Descovery of Electrons

J.J. Thomson became the third Cavendish Professor of Experimental Physics in 1884. One of the phenomena he studied was the conduction of electricity through gases.

One subject which interested Thomson was cathode rays. These rays are emitted at the cathode, or negative terminal, in a discharge tube. In 1879 Crookes had proposed that the cathode rays were 'radiant matter', negatively charged particles that were repelled from the negatively charged cathode and attracted to the positively charged anode.

The nature of the cathode rays was controversial. Although Thomson thought the rays must be particles, many Europeans thought they were an 'etherial disturbance', like light. In Germany Hertz had observed the rays passing through thin sheets of gold. It seemed impossible that particles could pass through solid matter.

Hertz had also found (wrongly) that the rays were not deflected by electric fields. In 1897 Thomson repeated Hertz's experiment.J.J. Thomson had balanced the cathode rays between the electric and magnetic forces.

The force (F) on a charged object in an electric field depends on the strength of the electric field (E), multipled by the charge (q) on the object.

F = Eq

The force (F) on a charged object in a magnetic field depends on the strength of the magnetic field (B), multipled by both the charge (q) and velocity (v) of the object.

F = Bqv

Since the forces were balanced:

Eq = Bqv

v = E/B

The velocity was equal to the electric field strength divided by the magnetic field strength. Thomson could measure these field strengths and use them to calculate the velocity of the rays.

Technical Education

Lincoln College of New England takes a unique approach to education. The college prides itself on helping students achieve more than just a degree... We provide an education that focuses on big results - results that help build a successful and fulfilling future while emphasizing the importance of lifelong learning. Our goal is to help each Lincoln College of New England student Be Amazing in all that they do.

Lincoln College of New England graduates have an excellent reputation among employers and are poised for success. On-campus housing is available at our three distinct small, campuses in Hartford, Southington and Suffield, CT.

Lincoln College of New England offers more than 30 undergraduate degree programs within the fields of Health Sciences, Business, Communications, Hospitality, and more. Bachelor Degrees, Associate Degrees, and Certificates are available depending on program area.

Nepal rich in hydropower

Nepal's most possessed natural resource is water. A land-locked country it may be, but the country is blessed by snow-capped mountains which feed Nepal land. Rivers are not only for rafters but also for businessmen. More than 80% of Nepal's electricity is generated by rivers. Snow-capped Himalayas are the main sources of Nepali rivers. Nepal has altitude ranging from 60 meter to all the way upto 8848 meters (Mt everest), the highest altitude of the earth. Some of the fastest running rivers in the world are located here.

There are three major rivers in Nepal namely Kosi River, Gandaki River and Karnali River which lie across east to west running from north to south. Surroundings of most rivers are in their natural settings. Nepali rivers are paradise to River Rafters who just can't have enough of angry and mad rivers. Need we mention Himalayan Water? It's all here in this beautiful country. No matter how many rivers you have rafted here, there is always a river waiting to be explored.Many of Nepal's rivers such as the Karnali, Seti, and Gandaki are fueled by the Himalayas. These rivers rush through 8848m altitude from sea level to 60m. Extreme elevation of the land helps these rivers fly! And they carry water to generate more than 90,000 mW of electricity. Currently Nepal produces less than 2% of it's capacity. So why hasn't anything been done to get closer to 98% of this open business ?
Many small sized hydro power plants are being currently setup. Lack of infrastructure such as roads, government policy, war and conflict in the region has slowed down many projects.

In Nepal, there are more plans than actions. There are plans to elevate poverty, such plans go through numbers like these.. Plan 1 to Plan 20. There are also plans to setup hydro-power projects to make nepal sufficient of electricity and also earn foreign revenue by selling it. Hydro power Plans have similar numbers like Plan 1 and Plan 2 and so on. They are as boring as the talks of political leaders. Everybody loves pointing their fingers at the other Government, and Every Government operates for about an year before it is replaced by another. When the new Government comes to office, they argue why plan-numbers were not long, so they add Plan 21 through Plan 9999 before saying good bye! In last 10 years alone, Nepal had more than 10 different governments, about one new government per year.

In case you were interested, there are hydro-power plans for upto year 2030, by which they believe Nepal will produce enough electricity for the entire country as well as start making some money by selling it!



Many small to medium sized, some privately owned hydropower plants are being setup in many part of the country, proving to all foreign investors that Nepal's rivers are good for business. Read about Nepal's War and how Nepal is unfolding, some argue is it really ? Also visit blogs by Nepalese who have good coverage on what Nepali Government really is. See Web Directory > Nepali Blogs

Also check out this nice PDF File which has Nepal Power Development Map, lot of small to large scale hydro power projects, some active and some just sleeping ones

Einstien photoelectric effect

In the photoelectric effect, electrons are emitted from matter (metals and non-metallic solids, liquids or gases) as a consequence of their absorption of energy from electromagnetic radiation of very short wavelength, such as visible or ultraviolet light. Electrons emitted in this manner may be referred to as "photoelectrons".First observed by Heinrich Hertz in 1887, the phenomenon is also known as the "Hertz effect", although the latter term has fallen out of general use. Hertz observed and then showed that electrodes illuminated with ultraviolet light create electric sparks more easily.

The photoelectric effect requires photons with energies from a few electronvolts to over 1 MeV in high atomic number elements. Study of the photoelectric effect led to important steps in understanding the quantum nature of light and electrons and influenced the formation of the concept of wave–particle duality. Other phenomena where light affects the movement of electric charges include the photoconductive effect (also known as photoconductivity or photoresistivity), the photovoltaic effect, and the photoelectrochemical effect.

The photons of a light beam have a characteristic energy determined by the frequency of the light. In the photoemission process, if an electron within some material absorbs the energy of one photon and thus has more energy than the work function (the electron binding energy) of the material, it is ejected. If the photon energy is too low, the electron is unable to escape the material. Increasing the intensity of the light beam increases the number of photons in the light beam, and thus increases the number of electrons excited, but does not increase the energy that each electron possesses. The energy of the emitted electrons does not depend on the intensity of the incoming light, but only on the energy or frequency of the individual photons. It is an interaction between the incident photon and the outermost electron.

Electrons can absorb energy from photons when irradiated, but they usually follow an "all or nothing" principle. All of the energy from one photon must be absorbed and used to liberate one electron from atomic binding, or else the energy is re-emitted. If the photon energy is absorbed, some of the energy liberates the electron from the atom, and the rest contributes to the electron's kinetic energy as a free particle.[citation needed]
Experimental results of the photoelectric emission
For a given metal and frequency of incident radiation, the rate at which photoelectrons are ejected is directly proportional to the intensity of the incident light.
For a given metal, there exists a certain minimum frequency of incident radiation below which no photoelectrons can be emitted. This frequency is called the threshold frequency.
For a given metal of particular work function, increase in intensity of incident beam increases the magnitude of the photoelectric current, though stoppage voltage remains the same.
For a given metal of particular work function, increase in frequency of incident beam increases the maximum kinetic energy with which the photoelectrons are emitted, but the photoelectric current remains the same, though stoppage voltage increases.
Above the threshold frequency, the maximum kinetic energy of the emitted photoelectron depends on the frequency of the incident light, but is independent of the intensity of the incident light so long as the latter is not too high [5]
The time lag between the incidence of radiation and the emission of a photoelectron is very small, less than 10−9 second.
The direction of distribution of emitted electrons peaks in the direction of polarization (the direction of the electric field) of the incident light, if it is linearly polarized.[citation needed]
Mathematical description

The maximum kinetic energy Kmax of an ejected electron is given by


where h is the Planck constant, f is the frequency of the incident photon, and φ = hf0 is the work function (sometimes denoted W), which is the minimum energy required to remove a delocalised electron from the surface of any given metal. The work function, in turn, can be written as


where f0 is called the threshold frequency for the metal. The maximum kinetic energy of an ejected electron is


Because the kinetic energy of the electron must be positive, it follows that the frequency f of the incident photon must be greater than f0 in order for the photoelectric effect to occur
Stopping potential

The relation between current through an illuminated photoelectric system and applied voltage illustrates the nature of the photoelectric effect. For discussion, a plate P is illuminated by a light source, and any emitted electrons are collected at another plate electrode Q. The potential between P and Q can be varied and the current flowing in the external circuit between P and Q is measured.

If the frequency and the intensity of the incident radiation are kept fixed, it is found that the photoelectric current increases gradually with the increase in positive potential until all the photoelectrons emitted are collected. The photoelectric current attains saturation value and it does not increase further for any increase in the positive potential. The saturation current depends on the intensity of illumination, but not its wavelength.

If we apply negative potential to plate Q with respect to plate P, and increases it gradually we note that photoelectric current decreases rapidly until it is zero, at a certain negative potential on plate Q.The minimum negative potential given to plate Q at which the photoelectric current becomes zero is called stopping potential or cut off potential.[7] i. For the given frequency of incident radiation, the stopping potential is independent of its intensity.

ii. For a given frequency of the incident radiation, the stopping potential V0 if related to the maximum kinetic energy of the photoelectron that is just stopped from reaching plate Q.

If m is the mass and vmax is the maximum velocity of photoelectron emitted, then



If e is the charge on the electron and V0is the stopping potential, then work done by the retarding potential in stopping the electron = eV0.

Therefore, we have, 1/2mv2max = eV0

The above relation shows that the maximum velocity of the emitted photoelectron is independent of the intensity of the incident light.

Hence, we have the next equality:
Kmax = eV0

The stopping voltage varies linearly with frequency of light, but depends on the type of material. For any particular material, there is a threshold frequency that must be exceeded, independent of light intensity, to observe any electron emission.
Three-step model

In the X-ray regime, the photoelectric effect in crystalline material is often decomposed into three steps:[8]
Inner photoelectric effect (see photodiode below). The hole left behind can give rise to auger effect, which is visible even when the electron does not leave the material. In molecular solids phonons are excited in this step and may be visible as lines in the final electron energy. The inner photoeffect has to be dipole allowed. The transition rules for atoms translate via the tight-binding model onto the crystal. They are similar in geometry to plasma oscillations in that they have to be transversal.
Ballistic transport of half of the electrons to the surface. Some electrons are scattered.
Electrons escape from the material at the surface.

In the three-step model, an electron can take multiple paths through these three steps. All paths can interfere in the sense of the path integral formulation. For surface states and molecules the three-step model does still make some sense as even most atoms have multiple electrons which can scatter the one electron leaving.[citation needed]

Hurricane(Destruction)

Hurricane Katrina of the 2005 Atlantic hurricane season was the costliest natural disaster, as well as one of the five deadliest hurricanes, in the history of the United States.[2] Among recorded Atlantic hurricanes, it was the sixth strongest overall. At least 1,836 people died in the actual hurricane and in the subsequent floods, making it the deadliest U.S. hurricane since the 1928 Okeechobee hurricane; total property damage was estimated at $81 billion (2005 USD) nearly triple the damage wrought by Hurricane Andrew in 1992.

Hurricane Katrina formed over the Bahamas on August 23, 2005 and crossed southern Florida as a moderate Category 1 hurricane, causing some deaths and flooding there before strengthening rapidly in the Gulf of Mexico. The storm weakened before making its second landfall as a Category storm on the morning of Monday, August 29 in southeast Louisiana. It caused severe destruction along the Gulf coast from central Florida to Texas, much of it due to the storm surge. The most significant amount of deaths occurred in New Orleans, Louisiana, which flooded as the levee system catastrophically failed, in many cases hours after the storm had moved inland. Eventually 80% of the city and large tracts of neighboring parishes became flooded, and the floodwaters lingered for weeks. However, the worst property damage occurred in coastal areas, such as all Mississippi beachfront towns, which were flooded over 90% in hours, as boats and casino barges rammed buildings, pushing cars and houses inland, with waters reaching 6–12 miles (10–19 km) from the beach.

The hurricane protection failures in New Orleans prompted a lawsuit against the US Army Corps of Engineers (USACE), the designers and builders of the levee system as mandated in the Flood Control Act of 1965. Responsibility for the failures and flooding was laid squarely on the Army Corps in January 2008, but the federal agency could not be held financially liable due to sovereign immunity in the Flood Control Act of 1928. There was also an investigation of the responses from federal, state and local governments, resulting in the resignation of Federal Emergency Management Agency (FEMA) director Michael D. Brown, and of New Orleans Police Department (NOPD) Superintendent Eddie Compass. Conversely, the United States Coast Guard (USCG), National Hurricane Center (NHC) and National Weather Service (NWS) were widely commended for their actions, accurate forecasts and abundant lead time.

Five years later, thousands of displaced residents in Mississippi and Louisiana are still living in temporary accommodation. Reconstruction of each section of the southern portion of Louisiana has been addressed in the Army Corps of Engineers LACPR Final Technical Report which identifies areas not to be rebuilt and areas and buildings that need to be elevated.

Black hole

A black hole is a region of space from which nothing, not even light, can escape. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is an undetectable surface called an event horizon that marks the point of no return. It is called "black" because it absorbs all the light that hits the horizon, reflecting nothing, just like a perfect black body in thermodynamics.[1] Quantum mechanics predicts that black holes emit radiation like a black body with a finite temperature. This temperature is inversely proportional to the mass of the black hole, making it difficult to observe this radiation for black holes of stellar mass or greater.

Objects whose gravity field is too strong for light to escape were first considered in the 18th century by John Michell and Pierre-Simon Laplace. The first modern prediction of a black hole in general relativity was found by Karl Schwarzschild in 1916, although its interpretation as a black hole was not fully appreciated for another four decades. Long considered a mathematical curiosity, it was during the 1960s that theoretical work showed black holes were a generic prediction of general relativity. The discovery of neutron stars sparked interest in gravitationally collapsed compact objects as a possible astrophysical reality.

Black holes of stellar mass are expected to form when heavy stars collapse in a supernova at the end of their life cycle. After a black hole has formed it can continue to grow by absorbing mass from its surroundings. By absorbing other stars and merging with other black holes, supermassive black holes of millions of solar masses may be formed.

Despite its invisible interior, the presence of a black hole can be inferred through its interaction with other matter. Astronomers have identified numerous stellar black hole candidates in binary systems, by studying their interaction with their companion stars. There is growing consensus that supermassive black holes exist in the centers of most galaxies. In particular, there is strong evidence of a black hole of more than 4 million solar masses at the center of our Milky Way.
In 1915, Albert Einstein developed his theory of general relativity, having earlier shown that gravity does influence light's motion. Only a few months later, Karl Schwarzschild found a solution to Einstein field equations, which describes the gravitational field of a point mass and a spherical mass. A few months after Schwarzschild, Johannes Droste, a student of Hendrik Lorentz, independently gave the same solution for the point mass and wrote more extensively about its properties.[ This solution had a peculiar behaviour at what is now called the Schwarzschild radius, where it became singular, meaning that some of the terms in the Einstein equations became infinite. The nature of this surface was not quite understood at the time. In 1924, Arthur Eddington showed that the singularity disappeared after a change of coordinates (see Eddington–Finkelstein coordinates), although it took until 1933 for Georges Lemaître to realize that this meant the singularity at the Schwarzschild radius was an unphysical coordinate singularity.

In 1931, Subrahmanyan Chandrasekhar calculated, using general relativity, that a non-rotating body of electron-degenerate matter above a certain limiting mass (now called the Chandrasekhar limit at 1.4 solar masses) must have an infinite density. In other words, the object must have a radius of zero. His arguments were opposed by many of his contemporaries like Eddington and Lev Landau, who argued that some yet unknown mechanism would stop the collapse.They were partly correct: a white dwarf slightly more massive than the Chandrasekhar limit will collapse into a neutron star,] which is itself stable because of the Pauli exclusion principle. But in 1939, Robert Oppenheimer and others predicted that neutron stars above approximately three solar masses (the Tolman–Oppenheimer–Volkoff limit) would collapse into black holes for the reasons presented by Chandrasekhar, and concluded that no law of physics was likely to intervene and stop at least some stars from collapsing to black holes.

Oppenheimer and his co-authors interpreted the singularity at the boundary of the Schwarzschild radius as indicating that this was the boundary of a bubble in which time stopped. This is a valid point of view for external observers, but not for infalling observers. Because of this property, the collapsed stars were called "frozen stars,"[13] because an outside observer would see the surface of the star frozen in time at the instant where its collapse takes it inside the Schwarzschild radius.
Golden age
See also: Golden age of general relativity

In 1958, David Finkelstein identified the Schwarzschild surface as an event horizon, "a perfect unidirectional membrane: causal influences can cross it in only one direction" This did not strictly contradict Oppenheimer's results, but extended them to include the point of view of infalling observers. Finkelstein's solution extended the Schwarzschild solution for the future of observers falling into a black hole. A complete extension had already been found by Martin Kruskal, who was urged to publish it.

These results came at the beginning of the golden age of general relativity, which was marked by general relativity and black holes becoming mainstream subjects of research. This process was helped by the discovery of pulsars in 1967, which were shown to be rapidly rotating neutron stars by 1969. Until that time, neutron stars, like black holes, were regarded as just theoretical curiosities; but the discovery of pulsars showed their physical relevance and spurred a further interest in all types of compact objects that might be formed by gravitational collapse.

In this period more general black hole solutions were found. In 1963, Roy Kerr found the exact solution for a rotating black hole. Two years later, Ezra Newman found the axisymmetric solution for a black hole that is both rotating and electrically charged. Through the work of Werner Israel Brandon Carter] and David Robinson[23] the no-hair theorem emerged, stating that a stationary black hole solution is completely described by the three parameters of the Kerr–Newman metric; mass, angular momentum, and electric charge.

For a long time, it was suspected that the strange features of the black hole solutions were pathological artefacts from the symmetry conditions imposed, and that the singularities would not appear in generic situations. This view was held in particular by Vladimir Belinsky, Isaak Khalatnikov, and Evgeny Lifshitz, who tried to prove that no singularities appear in generic solutions. However, in the late sixties Roger Penrose[25] and Stephen Hawking used global techniques to prove that singularities are generic.

Work by James Bardeen, Jacob Bekenstein, Carter, and Hawking in the early 1970s led to the formulation of black hole thermodynamics. These laws describe the behaviour of a black hole in close analogy to the laws of thermodynamics by relating mass to energy, area to entropy, and surface gravity to temperature. The analogy was completed when Hawking, in 1974, showed that quantum field theory predicts that black holes should radiate like a black body with a temperature proportional to the surface gravity of the black hole.

The term "black hole" was first publicly used by John Wheeler during a lecture in 1967. Although he is usually credited with coining the phrase, he always insisted that it was suggested to him by somebody else. The first recorded use of the term is in a 1964 letter by Anne Ewing to the American Association for the Advancement of Science. After Wheeler's use of the term, it was quickly adopted in general use.
Properties and structure

The no-hair theorem states that, once it achieves a stable condition after formation, a black hole has only three independent physical properties: mass, charge, and angular momentum.[24] Any two black holes that share the same values for these properties, or parameters, are indistinguishable according to classical (i.e. non-quantum) mechanics.

These properties are special because they are visible from outside a black hole. For example, a charged black hole repels other like charges just like any other charged object. Similarly, the total mass inside a sphere containing a black hole can be found by using the gravitational analog of Gauss's law, the ADM mass, far away from the black hole. Likewise, the angular momentum can be measured from far away using frame dragging by the gravitomagnetic field.

When an object falls into a black hole, any information about the shape of the object or distribution of charge on it is evenly distributed along the horizon of the black hole, and is lost to outside observers. The behavior of the horizon in this situation is closely analogous to that of a conductive stretchy membrane with friction and electrical resistance, a dissipative system (see membrane paradigm).This is different from other field theories like electromagnetism, which do not have any friction or resistivity at the microscopic level, because they are time-reversible. Because a black hole eventually achieves a stable state with only three parameters, there is no way to avoid losing information about the initial conditions: the gravitational and electric fields of a black hole give very little information about what went in. The information that is lost includes every quantity that cannot be measured far away from the black hole horizon, including the total baryon number, lepton number, and all the other nearly conserved pseudo-charges of particle physics. This behavior is so puzzling that it has been called the black hole information loss paradox.

Aomic BOmbing in Hiroshima And Nakasami

During the final stages of World War II in 1945, the United States conducted two atomic bombings against the cities of Hiroshima and Nagasaki in Japan, the first on August 6, 1945 and the second on August 9, 1945. These two events are the only active deployments of nuclear weapons in war to date.

For six months, the United States had made use of intense strategic fire-bombing of 67 Japanese cities. Together with the United Kingdom and the Republic of China, the United States called for a surrender of Japan in the Potsdam Declaration on July 26, 1945. The Japanese government ignored this ultimatum. By executive order of President Harry S. Truman, the U.S. dropped the nuclear weapon "Little Boy" on the city of Hiroshima on Monday, August 6, 1945,[3][4] followed by the detonation of "Fat Man" over Nagasaki on August 9.

Within the first two to four months of the bombings, the acute effects killed 90,000–166,000 people in Hiroshima and 60,000–80,000 in Nagasaki,[1] with roughly half of the deaths in each city occurring on the first day. The Hiroshima prefectural health department estimates that, of the people who died on the day of the explosion, 60% died from flash or flame burns, 30% from falling debris and 10% from other causes. During the following months, large numbers died from the effect of burns, radiation sickness, and other injuries, compounded by illness. In a US estimate of the total immediate and short term cause of death, 15–20% died from radiation sickness, 20–30% from flash burns, and 50–60% from other injuries, compounded by illness. In both cities, most of the dead were civilians.

Six days after the detonation over Nagasaki, on August 15, Japan announced its surrender to the Allied Powers, signing the Instrument of Surrender on September 2, officially ending the Pacific War and therefore World War II. Germany had signed its Instrument of Surrender on May 7, ending the war in Europe. The bombings led, in part, to post-war Japan's adopting Three Non-Nuclear Principles, forbidding the nation from nuclear armament.[9] The role of the bombings in Japan's surrender and the U.S.'s ethical justification for them, as well as their strategical importance, is still debated.

At the time of its bombing, Hiroshima was a city of some industrial and military significance. A number of military camps were located nearby, including the headquarters of the Fifth Division and Field Marshal Shunroku Hata's 2nd General Army Headquarters, which commanded the defense of all of southern Japan.[22] Hiroshima was a minor supply and logistics base for the Japanese military. The city was a communications center, a storage point, and an assembly area for troops. It was one of several Japanese cities left deliberately untouched by American bombing, allowing a pristine environment to measure the damage caused by the atomic bomb.

The center of the city contained several reinforced concrete buildings and lighter structures. Outside the center, the area was congested by a dense collection of small wooden workshops set among Japanese houses. A few larger industrial plants lay near the outskirts of the city. The houses were constructed of wood with tile roofs, and many of the industrial buildings were also built around wood frames. The city as a whole was highly susceptible to fire damage.

The population of Hiroshima had reached a peak of over 381,000 earlier in the war, but prior to the atomic bombing the population had steadily decreased because of a systematic evacuation ordered by the Japanese government. At the time of the attack, the population was approximately 340,000–350,000.[1] Because official documents were burned, the exact population is uncertain.
The bombing

Seizo Yamada's ground level photo taken from approximately 7 km northeast of Hiroshima.
For the composition of the USAAF mission, see 509th Operations Group#Components.

Hiroshima was the primary target of the first nuclear bombing mission on August 6, with Kokura and Nagasaki being alternative targets. August 6 was chosen because clouds had previously obscured the target. The 393d Bombardment Squadron B-29 Enola Gay, piloted and commanded by 509th Composite Group commander Colonel Paul Tibbets, was launched from North Field airbase on Tinian in the West Pacific, about six hours flight time from Japan. The Enola Gay (named after Colonel Tibbets' mother) was accompanied by two other B-29s. The Great Artiste, commanded by Major Charles W. Sweeney, carried instrumentation; and a then-nameless aircraft later called Necessary Evil (the photography aircraft) was commanded by Captain George Marquardt.[

After leaving Tinian the aircraft made their way separately to Iwo Jima where they rendezvoused at 2,440 meters (8,010 ft) and set course for Japan. The aircraft arrived over the target in clear visibility at 9,855 meters (32,333 ft). During the journey, Navy Captain William Parsons had armed the bomb, which had been left unarmed to minimize the risks during takeoff. His assistant, 2nd Lt. Morris Jeppson, removed the safety devices 30 minutes before reaching the target area.

The dark portions of the garments this victim wore during the flash caused burns on their skin.[

About an hour before the bombing, Japanese early warning radar detected the approach of some American aircraft headed for the southern part of Japan. An alert was given and radio broadcasting stopped in many cities, among them Hiroshima. At nearly 08:00, the radar operator in Hiroshima determined that the number of planes coming in was very small—probably not more than three—and the air raid alert was lifted. To conserve fuel and aircraft, the Japanese had decided not to intercept small formations. The normal radio broadcast warning was given to the people that it might be advisable to go to air-raid shelters if B-29s were actually sighted. However a reconnaissance mission was assumed because at 07.31 the first B29 to fly over Hiroshima at 32,000 feet (9,800 m) had been the weather observation aircraft Straight Flush that sent a morse code message to the Enola Gay indicating that the weather was good over the primary target and because it then turned out to sea the 'all clear' was sounded in the city. At 08.09 Colonel Tibbets started his bomb run and handed control over to his bombardier.

The release at 08:15 (Hiroshima time) went as planned, and the gravity bomb known as "Little Boy", a gun-type fission weapon with 60 kilograms (130 lb) of uranium-235, took 43 seconds to fall from the aircraft flying at 31,060 feet (9,470 m)[29] to the predetermined detonation height about 1,900 feet (580 m) above the city. The Enola Gay had traveled 11.5 miles away before it felt the shock waves from the blast.[30]

Due to crosswind, it missed the aiming point, the Aioi Bridge, by almost 800 feet (240 m) and detonated directly over Shima Surgical Clinic.[31] It created a blast equivalent to about 13 kilotons of TNT (54 TJ). (The U-235 weapon was considered very inefficient, with only 1.38% of its material fissioning.)[32] The radius of total destruction was about one mile (1.6 km), with resulting fires across 4.4 square miles (11 km2).[33] Americans estimated that 4.7 square miles (12 km2) of the city were destroyed. Japanese officials determined that 69% of Hiroshima's buildings were destroyed and another 6–7% damaged.

70,000–80,000 people, or some 30%[35] of the population of Hiroshima were killed immediately, and another 70,000 injured.[36] Over 90% of the doctors and 93% of the nurses in Hiroshima were killed or injured—most had been in the downtown area which received the greatest damage.

Although the U.S. had previously dropped leaflets warning civilians of air raids on 35 Japanese cities, including Hiroshima and Nagasaki,[38] the residents of Hiroshima were given no notice of the atomic bomb.