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Physics Video Project

The video project was given in 2008 for XI class students of Meghalaya. The best video among them is posted here. Though there are some errors, the video is inspiring to note the work done entirely from scratch by the XI Class students.

Speed of Light – a thought experiment

Suppose that you are a person 1 standing on a planet.You could see a person 2 moving in a space craft.2 has a mirror(on the surface of planet) exactly at is down which is moving exactly with the same speed that space craft is moving(and also,the line of translatory motion of both the craft and mirror are parallel to each other).If 2 has shot a beam of light from the bottom of space ship,as the mirror is moving exactly with the space craft;for 2,the path of light is straight line and gets reflected back along the same path in time t.If you are observing the whole thing from the surface of planet,for you,the path of light would obviously be ‘V’ shaped(let the time taken be t’).As the ‘V’ shaped path is longer than straight path and speed of light is same for observers,the time measured by 2 is obviously not the same as you measure.If you are considered to be reference frame,will the clock of 2 appear to be moving slower than yours? (Asked Charan)

Magnetism of Earth – What’s the cause?

Selwyn posted this Message: I was recently showing my grandchildren the effect on a compass needle by the magnetic field surrounding a magnet placed in opposition to the earth’s magnetic field – Magnetism 1.01 no? Their mother then asked me how is it that the earth has a magnetic field. I confidently answered that it was due to the central molten iron core of the earth acting as a magnet, and of course as we learned in Magnetism 1.1 68 years ago on of the ways of creating a magnet is a) by striking it several blows with a hammer while holding it in alignment with the earth’s N/S axis or b) heating it. In both cases this allows the atoms to move more freely and align themselves similarly with the axis. But that started me thinking: We know that heating ANYTHING applies energy to the atoms/molecules and this then causes them to display greater and more violent movement within the body of the material. If these molecules are so agitated, how can this identical phenomenon allow the molecule/atoms of the earth’s core to “settle down quietly” into a N/S configuration and remain so?

The answer is not so simple. Nobody has actually drilled into the centre of earth. What we know is by analysing the seismic waves and the shockwaves.

The Earth’s magnetic field is believed to be generated by electric currents in the conductive material of its core, created by convection currents due to heat escaping from the core. However the process is complex, and computer models that reproduce some of its features have only been developed in the last few decades. (Wikipedia)

The Earth and most of the planets in the Solar System, as well as the Sun and other stars, all generate magnetic fields through the motion of highly conductive fluids. The Earth’s field originates in its core. This is a region of iron alloys extending to about 3400 km (the radius of the Earth is 6370 km). It is divided into a solid inner core, with a radius of 1220 km, and a liquid outer core. The motion of the liquid in the outer core is driven by heat flow from the inner core, which is about 6,000 K (5,730 °C; 10,340 °F), to the core-mantle boundary, which is about 3,800 K (3,530 °C; 6,380 °F). The pattern of flow is organized by the rotation of the Earth and the presence of the solid inner core.

The mechanism by which the Earth generates a magnetic field is known as a dynamo.

(Wikipedia)

The Dynamo Effect

The simple question “how does the Earth get its magnetic field?” does not have a simple answer. It does seem clear that the generation of the magnetic field is linked to the rotation of the earth, since Venus with a similar iron-core composition but a 243 Earth-day rotation period does not have a measurable magnetic field. It certainly seems plausible that it depends upon the rotation of the fluid metallic iron which makes up a large portion of the interior, and the rotating conductor model leads to the term “dynamo effect” or “geodynamo”, evoking the image of an electric generator.

Convection drives the outer-core fluid and it circulates relative to the earth. This means the electrically conducting material moves relative to the earth’s magnetic field. If it can obtain a charge by some interaction like friction between layers, an effective current loop could be produced. The magnetic field of a current loop could sustain the magnetic dipole type magnetic field of the earth. Large-scale computer models are approaching a realistic simulation of such a geodynamo.

Refer to this Wikipedia Page and this Hyperphysics page for more details

The GeoMag page is also very informative and useful(Natural Resources Canada)

Principle of Working of Microwave Oven

Why is it called a microwave oven?
Who invented it?
What is the principle of its working?

A microwave oven is used to cook (or heat) food with the help of microwaves produced by magnetron – the device producing microwaves in the oven. Microwave ovens are so quick and efficient because they channel heat energy directly to the molecules (tiny particles) inside food.

Who invented Microwave Oven?

Percy Spencer is generally credited with inventing the modern microwave oven after World War II from radar technology developed during the war. Named the “Radarange”, it was first sold in 1946. Raytheon later licensed its patents for a home-use microwave oven that was first introduced by Tappan in 1955, but these units were still too large and expensive for general home use. The countertop microwave oven was first introduced in 1967 by the Amana Corporation, and their use has spread into commercial and residential kitchens around the world.

Working of microwave oven

A microwave oven, commonly referred to as a microwave, is a kitchen appliance that heats and cooks food by exposing it to electromagnetic radiation in the microwave spectrum. This induces polar molecules in the food to rotate and produce thermal energy in a process known as dielectric heating. Microwave ovens heat foods quickly and efficiently because excitation is fairly uniform in the outer25–38 mm (1–1.5 inches) of a homogenous (high water content) food item; food is more evenly heated throughout (except in heterogeneous, dense objects) than generally occurs in other cooking techniques.

A microwave oven heats food by passing microwave radiation through it. Microwaves are a form of non-ionizing electromagnetic radiation with a frequency higher than ordinary radio waves but lower than infrared light. Microwave ovens use frequencies in one of the ISM (industrial, scientific, medical) bands, which are reserved for this use, so they don’t interfere with other vital radio services. Consumer ovens usually use 2.45 gigahertz (GHz)—a wavelength of 12.2 centimetres (4.80 in)—while large industrial/commercial ovens often use 915 megahertz (MHz)—32.8 centimetres (12.9 in). Water, fat, and other substances in the food absorb energy from the microwaves in a process called dielectric heating. Many molecules (such as those of water) are electric dipoles, meaning that they have a partial positive charge at one end and a partial negative charge at the other, and therefore rotate as they try to align themselves with the alternating electric field of the microwaves. Rotating molecules hit other molecules and put them into motion, thus dispersing energy. This energy, when dispersed as molecular vibration in solids and liquids (i.e. as both potential energy and kinetic energy of atoms), is heat.

For more details refer to :

Relativity?

Charan asked: Suppose that you are a person 1 standing on a planet.You could see a person 2 moving in a space craft.2 has a mirror(on the surface of planet) exactly at is down which is moving exactly with the same speed that space craft is moving(and also,the line of translatory motion of both the craft and mirror are parallel to each other).If 2 has shot a beam of light from the bottom of space ship,as the mirror is moving exactly with the space craft;for 2,the path of light is straight line and gets reflected back along the same path in time t.If you are observing the whole thing from the surface of planet,for you,the path of light would obviously be ‘V’ shaped(let the time taken be t’).As the ‘V’ shaped path is longer than straight path and speed of light is same for observers,the time measured by 2 is obviously not the same as you measure.If you are considered to be reference frame,will the clock of 2 appear to be moving slower than yours?

You can now Download AISSCE 2015 CBSE Physics Board Question Paper and solution HERE

Depletion Layer and Energy barrier

Explain the formation of depletion region and potential barrier in a pn junction diode.

The depletion region (also called depletion layer, depletion zone, junction region, space charge region or space charge layer) is an insulating region within a conductive, doped semiconductor material where the mobile charge carriers have been diffused away, or have been forced away by an electric field. The only elements left in the depletion region are ionized donor or acceptor impurities.

When the N-type semiconductor and P-type semiconductor materials are first joined together, a very large density gradient exists between both sides of the PN junction. The result is that some of the free electrons from the donor impurity atoms begin to migrate across this newly formed junction to fill up the holes in the P-type material producing negative ions.

A depletion region forms instantaneously across a p–n junction. It is most easily described when the junction is in thermal equilibrium or in a steady state: in both of these cases the properties of the system do not vary in time; they have been called dynamic equilibrium.

Electrons and holes diffuse into regions with lower concentrations of electrons and holes, much as ink diffuses into water until it is uniformly distributed. By definition, N-type semiconductor has an excess of free electrons compared to the P-type region, and P-type has an excess of holes compared to the N-type region. Therefore, when N-doped and P-doped pieces of semiconductor are placed together to form a junction, electrons migrate into the P-side and holes migrate into the N-side. Departure of an electron from the N-side to the P-side leaves a positive donor ion behind on the N-side, and likewise the hole leaves a negative acceptor ion on the P-side.

Following transfer, the diffused electrons come into contact with holes on the P-side and are eliminated by recombination. Likewise for the diffused holes on the N-side. The net result is the diffused electrons and holes are gone, leaving behind the charged ions adjacent to the interface in a region with no mobile carriers (called the depletion region). The uncompensated ions are positive on the N side and negative on the P side. This creates an electric field that provides a force opposing the continued exchange of charge carriers. When the electric field is sufficient to arrest further transfer of holes and electrons, the depletion region has reached its equilibrium dimensions. Integrating the electric field across the depletion region determines what is called the built-in voltage (also called the junction voltage or barrier voltage or contact potential).