Republic of the Philippines
Department of Education
Regional Office IX, Zamboanga Peninsula
11/12
Zest for Progress
Z P
eal of
artnership
General Physics 2
Quarter 4 – Module 6
The Photoelectric Effect & Radioactivity
Name of Learner:
Grade & Section:
Name of School:
Module
5
The Photoelectric Effect &
Radioactivity
What I Need to Know
This module was developed to strengthen your knowledge about
Photoelectric effect. It is designed to help you master in calculating the rate
of decay of any isotopes using Half-Life.
After comprehending this module, you are supposed to:
1. Explain the photoelectric effect using the idea of light quanta or
photons. (STEM_GP12MPIVh-45)
2. Explain qualitatively the properties of atomic emission and absorption
spectra using the concept of energy levels. (STEM_GP12MPIVh-46)
3. Calculating radioisotope activity using the concept of half-life.
(STEM_GP12MPIVh-47)
What’s In
When you were in Grade 10, you were able to describe how
electromagnetic wave were produced and propagated. All forms of
electromagnetic waves that include light have a dual nature, that it
considered as a wave and a particle.
Let us recall some concepts on electromagnetic wave and light.
ACTIVITY 1: DESCRIBING SOULMATE!
Directions: Choose the correct term that best describe the statement
and write your answer on the space provided before the number.(2pts each)
Wave
Light
Electromagnetic wave
Transverse
Wavelength
__________1.It is understood to be a transverse wave produced by a vibrating
electric charge.
__________2. Describe by Maxwell, as a propagating wave made up of electric
and magnetic field.
__________3.It is a wave in which the movement of the energy is perpendicular
to the movement of the particle of the wave.
__________4.Characteristic of EM, is the measurement of the distance between
the successive crests or troughs of a wave.
__________5. Moving oscillations, not carrying matter along.
What’s New
ACTIVITY 2A: PA EFFECT PA MORE!
Directions: Write T if the statement is true, F if the statement is false and
wirte the letter on the space provided before the number.
_____1. When EM light is incident on a surface, electrons are ejected.
_____2. Photoelectric effect demonstrates how electron are ejected.
_____3. In order for the electron to be ejected the energy of the photon must
be greater than to the amount of work.
_____4. Ejected electron receive the energy carried by the photons that hits
surface of the materials.
_____5. Light must have a sufficiently long wavelength to provide energy
large enough to eject electrons.
ACTIVITY 2B: MATCH ME!
Directions: Match column A with column B. Write the corresponding letter
on the space provided before the number.
Column A
_____1. The amount of time in which half of any
sample of identical nuclei will undergo
decomposition.
______2. It is made up of protons and neutrons.
______3. A term, which means changes ejected
electron into a nucleus of an element.
______4. It is dependent on the half-life of the
substance.
______5. A force that holds nucleus together.
Column B
a. Nuclear Force
b. Decay
c. Half-life
d. Radioactive decay
e. Nucleus
f. Electron
What is it
Photons or Light Quanta
The radiation beam acts like a stream of energy particles, when
electromagnetic radiation interacts with atoms and molecules.
Each photon possesses an amount of energy dependent on the
frequency (ƒ) or wavelength (λ) of the radiation in the beam.
In calculating the photon energy it can be mathematically determined
as
photon energy = hƒ = hc/λ
Where
h = Planck s constant with the value of 6.626 x 10 34 Joules (J)
ƒ = frequency
λ = wavelength
c = speed of light in a vacuum with the constant value of 3.0 x 108 m/s
Photoelectric Effect
A phenomenon when light is incident on a surface, electron are emitted
or ejected. These electron gain energy from the photon that hit the surface
of the materials. In order to eject the electron, light must have a sufficient
short wavelength to provide large energy. Lastly the energy of the photon
must be equal to the amount of work necessary to free an electron from the
surface of the material.
In many materials photoelectric effect can be observed but it can be
easily observed in the metals.
Figure 1. The emission of electrons from a
Metal plate caused by light quanta
Source: https://www.khanacademy.org/
Energy Levels, Atomic Emission, Atomic Absorption
Energy levels are fixed distances from the nucleus of an atom where
electrons may be found. Electrons at higher levels have more energy and it
jump from a lower to the next higher energy level when it absorb this amount
of energy. However this electron will move from higher energy level to a lower
energy level when it emit or given off energy in a form of light. Take note
that different atoms have different arrangement of electrons that is why, it
also give off different colors.
Figure 2. Absorption and Emission of electron
Source: https://www.wikipedia.org/
Radioactive Decay and Half Life
Radioactivity it is a property exhibited by certain types of emitting
energy and subatomic particles spontaneously. Mostly unstable nucleus will
decompose spontaneously, or known as decay, into a more stable
configuration. This decomposition of an unstable nucleus will takes places
in an specific ways by emitting certain particles or in the form of light.
Radioactive Decay is the property of several naturally occurring
elements
that
produced
isotopes
of
the
elements.
Figure 3: Radioactive Decay
Source: https://www.earth.com/
Types of radioactive decay
1. Alpha (α) particle decay - involves the ejection of two neutrons and two
protons from the nucleus of a radioactive
element.
2. Beta (β) particle - when a proton transforms into a neutron, emitting
anti-electron or positron.
3. Gamma (γ) ray - is given to photons emitted from the nucleus.
The spontaneous decay or radioactive decay can measured by its halflife. Half-life (t1/2) of an isotope in the time interval during which half of the
atoms originally present disintegrate. The time interval required for a onehalf of the atomic nuclei of a radioactive sample to decay.
The formula below shows the relationship between the initial number
of nuclei (N) and the number of nuclei after the decay( ∆N) in a short time
(∆t).
∆N = λN∆t
Eq. I
Where
N = initial number of nuclei before the decay
∆N = number of nuclei after the decay
∆t = time of decay
Λ = decay constant which is related to half life
Then
ΛT1/2 = 0.693
Rewrite equation I to derive the rate of decay
∆𝑁
=λN
Eq. II
∆𝑡
where
∆𝑁
= rate of decay
∆𝑡
Example # 1
If carbon 14 contains a 1.00 x 1022 nuclei with a half-life of 5730 yr.
How much nuclei will decay in a second?
Step 1: Given
N = 1.00 X 1022 Nuclei
Half-life = 5730 yr.
Step 2:Unknown
∆N = number of nuclei after the decay
Step 3: Formula: 3.1. ΛT1/2 = 0.693 & 3.2. ∆N = λN∆t
Step 4: Solution
Solution 4.1: Calculate the number of second in a year and the decay
constant
1.1. The number of second in a year
= seconds x minutes x hour x year
= 60 x 60 x 24 x 365
3.15 x 107 s/yr
1.2. Decay constant
Λ = 0.693 /T1/2
=
0.693
(5730 yr)(3.15 x 10 7 s/yr)
= 3.5 x 10 -8 s-1
Solution 4.2: Calculate the rate of decay
∆N /∆T = λN
= (3.5 x 10 -8 s-1)(1.00 x 1022nuclei)
= 4.2 x 10 6 nuclei/s
Table 5.1 Half-life of some radioisotopes
Radioactive substance
Polonium-214
Oxygen-15
Sodium-24
Protactinium-238
Radon
Iodine-131
Cobalt-60
Cesium-137
Carbon-14
Uranium-238
Half-life
0.00016s
2 minutes
15 hours
6.75 days
3.82 days
8 days
5.2 years
30 years
5730 years
4.5 billion years
Half life is a measure of the stability of a radionuclide. The shorter the
half-life, the more rapidly the nucleus decays and the more intense the
emission. Hence, a short half-life is associated with a relatively unstable
nucleus.
What’s More
Activity 3: Life Complete!
Directions:Complete the given table and show the solution .
Radioactive
Half -life
Initial
Time of
Decay
substance
Nuclei
decay
Constant(λ)
(∆t)
Oxygen-15
2 mins
Radon
3.82 days
Iodine-131
8 days
Sodium-24
15 days
1.2 x 1012
3.
3.4 x 10
7.
1s
1min
10
5.
hr
1hr
1.
s-1
4.
s-1
1.0x10-6s-1
8.
s-1
Number of
Nuclei
after the
decay (∆N)
2.
2.5x109
6.
1.5 x 108
Show your solution
What I Have Learned
Activity 4: Guess What!
Direction: Write the correct answer that best describe by the statement.
__________1. It is the property of several naturally occurring
elements that produced isotopes of the elements.
__________2. The half-life of a uranium-238.
__________3. This are fixed distances from the nucleus of an atom where
electron may be found.
__________4. A phenomenon when light is incident on a surface, electron
are emitted or ejected.
__________5. It is a measure of the stability of a radionuclide.
What I Can Do
Activity 5: Solve Me!
Direction: Solve the given Problem and show the solution needed.
1. Cobalt-60 has a half-life of 5.25 years. How many nuclei will remain after
a second of decay? If the initial nuclei of Cobalt-60 is 2.3x1020.
Step 1: What are the given in a problem?
Step 2: What is the unknown in the problem?
Step 3: What is the formula in finding the unknown in the given problem?
Step 4: Show the solution
Assessment
Directions. Encircle the letter of the best answer.
1. Which of the following radioactive substances with half-life of 2 minutes?
a. Cobalt -60 b. uranium-238
c. oxygen-15
d. sodium-24
2. What type of radioactive decay that proton transforms into a neutron,
emitting anti-electron or positron?
a. Alpha
b. Beta
c. Gamma
d. all of the above
3. In order to eject electron light must have _______ wavelength.
a. Short
b. Long
c. Wide
d. Both a & b
4. When does electron moves from a lower energy level to higher energy level?
a. Electrons emit energy.
c. Electrons absorb energy
b. Electrons emit and absorb energy
d. Electrons given off an energy
5. What is the half-life of iodine-131?
a.15 days
b. 6.75 days
c. 3.82 days
d. 8 days
6. It is used to measure the spontaneous radioactive decay of an isotopes.
a. Half-life
b. Carbon dating c. radioactivity
d. Energy Level
7. The electron will gain energy from_________ in order to eject from the
surface of the materials.
a. Proton
b. Neutron
c. Photon
d. Positron
8. A type of radioactive decay in which the photon emitted from the nucleus
of a atom.
a. Beta
b. Gamma
c. Alpha
d. all of the above
9. What will happen to the electron when it emits or given off an energy in a
form of light?
a. Electron will move to the higher energy level
b. Electron will move to the lowest energy level
c. Electron will stay on the same energy level
d. Both a & b
10. A radiation beam acts like a stream of energy particles.
a. Electron
b. Photon
c. Neutron
d. Proton
11. It represent the decay constant in a formula.
a. ∆N
b. N
c. λ
d.∆t
12. What is the equivalent value of a plank constant?
a. 6.626 x 10 14 Joules (J)
c. 6.626 x 10 34 Joules (J)
24
b. 6.626 x 10 Joules (J)
d. 6.626 x 10 44 Joules (J)
13. How many proton and neutron will be ejected during the alpha decay?
a. 1
b. 2
c. 3
d. 4
14. It represent the number of nuclei after the decay.
a. ∆N
b. N
c. λ
d.∆t
15. Electron will be ejected if the energy of the photon is ________to the work
necessary to free an electron.
a. greater
b. lesser
c. equal
d. all of the above
Additional Activity
Activity 6: More Practice!
Directions: Solve the following problem and show all the solution
1. Cesium-137 has a half-life of 30 years. How many nuclei will remain after a
minute? If the initial nuclei of Cesium-137 is 5.4 x 10 18.
2. Protactinium-238 has a half-life of 6.75 days. Suppose After a second of decay
the total nuclei is equal to 1.5 x 10 12. How many nuclei is present in a
protactinium-238 before a second of decay?
Activity 2B
1. c
2. e
3. b
4. D
5. a
Activity 5
1. Radioactive decay
2. 4.5 billion years
3. Energy level
4. Photoelectric effect
5. Half-life
Activity 2A
1. T
2. T
3. F
4. T
5. F
Activity 4
1. .0058
2. 6.96 x 10 9
3. 1.3 x 10 -4
4. 3.25 x 104
5. 19.25 hr
6. 3.4 x 104
7. 7.9 x 10 10
8. 5.34 x 10 -7
9.
Activity 1
1. Light
2. Electromagnetic wave
3. Transverse
4. Wavelength
5. wave
Answer Key- Gr12Q4W3 General Physics 2
References
Books:
Science 4 Teacher Guide, Page 115-120
Science Learning Guide Grade 8, Page 10 - 15
Electronic Resources:
https://www.wikipedia.org/
https://www.earth.com/
Development Team
Writer: Jonathan R. Linaza, T-III,
ZSNHS
Editors: Irmina C. Calibo, T-III, ZSNHS
Reviewer: Mila P. Arao, EPS
Illustrator:
Layout Artist:
Management Team:
DANNY B. CORDOVA, EdD, CESO VI
SDS
MA. COLLEEN L. EMORICHA, EdD,
CESE ASDS
MARIA DIOSA Z. PERALTA
CID Chief
MA. MADELENE P. MITUDA, EdD
EPS-LRMDS
MILA P. ARAO,
EPS-Science
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