place along with possession of kinetic energy by ejected electron.
Photoelectric Effect Experiment
The Phenomenon was discovered by Hertz and was experimentally proved by Thomsonand Millikan. Einstein added his photoelectric equation to support the phenomenon. The phenomenon is basically about light energy (photo) converting into electrical energy.
The Apparatus consists of an evacuated quartz tube having photosensitive plate called emitter A and collector B. Plate A is connected to the negative terminal and plate B is connected to the positive terminal of a battery via rheostat R. The potential difference between the plates AB can be adjusted by changing the value of the rheostat. When light of suitable wavelength is incident on the plate A then electrons are emitted and reaches plate B, thus measurable current flows through the circuit.
Factors on which Photoelectric effect depends :
The amount of current flow (number of electrons) and the kinetic energy of the emitted electrons depend upon :
1. Applied Potential difference between the plates:
For a given photo metal if the frequency and intensity of incident light kept constant and if the potential difference between the plates is increased then photo current also increases until it reaches a maximum (saturated) value. If the terminals are reversed and we increase the potential difference gradually then photo current decreases and at one point it becomes zero. This point where the current is zero is termed as Stopping Potential.
2. Intensity of incident radiation:
For a given photo metal if the frequency and applied voltage is kept constant with increasing intensity then we observe that photo current increases with increases in intensity with out change in the stopping potential.
4. Material of the Photo metal:
The experiment is repeated for different photo metal and their threshold frequencies are noted. The variation of frequency versus stopping potential is graphed and noticed that threshold frequency varies from metal to metal.
Photoelectric Effect Applications
Below are the applications of Photoelectric effect:
1. The photoelectric effect is used in the photoelectric cell which converts a light energy into electrical energy.
2. In cinematography photoelectric effect has a application of reproducing the sound.
3. Photoelectric effect also has an application in street lights for automatic switch on and off.
4. Traffic signals are using this effect for automatic controls and for count the machines.
5. Working of burglar alarm uses this photoelectric effect.
6. Television transmission is one of the applications of this photoelectric effect.
Threshold Frequency
Threshold frequency is defined as the minimum frequency of incident light which can cause photo electric emission i.e. this frequency is just able to eject electrons without giving them additional energy.
It is denoted by V0
Work Function
Minimum amount of energy which is necessary to start photo electric emission is called Work Function. If the amount of energy of incident radiation is less than the work function of metal, no photo electrons are emitted.
It is denoted by ϕ0. Work function of a material is given by ϕo = hv0 .
It is a property of material. Different materials have different values of work function. Generally, elements with low I.P values have low work function such as Li, Na, K, Rb, and Cs.
Einstein’s Equation
The mass-energy relationship announced by Albert Einstein in 1905 in the form E = mc 2, where E is a quantity of energy, m its mass, and c is the speed of light. It presents the concept that energy possesses mass. 2. The relationship E max = hf – W, where E max is the maximum kinetic energy of electrons emitted in the photoemissive effect, h is the Planck constant, f the frequency of the incident radiation, and W the work function of the emitter. This is also written E max = hf – φe, where e is the electronic charge and φ a potential difference, also called the work function. (Sometimes W and φ are distinguished as work function energy and work function potential.) The equation can also be applied to photoemission from gases, when it has the form: E = hf – I, where I is the ionization potential of the gas.
X-ray Production
When fast-moving electrons slam into a metal object, x-rays are produced. The kinetic energy of the electron is transformed into electromagnetic energy. The function of the x-ray machine is to provide a sufficient intensity of electron flow from the cathode to anode in a controlled manner. The three principal segments of an x-ray machine – a control panel, a high-voltage power supply, and the x-ray tube are all designed to provide a large number of electrons focused to a small spot in such a manner that when the electrons arrive at the target, they have acquired high kinetic energy.
Kinetic energy is the energy of motion. Stationary objects have no kinetic energy; objects in motion have kinetic energy proportional to their mass and the square of their velocity.
The equation used to calculate kinetic energy is:
KE = ½ mv2
where m is the mass in kilograms, v is the velocity in meters per second, and KE is the kinetic energy in joules. In determining the magnitude of the kinetic energy of a projectile, the velocity is more important than the mass.
In a x-ray tube, the projectile is the electron. As its kinetic energy is increased, both the intensity (number of x-rays) and the energy (their ability to penetrate) of the created x-rays are increased.
The x-ray machine is a remarkable instrument. It conveys to the target an enormous number of electrons at a precisely controlled kinetic energy. At 100 mA, for example, 6 x 1017 electrons travel from the cathode to the anode of the x-ray tube every second.
The distance between the filament and the target is only about 1 to 3 cm. Imagine the intensity of the accelerating force required to raise the velocity of the electrons from zero to half the speed of light in so short a distance.
The electrons traveling from the cathode to anode in a vacuum tube comprise the x-ray current and are sometimes called projectile electrons. When the projectile electrons impinge on the heavy metal atoms of the target, they interact with these atoms and transfer their kinetic energy to the target. These interactions occur within a very small depth of penetration into the target. As they occur, the projectile electrons slow down and finally come nearly to rest, at which time they can be conducted through the x-ray anode assembly and out into the associated electronic circuitry.
The projectile electron interacts with either the orbital electrons or the nuclei of target atoms. The interactions result in the conversion of kinetic energy into thermal energy and electromagnetic energy in the form of x-rays.
By far, most of the kinetic energy of projectile electrons is converted into heat. The projectile electrons interact with the outer-shell electrons of the target atoms but do not transfer sufficient energy to these outer-shell electrons to ionize them. Rather, the outer-shell electrons are simply raised to an excited, or higher, energy level. The outer-shell electrons immediately drop back to their normal energy state with the emission of infrared radiation. The constant excitation and restabilization of outer-shell electrons is responsible for the heat generated in the anodes of x-ray tubes.
Generally, more than 99% of the kinetic energy of projectile electrons is converted to thermal energy, leaving less than 1% available for the production of x-radiation. One must conclude, therefore, that, sophisticated as it is, the x-ray machine is a very inefficient apparatus.
The production of heat in the anode increases directly with increasing tube current. Doubling the tube current doubles the quantity of heat produced. Heat production also varies almost directly with varying kVp.
The efficiency of x-ray production is independent of the tube current. Regardless of what mA is selected, the efficiency of x-ray production remains constant. The efficiency of x-ray production increases with increasing projectile-electron energy. At 60 kVp, only 0.5% of the electron kinetic energy is converted to x-rays; at 120 MeV, it is 70%.
Characteristic Radiation
If the projectile electron interacts with an inner-shell electron of the target atom rather than an outer-shell electron, characteristic x-radiation can be produced. Characteristic x-radiation results when the interaction is sufficiently violent to ionize the target atom by total removal of the inner-shell electron. Excitation of an inner-shell electron does not produce characteristic x-radiation.
When the projectile electron ionizes a target atom by removal of a K-shell electron, a temporary electron hole is produced in the K shell. This is a highly unnatural state for the target atom and is corrected by an outer-shell electron falling into the hole in the K shell. The transition of an orbital electron from an outer shell to an inner shell is accompanied by the emission of an x-ray photon. the x-ray has energy equal to the difference in the binding energies of the orbital electrons involved.
Discrete X-ray Spectrum
We saw earlier that characteristic x-rays have precisely fixed, or discrete energies and that these energies are characteristic of the differences between electron binding energies of a particular element. A characteristic x-ray from tungsten, for example, can have one of fifteen energies and no others.
Bremsstrahlung Radiation
The production of heat and characteristic x-rays involves interactions between the projectile electrons and the electrons of target atoms. A third type of interaction in which the projectile electron can lose its kinetic energy is an interaction with the nucleus of a target atom. In this type of interaction, the kinetic energy of the projectile electron is converted into electromagnetic energy.
A projectile electron that completely avoids the orbital electrons on passing through an atom of the target may come sufficiently close to the nucleus of the atom to come under its influence. Since the electron is negatively charged and the nucleus is positively charged, there is an electrostatic force of attraction between them. As the projectile electron approaches the nucleus, it is influenced by a nuclear force much stronger than the electrostatic attraction. As it passes by the nucleus, it is slowed down and deviated in its course, leaving with reduced kinetic energy in a different direction. This loss in kinetic energy reappears as an x-ray photon. These types of x-rays are called bremsstrahlung radiation, or bremsstrahlung x-rays. Bremsstrahlung is the German word for slowing down or braking; bremsstrahlung radiation can be considered radiation resulting from the braking of projectile electrons by the nucleus.
A projectile electron can lose any amount of its kinetic energy in an interaction with the nucleus of a target atom, and the bremsstrahlung radiation associated with the loss can take on a corresponding range of values. For example, an electron with kinetic energy of 70 keV can lose all, none, or any intermediate level of that kinetic energy in a bremsstrahlung interaction; the bremsstrahlung x-ray produced can have an energy in the range of 0 to 70 keV. This is different from the production of characteristic x-rays that have specific energies.
Continuous X-ray Spectrum
If it were possible to identify and quantify the energy contained in each bremsstrahlung photon emitted from an x-ray tube, one would find that these energies extend from that associated with the peak electron energy all the way down to zero. In other words, when an x-ray tube is operated at 70 kVp, bremsstrahlung photons with energies ranging from 0 to 70 keV are emitted. Thus, creating a typical continuous, or bremsstrahlung, x-ray emission spectrum.
This emission spectrum is sometimes called the continuous emission spectrum because, unlike in the discrete spectrum, the energies of the photons emitted may range anywhere from zero to some maximum value. The general shape of the continuous x-ray spectrum is the same for all x-ray machines. The maximum energy that an x-ray can have is numerically equal to the kVp of operation. The greatest number of x-ray photons is emitted with energy approximately one-third of the maximum photon energy. The number of x-rays emitted decreases rapidly at very low photon energies and below 5 keV nearly reaches zero.
X-ray Tube
X-rays for medical diagnostic procedures or for research purposes are produced in a standard way; by accelerating electrons with a high voltage and allowing them to collide with a metal target. X-rays are produced when the electrons are suddenly decelerated upon collision with the metal target; these x-rays are commonly called brehmsstrahlungor “braking radiation”. If the bombarding electrons have sufficient energy, they can knock an electron out of an inner shell of the target metal atoms. Then electrons from higher states drop down to fill the vacancy, emitting x-ray photons with precise energies determined by the electron energy levels. These x-rays are called characteristic x-rays.
Radiation and radioactive materials are naturally part of our environment. Radiation is energy that moves in a very high speed. It can move through space or through matter. It is common knowledge that radiation can cause adverse effects to humans. Radiological safety hazards are potential harmful threats to human health that must be regulated by safety controls and precautions.
There are several health problems radiation poses to humans. It starts by breaking chemical bonds that hold molecules together. This then starts the cells of the body to change. The adverse effects of radiation depend upon the dosage and time of exposure of the person to radiation. The most dangerous will probably be getting large doses of radiation in a shorter period of time. A cell can instantaneously die at high radiation doses; therefore, a person can instantaneously die. On lower doses, the cells can repair themselves first and the person will recuperate. However, if the person has a preexisting disease that causes malfunctions with the cell repairs, the person can be in grave danger.
Threshold effects are those immediate noticeable effects due to radiation exposure. These include radiation sickness, cataracts, sterility and fetal effects. Signs of radiation sickness are nausea, vomiting, headache and loss of white blood cells. If the person does not receive medical treatment, there is a chance that he or she will die within 60 days. Hair loss can also be a sign of radiation sickness.
Radiation is highly beneficial in the medical, research, and industrial fields and can even be used in communications as well. Different types of radiation are used in different applications. The two main classifications of radiation are the ionizing and non-ionizing radiations.
Radiological safety hazards are much more evident in the ionizing type of radiation. This is because it carries more amount of energy than the non-ionizing radiation. This type of radiation exists in two forms: the electromagnetic rays or the particles. X-rays, gamma rays, alpha and beta particles are just a few examples of ionizing radiation.
Non-ionizing radiation, on the other hand, refers to two main regions of the electromagnetic spectrum. These are the optical radiation and electromagnetic fields. This type of radiation is used extensively in the manufacturing and telecommunications industry. There also have been no significant related health problems. Despite this, the ultra-violet light, which can be natural or man-made, can still cause health problems like the cancer of the skin.
Radiological safety hazards call for different precautions and safety measures. Different types of radiation call for different types of shielding protection. The amount and intensity of radiation also matters in this. For example, one can be shielded from alpha particles with just a sheet of paper while aluminum sheets can protect you from beta particles. Also, the thicker the shielding protection is, the lesser the intensity of the radiation becomes.
There are three types of radiation protection: occupational radiation protection, medical radiation protection, and public radiation protection. Occupational radiation protection specifically deals with the workers in the different industries that make use of radiation. Medical radiation protection is the protection of the patients and the radiographer. For example, when a patient undergoes an X-ray testing, both the patient and the radiologist take preliminary precautions. Lastly, public radiation protection is directed towards the public, the population.
Questions:
1. In Photoelectric Effect, when we passing a light into a material, the material should emit
A. Electrons B. Protons C. Positrons D. Neutrons
2. ……………………….. amount of energy which is necessary to start photo electric emission is called Work Function
A. Maximum B. Average C. Minimum D. Littlest
3. Threshold frequency is defined as the ……………………. frequency of incident light which can cause photo electric emission
A. Maximum B. Minimum C. Average D. highest
4. Which is not an example of ionizing radiation?
A. X-Rays B. Gamma Rays C. Projectile Gamma Rays D. Alpha and Beta Particles
5. The following parameters are related to Photoelectric effect, except
A. Photoelectric Current B. Threshold Energy C. Work Function D. Frictionless Electrons
Answers:
1. A 2. C. 3. B 4. C 5. D
