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"Now I am become Death, the destroyer of worlds." Robert Oppenheimer after the first test of the atomic bomb.

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Presentation on theme: ""Now I am become Death, the destroyer of worlds." Robert Oppenheimer after the first test of the atomic bomb."— Presentation transcript:

1 "Now I am become Death, the destroyer of worlds." Robert Oppenheimer after the first test of the atomic bomb.

2 Elementary Particles  By the mid - 1930s, experimental evidence confirmed the existence of three sub-atomic particles.  These are protons, electrons, and neutrons.

3 The Four Forces of Nature  The four basic forces of nature are electromagnetism, gravity, strong nuclear force, and the weak nuclear force

4 The Strong Force  This force is responsible for holding the nucleus together.  This force is so strong that it binds and stabilizes the protons of similar charges within a nucleus.  However, it is very short range. No such force will be felt beyond the order of 1 fm (femtometer or 10 - 15 m).

5 Electromagnetic Force  This is the force which exists between all particles which have an electric charge.  For example, electrons (negative charge) bind with nucleus of an atom, due to the presence of protons (positive charge).  The force is long range, in principle extending over infinite distance.  However, the strength can quickly diminishes due to shielding effect.

6 Electromagnetic Force continued  Many everyday experiences such as friction and air resistance are due to this force.  This is also the resistant force that we feel when pressing our palm against a wall.  This is originated from the fact that no two atoms can occupy the same space.

7 Electromagnetic Force 3  its strength is about 100 times weaker within the range of 1 fm, where the strong force dominates.  But because there is no shielding within the nucleus, the force can be cumulative and can compete with the strong force.  This competition determines the stability structure of nuclei.

8 The Weak Force  This force is responsible for nuclear beta decay and other decay processes involving fundamental particles.  The range of this force is smaller than 1 fm and is 10 -7 weaker than the strong force.  It is important in understanding the behavior of fundamental particles.

9 Gravitational Force  This is the force that holds us onto the Earth.  It is important in our daily life.  On the scale of the atomic world it is of negligible or no importance at all.  Gravitational force is cumulative and extended to infinity.  It exists whenever there is matter.

10 Gravitational Force  Your body is experiencing a gravitational pull with, say, your computer (or anything close to you or as far away as stars and galaxies) but the effect is so small you will never sense it.  However, you can sense the gravitational pull with the Earth (that is, your weight).  This is due to the cumulative effect of billions of billions of the atoms that make up your body with those atoms of the Earth.

11 Gravitational Force 3  This means that the larger the body (contains more matter), the stronger the force.  But on the scale of individual particles, the force is extremely small, only in the order of 10 -38 times that of the strong force.

12 Familiar and unfamiliar Forces  Of the 4 basic forces two of them can be experienced in our daily life.  They are also called the familiar forces which are the electromagnetic and gravitaional forces.  Similarly, the strong force and the weak force are called the unfamiliar forces.

13 Mystery of the Forces  How do the forces work?  Why do we sense forces - pushing and pulling?  The basic forces between particles of matter all act through a 'force carrier', which is exchanged between the interacting particles.  These exchange carriers are also known as field particles, or gauge boson

14 The Strong Force Carrier: Gluons (g)  Gluons have no mass and no electric charge.  They carry a special 'charge-like' property that hold quarks (constituents of protons and neutrons) together.  The theoretical treatment of this carrier is described in the theory of quantum chromodynamics (QCD)

15 The electromagnetic force Carrier: Photons (g)  Photons are responsible to 'carry' electromagnetic forces.  It is also known as the particle of 'light' as they also 'carry’ the light that we see.  They have no mass, and no charge and can exchange between two particles over infinite distance.  This is also the reason why light can travel to infinite distance and we can see stars that are far away.  Detailed theoretical description of photons is given in quantum electrodynamics (QED).

16 The weak force Carrier: W and Z (W +, W -, Z 0 )  The carrier W can be either positively charged (W + ) or negatively charged (W - ), while Z is neutral (Z 0 ). The reason why the force is weak is because these carriers are massive, about 100 times that of the weight of a proton.

17 The gravitaional force Carrier: Graviton (?)  The existence of this carrier is yet to be confirmed.  If it exists it should have zero mass and zero charge.  It may will be the discovery of the century if its presence is ever detected.

18 Antiparticles  Particles have their antiparticles.  Antiparticles are produced in nuclear reactions when enough energy is available.  When an antiparticle and a particle meet they annihilate each other.

19 Fate and origin  The energy of their vanished mass along with any kinetic energy they possessed is converted to electromagnetic energy.  Antiparticles are extremely rare in nature; they are mostly produced in particle accelerators.  This picture shows the Large Hadron Accelerator located between France and Switzerland.  People use magnetic and electric fields to accelerate protons and have them collide.  These collisions result in many new subatomic particles.

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