FORM FOUR PHYSICS STUDY NOTES TOPIC 3: RADIOACTIVITY & TOPIC 4: THERMIONIC EMISSION

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TOPIC 3: RADIOACTIVITY

The Nucleus of an Atom
The Structure of the Nucleus of an Atom
Describe the structure of the nucleus of an atom
The word atom is derived from the Greek word atom which means indivisible. The Greeks concluded that matter could be broken down into particles to small to be seen. These particles were called atoms.
Atoms are composed of three type of particles: protons, neutrons, and electron.Protons and neutrons are responsible for most of the atomic mass e.g in a 150 person 149 lbs, 15 oz are protons and neutrons while only 1 oz. is electrons.The mass of an electron is very small (9.108 X 10-28 grams).
Both the protons and neutrons reside in the nucleus. Protons have a positive (+) charge, neutrons have no charge i.e they are neutral. Electrons reside in orbitals around the nucleus. They have a negative charge (-).
It is the number of protons that determines the atomic number, e.g., H = 1. The number of protons in an element is constant (e.g., H=1, Ur=92) but neutron number may vary, so mass number (protons + neutrons) may vary.
The same element may contain varying numbers of neutrons; these forms of an element are called isotopes. The chemical properties of isotopes are the same, although the physical properties of some isotopes may be different.
Some isotopes are radioactive-meaning they "radiate" energy as they decay to a more stable form, perhaps another element half-life: time required for half of the atoms of an element to decay into stable form. Another example is oxygen, with atomic number of 8 can have 8, 9, or 10 neutrons.
The Atomic Number, Mass Number and Isotopes of an Element and their Symbols
Explain the atomic number, mass number and isotopes of an element and their symbols
The atomic number of a chemical element (also known as its proton number) is the number of protons found in the nucleus of an atom of that element.Therefore it is identical to the charge number of the nucleus. It is conventionally represented by the symbol Z.
The atomic number uniquely identifies a chemical element. In an uncharged atom, the atomic number is also equal to the number of electrons.
The atomic number, Z, should not be confused with the mass number, A.
Mass number is the number of nucleons, i. e the total number of protons and neutrons in the nucleus of an atom. ---The number of neutrons, N, is known as the neutron number of the atom; thus, A = Z + N (these quantities are always whole numbers).
Since protons and neutrons have approximately the same mass (and the mass of the electrons is negligible for many purposes) and the mass defect of nucleon binding is always small compared to the nucleon mass, the atomic mass of any atom, when expressed in unified atomic mass units (making a quantity called the "relative isotopic mass"), is roughly (to within 1%) equal to the whole number A.
Isotopes
Isotopes are atoms with the same atomic number Z but different neutron numbers N, and hence different atomic masses.
A little more than three-quarters of naturally occurring elements exist as a mixture of isotopes (see monoisotopic elements), and the average isotopic mass of an isotopic mixture for an element (called the relative atomic mass) in a defined environment on Earth, determines the element's standard atomic weight.
Historically, it was these atomic weights of elements (in comparison to hydrogen) that were the quantities measurable by chemists in the 19th century.The chemical properties of isotopes are the same, although the physical properties of some isotopes may be different.
Some isotopes are radioactive-meaning they "radiate" energy as they decay to a more stable form, perhaps another element half-life: time required for half of the atoms of an element to decay into stable form. Another example is oxygen, with atomic number of 8 can have 8, 9, or 10 neutrons.
Forces Holding the Nucleus
Mention forces holding the nucleus
Stable and unstable atoms
There are forces within the atom that account for the behavior of the protons, neutrons, and electrons. Without these forces, an atom could not stay together.
Recall that protons have a positive charge, electrons a negative charge, and neutrons are neutral. According to the laws of physics, like charges repel each other and unlike charges attract each other. A force called the strong force opposes and overcomes the force of repulsion between the protons and holds the nucleus together.
The net energy associated with the balance of the strong force and the force of repulsion is called the binding energy. The electrons are kept in orbit around the nucleus because there is an electromagnetic field of attraction between the positive charge of the protons and the negative charge of the electrons.
In some atoms, the binding energy is great enough to hold the nucleus together. The nucleus of this kind of atom is said to be stable. In some atoms the binding energy is not strong enough to hold the nucleus together, and the nuclei of these atoms are said to be unstable. Unstable atoms will lose neutrons and protons as they attempt to become stable.
  1. Binding energy is the net energy that is the result of the balance with the strong force and the repulsive force, and this is the amount of energy that holds the nucleus together.
  2. A stable atom is an atom that has enough binding energy to hold the nucleus together permanently.
  3. An unstable atom does not have enough binding energy to hold the nucleus together permanently and is called a radioactive atom.
TOPIC 4: THERMIONIC EMISSION
Thermionic Emission
Thermionic emission is the discharge of electrons from heated materials, widely used as a source of electrons in conventional electron tubes (e.g., television picture tubes) in the fields of electronics and communications. The phenomenon was first observed (1883) by Thomas A. Edison as a passage of electricity from a filament to a plate of metal inside an incandescent lamp. The classical example of thermionic emission is the emission of electrons from a hot cathode into a vacuum (also known as thermal electron emission or the Edison effect) in a vacuum tube. The hot cathode can be a metal filament, a coated metal filament, or a separate structure of metal or carbides or borides of transition metals. Vacuum emission from metals tends to become significant only for temperatures over 1000 K. The science dealing with this phenomenon has been known as "thermionics," but this name seems to be gradually falling into disuse.
Cathode rays (also called an electron beam or e-beam) are streams of electrons observed in vacuum tubes.Electrons were first discovered as the constituents of cathode rays. In 1897 British physicist J. J. Thomson showed the rays were composed of a previously unknown negatively charged particle, which was later named the electron. Cathode ray tubes (CRTs) use a focused beam of electrons deflected by electric or magnetic fields to create the image in a classic television set.
The Production of Cathode Rays
Explain the production of cathode rays
Cathode rays are so named because they are emitted by the negative electrode, or cathode, in a vacuum tube. To release electrons into the tube, they first must be detached from the atoms of the cathode.
Modern vacuum tubes use thermionic emission, in which the cathode is made of a thin wire filament which is heated by a separate electric current passing through it. The increased random heat motion of the filament atoms knocks electrons out of the atoms at the surface of the filament, into the evacuated space of the tube.
Since the electrons have a negative charge, they are repelled by the cathode and attracted to the anode. They travel in straight lines through the empty tube. The voltage applied between the electrodes accelerates these low mass particles to high velocities. Cathode rays are invisible, but their presence was first detected in early vacuum tubes when they struck the glass wall of the tube, exciting the atoms of the glass and causing them to emit light, a glow called fluorescence.
Researchers noticed that objects placed in the tube in front of the cathode could cast a shadow on the glowing wall, and realized that something must be travelling in straight lines from the cathode.
After the electrons reach the anode, they travel through the anode wire to the power supply and back to the cathode, so cathode rays carry electric current through the tube. The current in a beam of cathode rays through a tube can be controlled by passing it through a metal screen of wires (a grid) to which a small voltage is applied.
The electric field of the wires deflects some of the electrons, preventing them from reaching the anode. Thus a small voltage on the grid can be made to control a much larger voltage on the anode. This is the principle used in vacuum tubes to amplify electrical signals.
High speed beams of cathode rays can also be steered and manipulated by electric fields created by additional metal plates in the tube to which voltage is applied, or magnetic fields created by coils of wire (electromagnets). These are used in cathode ray tubes, found in televisions and computer monitors, and in electron microscopes.
The Properties of Cathode Rays
State the properties of cathode rays
Properties of Cathode Rays Include:
  1. Cathode rays travel in straight lines. That is why, cathode rays cast shadow of any solid object placed in their path. The path cathode rays travel is not affected by the position of the anode.
  2. Cathode rays consist of matter particles, and posses energy by the virtue of its mass and velocity. Cathode rays set a paddle wheel into motion when it is placed in the path of these rays one the bladder of the paddle wheel.
  3. Cathode rays consist of negatively charged particles. When cathode rays are subjected to an electrical field, these get deflected towards the positively charge plate (Anode).We know that a positively charged body would attract only a negatively charged body, therefore the particles of cathode rays carry negative charge.Cathode rays also get deflected when these are subjected to a strong magnetic field.
  4. Cathode rays heat the object only which they fall. The cathode ray particles possess kinetic energy. When these particles strike an object, a part of the kinetic energy is transferred to the object. The causes a rise in the temperature of the object.
  5. Cathode rays cause green fluorescence on glass surface, i.e., the glass surface only which the cathode rays strike show a colored shine.
  6. Cathode rays can penetrate through thin metallic sheets.
  7. Cathode rays ionize the gases through which they travel.
  8. Cathode rays when fall only certain metals such as copper, but rays produced. The X-rays are not deflected by electrical or magnetic fields. X-rays pass through opaque materials such as black paper, but stopped by solid objects such as bones.
  9. Cathode rays travel with speed nearly equal to that of light.
The Application of Cathode Ray Tube
State the application of cathode ray tube
Application of cathode ray tube includes:
Televisions
Before LCD or Plasma television, the CRT was used to create a moving image.It used the same principle as a CRT, and for Black and White televisions, that worked fine. B&W TVs were essentially the same thing as a CRT, as all that's needed is the control of the brightness of the beam.
A CRT TV works by having the electron beam "scan" the screen at an rate faster than our eyes can perceive.This means that it shoots across the screen like a machine gun, and the images we see are actually made from many fluorescent dots.
The fluorescence caused by the beam striking the screen lasts a bit longer so that the next scan can be made without the previous image disappearing.It scans twice each time, first filling in the odd "holes" then the even ones.Each scan is about 1/50 of a second.
Colour CRT TVs had 3 electron guns rather than a single one, a shadow mask, and a modified fluorescent screen.The 3 electron guns were needed as there were three primary colours (Red, Green and Blue) that could be adjusted in different amounts to create any colour.
The colours are formed as a result of the shadow mask, which is a layer with holes in it that controls the angle of the incoming electron beams. This is because the fluorescent screen is separated into multi-coloured phosphors that are placed adjacent to each other at small intervals.
Thus it isn't actually a single coloured pixel, but rather 3 very small pixels that join together to form a larger dot.
Cathode Ray Oscilloscopes
A Cathode Ray Oscilloscope (CRO) is a diagnostic device that allows one to "see" voltage.It is essential a Cathode Ray Tube with two perpendicular sets of deflecting electric plates.The vertical set is where an input voltage is plugged in for the oscilloscope to display.
However, the horizontal set is connected to a "sweep generator".This is what provides a constant, but adjustable, timebase for the sweeping.It essentially creates a "sawtooth voltage."This is what causes the image to be animated, and measured with a linear scale.


O'LEVEL PHYSICS
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PHYSICS FORM TWO 
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