MODERN PHYSICS

STRUCTURE OF AN ATOM

Electron shells/orbits

Nucleus Neutron Electron

Proton

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An atom is the smallest particle of an element that can take part in a chemical
reaction. An atom consists of three particles nanmely;
Electrons

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Neutrons
Protons
An atom is made of a central part called the nucleus around which electrons revolve.
The nucleus is positively charged because it consists of protons which are
positively charged and neutrons which have no charge. The properties of the
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particles of an atom are as shown in the table below.

Name Symbol Mass Charge

Protons D Positive
Neutrons n No charge
Electrons Negative

Note:
The number of protons in the nucleus of an Atom is the same as the number of electrons
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revolving in shells, so the atom is said to be neutral.
The sum of protons and neutrons in the nucleus of an atom is referred to as Nucleons (or
nucleon number)
Atoms can combine with other atoms to form molecules.
A nuclide is a species of an atom with a specific number of protons and neutrons in the nucleus
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of an atom for example Carbon-13 with 6 protons and 7 neutrons.

Atomic number, Z:
This is the number of protons in the nucleus of an atom.
Atomic number, Z= Number of protons in the nucleus of the atom

Mass number [atomic massl. A: (nucleon number):

This is the total number of protons and neutrons the nucleus of an atom.
Mass number = Atomic number + Number of neutrons.
A = Z+ n

If an atom of an element X is represented as

where A is the Mass number and Z is the Atomic number
X

Examples:

1. Given a chloride atom 17c, Find the number of electrons and neutrons in the atom.

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2. State the composition of the atom
i. 285U
i. 1c

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ISOTOPES:
These are atoms of the same element with the same atomic number but different mass
numbers.
Therefore, isotopes of an element have the same number of protons and electrons and
different number of neutrons. Changing the number of neutrons in an element does not
change the element. To write an isotope, we write the name of an element followed by its
mass number. For example, Carbon-14.
Examples
1. (a) Which of the following nuclei belong to the same element:
X,Y, 2. Explain

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your answer. What name is given to such a nuclei?
(b) what is the Atomic number of element Z?
(c) what is the mass number of element X?
2. An Atom of cobalt has an atomic number of 27 and a mass number of 59. How many

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protons and neutrons does it have?
3. Uranium-235 and Uranium-238 are isotopes of Uranium and have the same atomic
number 92.
(a) What do the numbers 235 and 238 represent?
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(b) What does the number 92 tell you about the nucleus of either of these atoms?
(c) What else does the number tell you about the atoms as a whole?
(d) In what way does the nucleus of Uranium-235 differ from the nucleus of
Uranium-238?

ATOMICMODELS
There are various atomic models, some of which are improvements of the
previous theories.
1. Daltons' Atomic model
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A theory of chemical combination, first stated by John Dalton in 1803. It
involves the following assumptions (postulates):
(1) Elements consist of indivisible small particles (atoms).
(2) All atoms of the same element are identical; different elements have
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different types of atoms.

(3) Atoms can neither be created nor destroyed.
(4) 'Compound elements' (i.e. compounds) are formed when atoms of
different elements join in simple ratios to form 'compound atoms' (i.e.
molecules).
Dalton also proposed symbols for atoms of different elements (later
replaced by the present notation using letters).

From his own experiments and observations, as well as the work of his peers, Dalton proposed a
theory of the atom known as Dalton's atomic theory. The general main aspects of this theory
are as follows:

All matter is composed of extremely small particles called atoms.

Atoms of a given element are identical in size, mass, and other properties. Atoms of
different elements differ in size, mass, and other properties.

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 Atoms cannot be subdivided, created, or destroyed.
Atoms of different elements can combine in simple whole number ratios to form
chemical compounds.
In chemical reactions, atoms are combined, separated, or rearranged.

Dalton's atomic theory has been largely accepted by the scientific community, with the exception
of three changes. We now know that (1) an atom can be further subdivided, (2) all atoms of an
element are not identical in mass, and (3) using nuclear fission and fusion techniques, we can
create or destroy atoms by changing them into other atoms.

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2. Rutherford's nmodel
Rutherford's atomic model became known as the nuclear model. In the
nuclear atom, the protons and neutrons, which comprise nearly all of the

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mass of the atom, are located in the nucleus at the center of the atom. The
electrons are distributed around the nucleus and occupy most of the volume
of the atom
An atom consists of a very small central core called the nucleus with strong
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electric charge surrounded by electrons of the opposite charge which fill the
rest of the atom.
Rutherford's model states: that the positive charge of the atom and nearly
all its mass is concentrated in a very small volume at the centre with
electrons in motion in a circular orbit around the nucleus.

According to the Rutherford atomic model:

1.The positive charge and most of the mass of an atom is concentrated in an extremely
small volume. He called this region of the atom as a nucleus.
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2. Rutherford's model proposed that the negatively charged electrons surround the nucleus
of an atom. He also claimed that the electrons surrounding the nucleus revolve around it
with very high speed in circular paths. He named these circular paths as orbits.
3. Electrons being negatively charged and nucleus being a densely concentrated mass of
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positively charged particles are held together by a strong electrostatic force of attraction.
Electron orbits
Nucleus

Proton
Neutron
Electrons

EJECTION OF ELECTRONS FROM ATOMS

Electrons can be removed from an atom when an energy is applied. The process of removing an
electron from an atom is called ionization and the energy needed is the ionization energy.
Electrons may be ejected due to heat (thermionic emission) or light (photoelectric emission).

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Thermionic emission.
When heating water, it will start boiling only when the heat energy gained is sufficiently high. If
the heat energy is not high enough to attain its boiling point, the water will not boil.
In comparison to the above;
Metals contain free electrons in their lattice that are loosely bound to their parent nuclei.
As the temperature of the metal is raised, velocities of the electrons increase, some of the surface
electrons acquire sufficient kinetic energy to overcome the electrostatics attraction force of the
atomic nucleic and consequently escape from the metal surface.

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Thermionic emission
is a process by which free electrons are emitted from a hot metal surface.
But there is a minimum temperature and energy needed to remove an electron since they are
always attracted by the positively charged nucleus.

surface.
Research questions.

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NB; Work function is the minimum energy required to release an electron from the metal

1.
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How does surface area of a metal affect the rate of thermionic emission?
2. Briefly discuss the effect of varying temperature on the rate of thermionic
emission.
3. What is the difference between threshold temperature and work function.
4. How does work function of a metal affect thermionic emission?

Photoelectric emission.
The photoelectric effect is the emission of electrons from a material caused by electromagnetic
radiation (light) of high enough frequency (energy). Electrons emitted in this manner are called
photoelectrons.
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It is important to note that the emission of photoelectrons and the kinetic energy of the ejected
photoelectrons is dependent on the frequency of the light that is incident on the metal's surface.
The photoelectric effect occurs because the electrons at the surface of the metal tend to absorb
energy from the incident radiation (light) and use it to overcome the attractive forces that bind
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them to the metallic nuclei and the remainder of the energy is used as the kinetic energy with
which the electron leaves.

APPLICATIONS OF EJECTED ELECTRONS.

1. Cathode rays
These are streams of fast-moving electrons.
They are produced from the cathode by thermionic emission. Cathode rays carry energy
since they possess speed.
Cathode rays or electron beams (e-beam) are streams of electrons observed in
discharge tubes. If an evacuated glass tube is equipped with two clectrodes and a voltage
is applied between them, glass behind the positive electrode is observed to glow, due to
electrons emitted from the cathode (the electrode connected to the negative terminal of
the voltage supply).

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 NB;
Research on the process of production of Cathode rays in the Cathode ray tube and the
properties of Cathode rays.
Application of Cathode rays in a Cathode ray osciloscope (CRO) and the uses of

2. X-rays
These are electromagnetic waves of short wavelength which are produced when cathode
rays are stopped by a metal surface.
Medical uses:
• They are used to investigate the broken bones in X ray photography.

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•They are used to treat cancer cells.
• They used to detect the complicated organs of the body.
• They are used to detect tuberculosis of the lungs.

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• They are used to diagnose stomach ulcers.
When X-rays are passed through the body onto the photographic plate or film,
the bones which are composed of a much denser material than the flesh absorb most of
the X-rays and appear white on the photographic plate or film.
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The flesh which is composed of less dense material allows most of the X-rays to pass
through it hence darkening the photographic film or plate.
These shadows are studied in order to locate the broken part.
X-rays are produced in an X-ray tube shown below
Tungsten target gh voltage Vacuum

Evacuated
glass
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Low voltage

Focusing cup
Cooliríg fins Cathòde filament
rays cathode:
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Copper anode
Lead shield rayS

The cathode filament is heated using a low voltage source.

The cathode emits electrons by thermionic emission.
The emitted electrons are accelerated towards the target by the high voltage applied at the
anode.
The focus cup focuses the fast-moving electrons to the target
When electrons hit the target, they are brought to rest an most of their kinetic energy is
converted heat and the rest becomes X-rays.
The heat generated around the anode is conducted away through the anode to the cooling
fins (radiator).
Health Hazards caused by X-ray
Destroys living cells in our bodies.
Can cause hereditary defects (genetic mutation).
Can cause serious diseases like cancer.
Can cause sterility.

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 Can damage eye sight.

Safety Precautions taken when using X-rays

Avoid unnecessary exposure to X-rays.
One should protect himself by wearing lead jackets.
X-ray apparatus must be well shielded using thick lead.
Any cuts on the body should be covered.

NB; Research on the differences between Cathode rays and X-rays.

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NUCLEAR PROCESSES
The term Nuclear reaction is used to refer to the externally induced changes brought onto the
nucleus of an atom. The reactions which involve the nucleus of an atom occur when it is

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unstable. Scientific studies have proved that iron-56 is the most stable atom, so small mass
number atoms combine to obtain a mass number close to 56 and larger mass number nuclei split
to also attain a mass number near 56. The processes result into a nuclear fusion and fission
respectively. In both processes, energy is released.
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Nuclear fission
This is the splitting of heavy unstable nucleus to form two or more lighter stable nuclei with the
release of energy
The energy produced is converted into electricity in a nuclear plant.
For example, when Uranium U
is hit by high-speed neutron it splits into l4Ba., Kr and 3hn.
Nuclear fission occurs naturally in radioactive elements

Conditions for nuclear fission to occur

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Parent nuclide is heavy and unstable.
Occurs at a very low temperature.
Slow moving neutrons.
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Examples where nuclear fission takes place

At the sun and other stars
In hydrogen bomb
Takes place in a nuclear reactor

Nuclear fusion
This is the union of two or more lighter unstable nuclei to form a heavy stable nucleus with the
release of energy e.g

He + n + energy
This also leads to loss of mass and results in release of energy.

Conditions for nuclear fusion to occur

This requires extremely high temperature.

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 Requires particles with small nuclear charge so that the nuclear repulsive force between
nuclei to be combined is minimum.
Fusing nuclides are smaller and unstable.
There should be very high pressure.
Nuclear fusion also requires that the particles approaching each other should be at very
high speed so that the two light nuclides can overcome the strong nuclear repulsion.

NUCLEAR REACTOR:
The diagram below shows a nuclear reactor that produces electricity by nuclear fission.

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Hot gas

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Steamn out
Control rod
Uranium rod Cold water in
Moderator
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gas
Concrete Pump

How electricity is produced from the nuclear reactor:

The uranium rod undergoes fission and fast neutrons are produced as a result.
The neutrons collide with other atoms and the energy possessed by the neutrons changes to heat
energy.
The heat energy is then absorbed by the coolant and is made to heat water to produce steam.
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The steam then drives the turbines connected to a generator and electricity is produced.

Functions of the parts:

Control rods are made of boron or cadmium and they control the fission rate that is
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they absorb neutrons that would initiate a fission reaction.

ii) The moderator is made of graphite or heavy water at very high pressure and it is used
to slow the neutrons there by making them to be absorbed by the uranium atom.
ii) Coolant is made of water at very high pressure, liquid sodiun or carbon-dioxide and
is used to absorb heat from the reactor core.
iv) Uranium rods are made of uranium and they are as fuel which contains uranium-235.
The atoms of uranium undergo fission and produce energy
V) The concrete shield absorbs any radiation from the fission fragments or from the fuel
directly.

RADIOACTIVITY
Is the spontaneous disintegration of the unstable nucleus by the emission of radiations.
There are three radiations emitted during radioactive decay, they include;
(i) alpha particles (a)
(iü) beta particles (B)
(iii) gamma rays(y)

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A radioactive element
Is one whose nucleus spontaneously disintegrates and continuously emits radiations
Alpha particles (a)
An Alpha particle is Helium atom.
Alpha particles have a mass number 4 and an atomic number of 2 and are positively
charged.
The symbol for an Alpha particle is He
If a radioactive source X undergoes decay to nuclide "Y" by emitting an alpha particle.

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The equation is written as follows:

He
Radioactive Source
(Parent)
Beta particle (B)

p daughter nuclide particles emitted

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Beta particles are high speed electrons.
The mass of Beta particles is almost zero and are negative charge.
Symbol of Beta particle is -e
If a radioactive source undergoes decay to emit a Beta particleand turns to nuclide Y then the
X

equation of the nuclear charge is

X
HY
n+1
+ -e
Gamma rays
Gamma rays are electromagnetic waves of the shortest wave length.
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Gamma rays have no mass number and carry no charge.
Gamma rays are produced when an excited atomic nucleus loses energy and the energy is
given out as gamma rays.
X undergoes a gamnma decay to form a stable element X. Then the nuclear reaction
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equation is given by:

x + + Energy

Penetrating power of the radiations:

Alpha particles have the least penetrating power and can be stopped by a thick sheet of
paper.
Beta particles have greater penetrating power than alpha particles and can be stopped by a
thin sheet of aluminum.
Gamma rays have the greatest penetrating power and can be stopped by thick block of
lead as shown below.

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 Radioactive Thick paper Thick lead
Souce

Aluminium sheet
NB; Solve questions on age 101, Baroque, book 4.

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HALF LIFE
Is the time taken for a radioactive material to decay to half the original mass.
OR:
Is the time taken for the activity of a radioactive material to decay to half the original value.

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OR:
The time taken for half the number of atoms of a radioactive substance to decay.

Activity, A of a radioactive source

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Is the number of disintegrations taking place in the source per second.
When the mass of a radioactive material is increased, the activity of the radioactive
material increases because the activity of a radioactive material is directly proportional to
the mass of the radioactive material present at that time.
The activity of the radioactive material remains constant when temperature is increased
because the activity of a radioactive material is not affected by temperature.

Example 1
A radioactive material has a half-life of 4 hours and the initial mass of the substance is 9.6g.
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Calculate
(i) Mass remaining after 24 hours.
(ii) Mass decayed after 24 hours.
(i)
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4hours 4hour 4hours 4hours

9.6g 4.8g +2.4g +1.2g 4hors
+0.6g +0.3g 4hours
+0.15g
Mass remaining = 0.15g

(i)
Mass decayed = original mass remaining mass
=9.6g– 0.15g
=9.45g

Alternatively
You can use the table with two columns i.e. one column for amount remaining and the other the
time taken.

Amount remaining(grams) Time taken

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 9.6
4.8
2.4
1.2 12
0.6 16
0.3 20
0.15 24

From the table,

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(i) Mass remaining after 24 hours =0.15g
(ii) Mass decayed =9.6g -0.15g =9.45g

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Half-life from the graph:
• The graph
of amount of an element, N against time, t is plotted.
• Draw a horizontal line from
half of the original amount to meet the curve.
• Draw a vertical line from the point on the curve to meet the time axis.
• Read the half-life from where the vertical line meets the time axis.

AmountNo
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Ty Time
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Example:
The table below shows results obtained in an experiment to determine the half-life of a
radioactive substance.
Count rate 250 140 76 38 25
Time (min.) 10 |15 20
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a
Draw graph of count rate against time and use to
it determine the half-life of the radioactive
substance.

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 A
araphof count rate against timne
4
250

200
From the graph,
No 250
= 125
150 22=6
Ty minutes

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100

50

p 10 15
Time (minutes)
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Background radiations
In the absence of a radioactive source, the radiation detector will be recording a presence of
some radioactive radiations; these are known as background radiations.
These are radiations in the atmosphere caused by the naturally existing radioactive sources.
Background radiation is all around us, most background radiation comes from natural sources,
while most artificial background radiation comes from medical examinations, such as x-ray
photographs.

(a) Natural sources:

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Natural sources of background radiation include;
(i) Cosmic rays (Radiations), These are radiations that reach the Earth from space from
majorly the sun and stars.
(ii) Terrestrial Radiation; These are radiations from the Earth itself. Radioactive materials
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(e.g uranium, thorium, and radium) exist naturally in soil and rock which undergoes
spontaneous disintegration emitting radiations.
(iii) Internal Radiation; Small traces of radioactive materials are present in the human
body. These come from natural radioactive sources such as Carbon-14 in the air we
breathe.

(b)Artificial sources:
Human activity has added to background radiation by using artificial sources of radiation which
include the following:
(i) Nuclear power stations:
Major incidents from nuclear power stations have released radiations into the environment.
Nuclear waste from power station also accounts for a proportion of artificial background
radiation
(ii) Nuclear weapons:

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Nuclear weapon testing resulted in an increase of radiation in the environment because of
radioactive fallout from nuclear weapons testing.

(c) Medical sources:

Humans are exposed to radiations by medical procedures such as x-rays and radiotherapy. Nearly
all artificial background radiation comes from medical procedures such as receiving x-rays for x
ray photographs.
Dangers of the radioactive emissions (Health hazards)
May kill body cells leading to deep burns and death.

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May mutate body cells by altering their genetic nature.
May cause cancer.
May damage sight.

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Precautions taken when handling radioactive source.
They should be handled using long pair of tongs.
They should be transported in thick lead containers.
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Avoid unnecessary exposure to the radiations.
Wear protective clothing when handling radioactive elements.
Avoid eating or drinking in places where radioactive sources are in use.
Cover any wound before using radioactive source.

Uses of radioactivity
Industrial use
To detect flaws, leakages in pipes and welded joints.
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In detection of the grades of oil in a pipeline i.e the grade of oil is changed at one end of a
pipeline, radioactive tracers are added to the oil so that it can be detected at the receiving
end.
Hardening polythene and petroleum.
Radioactivity is used to control automatically the thickness of paper, plastics and metal
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sheets during manufacture.

This is done by placing the radioisotope on one side of a moving sheet and the Geiger Muller
(detector of radiations) on the other side. As the count rate decreases, the thickness increases.
Medical uses of radioactivity
In short medical uses of radioactivity include:
It is used in the treatment of cancer.
It is used in sterilization of medical equipment like syringe etc.
It is used in investigation of the lung and heart conditions.
Tracer,the progress of a small amount of weak radioisotope injected into a system can be
traced by G-M tube. The method is used in medicine to detect brain tunmors.
Agricultural uses of radioactivity
Used as tracers to study the uptake of fertilizers by plants
This is done as follows

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 Mineral fertilizer with the radioactive isotope of the required element is put in the
soil near the plant.
Using radiation detector (Geiger Muller), radiations in different parts of the plant is
detected.
V The part of the plant where most c activity is detected by the Geiger Muller counter is
the part with the highest concentration of the radioactive element and the part where
the plant uses the element.
Used in controlling pest this is done by sterilizing males.
Used to produce new varieties of plants with new characteristics.

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Carbon dating
Is the finding of the age of fossils and rocks.
This is done basically depending on the half-life and proportion of radioactive carbon 14 present

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in fossil.
NB; Research on the social and political implications and regulations and environmental
implications on the handling of nuclear power establishments and further discoveries.
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 DIGITAL ELECTRONICS
With the help of electronics, we can use machines in money transactions, in information storage,
in information delivery and so on. Sophisticated and automated innovations are being developed,
all for the friendly use of electronics.
Electronics concept can be divided into analog and digital.

Analog signals
These are signals which convey information in a wave form. It uses a variation of a given
property to convey the signals over a period of time eg variation of frequency from the human

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voices, variation of voltage in electricity through a wire etc.
An analog signal is simply considered as a continuous electrical signal that varies in amplitude
and frequency, representing information such as sound, temperature or pressure. The signal can
have an infinite number of values within a certain range.

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An analog signal generally refers to any natural phenomenon that varies its own property over a
period of time.

Digital signals
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In these signals, there are only two distinct and opposite properties, its either ON or OFF. These
are the cases involved in the inside circuits of switches of lights, a calculator, a computer, a smart
phone and other electronic devices.
A digital signal is a discrete signal represented by a sequence of
binary digits (Os and ls). It's
used encode and transmit information in digital communication systems. Digital signals are
to
characterized by discrete values and are less susceptible to noise compared to analog signals,
making them more reliable for long-distance transmission and storage.
The figure below shows the appearance for the analog and digital signals.
nalog Signal
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ituce
Tim

Digital sgnal
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Tim

Electronic components
Electronic devices are components for controlling the flow of electrical currents for the purpose
of information processing and system control.
Electronic devices are usually very small and can be grouped together into packages called
integrated circuits (1C). Each component does a different job and they linked together by cables
or printed metal connections already on the board.
Some of the most common important components in the design of digital circuits include:
Component Application
Resistor Rectification of AC and voltage regulation.
Diode Energy storage, filtering and smoothening of AC
Transistor Power amplification and switching
Capacitor Potential divider and energy storage.

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NB; the components can be identified on page 114, Baroque learners' book 4.

Resistors as potential dividers.

A resistor can be used as a potential divider by connecting it in serries with another resistor or a
load eg a component or a device.
The output voltage is taken across one of the resistors, which divides the input voltage
proportionally based on the ratio of the resistances. This division of voltage is in accordance to
Ohm's law, where the voltage across a resistor is proportional to the current flowing through it
and its resistance.

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R,

p R, VoUT
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Where VIN is the voltage supplied by the cell, VoUr is the voltage across the resistor of interest,
R and R2 are the resistances of the resistors in the divider circuit.
R

Using the knowledge of voltage across serries resistors, Vour=,-VN (From V=IR )
R+Rz

Resistors as potential dividers find various applications in electronic devices and circuits such as;
Voltage regulators; in power supplies, voltage dividers can be used to create a stable output
voltage from a varying input voltage.
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Sensor circuits; Sensors such as temperature sensors, the divider can be used to scale up or
down the voltage passing through a given load for an automated action to occur
Volume regulators; the volume knobs in electrical devices such as radios use potential dividers
to increase or decrease the volume by adjusting the potential difference supplied to the speaker.
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Example
1. The figure below shows an arrangement of a system operating a lighting network using a
potential divider.

2 k2

S
12 \

4 kQ

Calculate the:
The potential the output (bulb)
ii The current passing through the bulb system if it has a total resistance of 100 2.
NB; other examples can be accessed in Baroque, Learners' book 4 page 117

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 Volume control on a radio as a potential divider.
A potential divider used in volume knobs divides voltage across two resistors in series. By
adjusting the position of the knob, the ratio of the resistance also changes thereby altering the
voltage output and thus controlling the volume.

Elementary logic and memory circuits.

A logic circuit is an electronic circuit designed to perform logical operations on input signals and
produce output signals based on predetermined logic rules. These circuits manipulate binary
signals (0s and ls) using logic gates. A logic gate is a barrier which either allows or blocks

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clectricity passing through a given point basing on its operating rules.
Logic circuits are fundamental components of digital systems, including computers, calculators
and digital control systems, enabling them to process and manipulate data according to logical
rules. With the use of one or more logic gates, a machine is capable of performing basic

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mathematical reasoning.
Calculators and computers store numbers (0-9) as a long string of zeros and ones in a form called
a binary code (0's and 1's). Each number is stored using microscopic electronic switches called
transistors. It is easy to store binary numbers simply by switching transistors on and off.
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Switching it on stores a one, switching it off stores a zero. Therefore, storing numbers is easy.
But to add, subtract, multiply and divide, they must use logic gates.
The storing of this binary numbers and switching of state between ON and OFF is useful in
bistable and an astable switch.
A bistable switch; also known as flip-flop, is an electronic circuit with two stable states. It can
remain in either state indefinitely until it receives a trigger to change its state. Bistable switches
are usually used in digital electronics for storing binary information or simple memory in circuits
like registers or counters.
An astable switch; also known as astable multivibrator, is an electronic circuit that continuously
alternates between two states without an external trigger. They can be made from NAND gate
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and transistors.
Its commonly used in oscillators, timers and pulse generators. The simplest example being the
LED flash light circuits in decorations.
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LOGIC GATES.
Logic gates are built from transistors, resistors and diodes. When current flowing is above a
certain value, then it is ON and represented by l and when the current flowing is below a certain
value, it is OFF and is represented by 0. Other similar descriptions can be indicated as below.
Logic 0 Logic 1
Off On
False True
Low High
No Yes
Open switch Closed switch

A logic gate is usually represented by;

A diagram which is used in circuit drawings
A truth table that represents its operation
> Algebraic equation representing its mode of operation.

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 The truth table.
This is a table that shows all possible outcomes that would occur form all possible scenarios that
are considered factual. This is a simple way of describing all possible input and output decisions
produced by all the possible combinations.
To construct a truth table, we must first investigate the rules involved and the switch in the
digital information.

For-example

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Take a look at the following story;
Jane and Claire are great friends. However, Jane's parents are rich with good facilities for
reading and playing, but they are strict, limited visitation is allowed for Jane when they are at
home.

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One evening, they plan to use Security and Bedroom lights for communication, for Claire to
decide either to ENTER or NOT TO ENTER home. For effective communication, Jane gave
Claire the following instructions:
When you see both the security light and the bedroom light OFF, it means that my parents are
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still homne, so don't come in.
If the bedroom light is ON and the security light is OFF, am reading so you can join me.
If the bedroom light is OFF and security light ON, then it means we are all watching in the living
room, so don't enter.
Then finally when both the bedroom and security lights are ON, means they have allowed me to
play, you can come in.
Use the above information to complete the table below:
SECURITY LIGHT BEDROOM LIGHT MESSAGE
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NB; such is an example of a truth table.
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TYPES OF LOGIC GATES.

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18| Pa g @TGS PHYSICS DEPARTMENT
 Algebraie
Name Graphical Symbol Function Truth Table
AB|E
F=AB
AND or 01
F= AB 10
11 1
A
B|
00
F=A + B

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OR F 1
10 1
11 1

p
F=
NOT or
F=A
AB|E
NAND
fli F= AB 0 1 1
1

1 0
11
A
B|F
A 001
NOR F=A+B 010
B 1 0 0
1 1 l0
Alternatively, the NAND and NOR gates can be illustrated as below
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NAND GATE
Inputs Output
A B F
or 0
0 1
Ja

1
0 1

1 1
NAND 1 1
0
NOR GATE
Inputs Output
A B F
A
or
0 0 1

1 0
B
0 0
NOR 1 0
NB; the operation of the AND and OR gates can be illustrated using the electric circuit switching
system on page 121 and 124 of Baroque Learner's book 4.

19 | Pag e @TGS PHYSICS DEPA RTMENT

The basic logic gates are the building blocks of more complex logic circuits. These logic gates
perform the basic Boolean functions, such as AND (1C 7408), OR (IC 7432), NAND (1C 7400),
NOR (IC 7402), Inversion (IC 7404), Exclusive-OR (IC 7486), Exclusive-NOR. The table above
shows the circuit symbol, Boolean function, and truth table. It is seen that each gate has one or
two binary inputs, A and B, and one binary output, F. The small circle on the output of the circuit
symbols designates the logic complement. The AND,OR, NAND, and NOR gates can be
extended to have more than two inputs. A gate can be extended to have multiple inputs if the
binary operation it represents is commutative and associative.

s
Real life applications of logic gate manipulation
ONOFF
Front Doorbell Switch
Doorbel

p
Back Doorbell Switch
ONOFE
ONOFF
fliIether the Font
OR the Back Doorbel
pressed then the
Doorbell Switch
Switch is
Doorbet rings.

oNOEF
Person Sesor
Burglar Alarrm
Alarm Switch
ONOFF NOEF

both the Person Sensor AND

the Aarm Switch are on then the
Burglar Alan is activatod
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Example
The input and output graph for a logic gate can be illustrated as below. Analyze it and make
Ja

conclusions of the gate being considered.

A Logic gate
B circuit

A 1

B1

Y 0

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20| Pa g @TGS PHYSICS DEPARTMENT
 USE OF LOGICGATES IN CONTROL CIRCUITS.
Logic gates are mainly decision making devices. They can be used to control an electronic
system. Many electronic systems are designed in three or four parts. These include;
1. The input sensor
2. The processor (electronic system)
3. The memory (absent in some systems)
4. The output (the transducer)

OUTPUT
INPUT SENSORS PROCESSOR

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SENSORS

The input sensor converts physical effects to electronic signals for processing by the

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system. The output device converts the processed electrical information from the system
into useful action.

For example,
1. Consider a calculator as an electronic system, the operation is carried out as shown
below;
fli
The input is data The prOcessor The output device
fed in the calculator performs the tasks which is the screen
through the key according to the displays the results
board instructions given
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Some of the data is
stored in the
memory unit for
further processing.
Ja

2. The circuit diagram below shows an automatic switch for the fan for the cooling in
the house.

HOT=1 Temperature
COLD=0 sensor

ON=1
Relay Main Fan
OFF=0
CONTROL
L SYSTEM
BRIGHT=1 Light sensor
DARK=0

a. Under what condition does the system turn on the fan?

b. What is the disadvantage of the switch?

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Pa g e @TGS PHYSICS DEPARTMENT
 Solution
a. The system requires that both inputs to the AND gate are 1 for the fan to be
turned on. This can be obtained from the truth table of the AND gate. The fan
will be switched on when the temperature indicates HOT and the light sensor
indicate BRIGHT.
b. The fan will not be switched on when the temperature indicates HOT and the
light sensor indicates DARK even when the fan is required.

NB;

s
Solve more on page 132, Baroque Learners' book 4.
Robots are machines connected to an electronic control system with a memory
that can learn, store and replicate a given behavior repeatedly with consistency
and accuracy.

p
fli
y
Ja

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22| Pa @TGS PHYSICS DEPARTMENT