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Talha's Physics Academy

Talha's Physics Academy is an exploration environment for concepts in physics which employs free Physics Books and other linking strategies to facilitate smooth navigationThe entire environment is interconnected with thousands of links, reminiscent of a neural network.

Talha's Physics Academy

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Talha's Physics Academy

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How weightlessness experiment in a satellite overcome? Give relevant formula

In order to produce an artificial gravity in the space craft, the laboratory of space craft is rotated with suitable frequency about its own axis. The rotation is so maintained that the astronaut do not feel weightlessness. The frequency of rotation depends on the length of laboratory of space craft.


  Consider a space craft whose laboratory is 'L' meter long consisting of two chambers connected by a tunnel. Let us see how many revloutions per second must the space craft make in order to supply artificial gravity for the astronauts.
Let 'T' be the time for one revolution and 'f' be the frequency of rotation.

R = L / 2 --------(i)
  When the laboratory revolves, a centripetal force is experienced by the astronauts.
  Fc = mv2/r --------(ii)
  According to Newton's second law of motion
Fc = mac --------(iii)
  Comparing equations (ii) and (iii)
mac = mv2/r
ac = v2/r
  Where ac is the centripetal acceleration
  Since radius of laboratory is R , therefore,
ac = v2/R --------(iv)
  Now we will determine the linear speed of the laboratory.
  In one rotation of the laboratory
Distance = 2pR
time = T
velocity = ?
  Using the relation        s = vt
                           2pR = vT
                               v = 2pR/T
  Putting the value of v in equation (iv), we get,
ac = (2pR/T)2/R
ac = (4p2R2/T2)/R
ac = (4p2R/T2)
ac = 4p2R x 1/T2
  But 1/T = frequency (f)
ac = 4p2R x f2
f2 = ac/4p2R

  For natural gravity acceleration must be equal to 9.8m/s2 i.e. ac = g , thus

  This expression indicates that the frequency of rotation depends on the length of the laboratory of space craft. Larger is the laboratory, smaller should be the number of rotation per second to obtain the natural gravity effect.

If the tension in string is doubled what will be the effect on the speed of standing waves in the string?

The velocity of a travelling wave in a stretched string is determined by the tension and the mass per unit length of the string.

The wave velocity is given by

When the wave relationship is applied to a stretched string, it is seen that resonant standing wave modes are produced.

So, if we double the tension the speed will increase 1.41 times or root (2) times.

At what condition a convex lens can behave as a diverging lens?

The ability of a lens to act as converging or diverging depends upon its refractive index. The refractive index n of the material of a lens depends upon the medium in which the lens is placed. Generally the lens is in air so n gives refractive index of material of lens with respect to air. If you place the convex lens in a medium other than air, then due to change in n, the focal length f of lens changes. if n decreases, f increases.

So if you immerse the convex lens in a liquid whose refractive index is greater than refractive index of material of lens then it will change its nature and become concave lens and thus it will start diverging the rays rather than focusing them on a single point. Its true the other way round also that is concave also becomes convex.

We show degree on centigrade and not show on kelvin why??..what is reason behind it??

The Kelvin temperature scale is sometimes called the absolute temperature scale, especially in older books. It was developed byWilliam Thomson, also known as Lord Kelvin, in 1848. A Kelvin degree is the same size as a Centigrade degree. This temperature scale however uses absolute zero, rather than the freezing point of water, as the zero point. In this temperature scale water freezes at 273.15 Kelvins and boils at 373.15 Kelvins. The Kelvin temperature scale should be used in thermodynamic calculations. Its principle difference is that kelvin measurements, written as K have a much lower starting point: 0K or 0 Kelvin (note the absence of the degree symbol °).

It bears additional mention that kelvins are not measured by degrees. They were considered so until 1968, when the 13th General Conference on Weights and Measures determined to drop the degree reference. This decision was made because Thomson’s measurement referred to an absolute and specific temperature (where no heat energy exists). Celsius, conversely, uses the point of reference of water freezing at the bottom of its scale, and this doesn’t accurately account for heat energy left in the water at this point (273.15 K). Instead, these temperature units are thought of as kelvins. When you measure something by Celsius, for example the boiling point of water, you are measuring in degrees (approximately 100° C). The boiling point of water on Thomson’s scale is approximately 373 kelvins or written as 373 K.

There are some important marking points for Thomson’s scale. Absolute zero is 0 K, and the triple point of water, where water can exist as gas, liquid and solid is 273.16 K (.01° C or 32.018° F). The melting point of ice, 0° C or 32°F, is 273.15 K. The boiling point of water, approximately 100° C or 212° F, is exactly 373.1339 K.

The scientific community often uses kelvin and Celsius measurements interchangeably or at the same time. You may see data on temperature given both a C degrees measurement and a kelvin measurement. This is especially the case when discussing heat energy units between the melting point of ice and absolute zero.

Give difference between inertial and non inertial frames of references?

Inertial Frame of Reference:

Inertial frames are ones which are at rest or moving with uniform velocity with respect to fixed stars.
For an observer in such a frame of reference Newton's second law of motion is valid.

Ex: A train moving with uniform velocity is an inertial frame.

Non Inertial Frame of Reference:

A non inertial frame is one which is accelerated ( linearly or due to rotation) with respect to fixed stars.
Newton's second law of motion is not valid in such a frame of reference, unless we introduce a force called pseudo - force.

Ex: A freely falling elevator may be taken as a non inertial frame.

For Example:

Wave Particle Duality and Uncertainty Principle

The wave-like nature of light explains most of its properties:

1. Reflection/refraction


3.Doppler effect

But, the results from stellar spectroscopy (emission and absorption spectra) can only be explained if light has a particle nature as shown by Bohr's atom and the photon description of light.
This dualism to the nature of light is best demonstrated by the photoelectric effect, where a weak UV light produces a current flow (releases electrons) but a strong red light does not release electrons no matter how intense the red light.

Einstein explained the photoelectric effect by assuming that light exists in a particle-like state, packets of energy (quanta) called photons. There is no current flow for red light because the packets of energy carried by each individual red photons are too weak to knock the electrons off the atoms no matter how many red photons you beamed onto the cathode. But the individual UV photons were each strong enough to release the electron and cause a current flow.

It is one of the strange, but fundamental, concepts in modern physics that light has both a wave and particle state (but not at the same time), called wave-particle dualism.

De Broglie Matter Waves:

Perhaps one of the key questions when Einstein offered his photon description of light is, does an electron have wave-like properties? The response to this question arrived from the Ph.D. thesis of Louis de Broglie in 1923. de Broglie argued that since light can display wave and particle properties, then matter can also be a particle and a wave too.

One way of thinking of a matter wave (or a photon) is to think of a wave packet. Normal waves look with this:
having no beginning and no end. A composition of several waves of different wavelength can produce a wave packet that looks like this:

So a photon, or a free moving electron, can be thought of as a wave packet, having both wave-like properties and also the single position and size we associate with a particle. There are some slight problems, such as the wave packet doesn't really stop at a finite distance from its peak, it also goes on for every and every. Does this mean an electron exists at all places in its trajectory?

de Broglie also produced a simple formula that the wavelength of a matter particle is related to the momentum of the particle. So energy is also connected to the wave property of matter.

While de Broglie waves were difficult to accept after centuries of thinking of particles are solid things with definite size and positions, electron waves were confirmed in the laboratory by running electron beams through slits and demonstrating that interference patterns formed.

How does the de Broglie idea fit into the macroscopic world? The length of the wave diminishes in proportion to the momentum of the object. So the greater the mass of the object involved, the shorter the waves. The wavelength of a person, for example, is only one millionth of a centimeter, much to short to be measured. This is why people don't `tunnel' through chairs when they sit down.

Uncertainty Principle:

Classical physics was on loose footing with problems of wave/particle duality, but was caught completely off-guard with the discovery of the uncertainty principle.

The uncertainty principle, developed by W. Heisenberg, is a statement of the effects of wave-particle duality on the properties of subatomic objects. Consider the concept of momentum in the wave-like microscopic world. The momentum of wave is given by its wavelength. A wave packet like a photon or electron is a composite of many waves. Therefore, it must be made of many momentums. But how can an object have many momentums?

Of course, once a measurement of the particle is made, a single momentum is observed. But, like fuzzy position, momentum before the observation is intrinsically uncertain. This is what is know as the uncertainty principle, that certain quantities, such as position, energy and time, are unknown, except by probabilities. In its purest form, the uncertainty principle states that accurate knowledge of complementarity pairs is impossible. For example, you can measure the location of an electron, but not its momentum (energy) at the same time.
Mathematically we describe the uncertainty principle as the following, where `x' is position and `p' is momentum:

This is perhaps the most famous equation next to E=mc2 in physics. It basically says that the combination of the error in position times the error in momentum must always be greater than Planck's constant. So, you can measure the position of an electron to some accuracy, but then its momentum will be inside a very large range of values. Likewise, you can measure the momentum precisely, but then its position is unknown.

Also notice that the uncertainty principle is unimportant to macroscopic objects since Planck's constant, h, is so small (10-34). For example, the uncertainty in position of a thrown baseball is 10-30 millimeters.

The depth of the uncertainty principle is realized when we ask the question; is our knowledge of reality unlimited? The answer is no, because the uncertainty principle states that there is a built-in uncertainty, indeterminacy, unpredictability to Nature.

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