### Welcome to Talha's Physics Academy

To Help Teachers and Students.

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.

New content for Talha's Physics Academy will be posted as it is developed,It is my intent to keep this material continuously available except for brief maintenance times.

All the Branches of Physics are covered.

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### How does GPS(Global Positioning System) work?

Q. How does GPS(Global Positioning System) work?

A. The Global Positioning System (GPS) is a network of about 30 satellites orbiting the Earth at an altitude of 20,000 km. The system was originally developed by the US government for military navigation but now anyone with a GPS device, be it a SatNav, mobile phone or handheld GPS unit, can receive the radio signals that the satellites broadcast.

Wherever you are on the planet, at least four GPS satellites are ‘visible’ at any time. Each one transmits information about its position and the current time at regular intervals. These signals, travelling at the speed of light, are intercepted by your GPS receiver, which calculates how far away each satellite is based on how long it took for the messages to arrive.

Once it has information on how far away at least three satellites are, your GPS receiver can pinpoint your location using a process called trilateration.

Trilateration

Imagine you are standing somewhere on Earth with three satellites in the sky above you. If you know how far away you are from satellite A, then you know you must be located somewhere on the red circle. If you do the same for satellites B and C, you can work out your location by seeing where the three circles intersect. This is just what your GPS receiver does, although it uses overlapping spheres rather than circles.

The more satellites there are above the horizon the more accurately your GPS unit can determine where you are.

GPS and Relativity

GPS satellites have atomic clocks on board to keep accurate time. General and Special Relativity however predict that differences will appear between these clocks and an identical clock on Earth.

General Relativity predicts that time will appear to run slower under stronger gravitational pull – the clocks on board the satellites will therefore seem to run faster than a clock on Earth.

Furthermore, Special Relativity predicts that because the satellites’ clocks are moving relative to a clock on Earth, they will appear to run slower.

The whole GPS network has to make allowances for these effects –  proof that Relativity has a real impact.

### Why Do We Always See the Same Side of the Moon?

Q: Why does the moon always present the same face to us? I find it impossible to believe that this could happen by chance.

A: Nope, not by chance — it’s pure physics.

For starters, the moon is not stuck in place with one side facing us. Our lunar companion rotates while it orbits Earth. It’s just that the amount of time it takes the moon to complete a revolution on its axis is the same it takes to circle our planet — about 27 days. As a result, the same lunar hemisphere always faces Earth.

How’d this come to be? In a word: gravity. The moon’s gravity slightly warps our planet’s shape and gives us tides. Likewise, Earth tugs at the moon, creating a rocky, high-tide “bulge” facing us. That bulge ended up working like a brake, slowing the moon’s spin down to the current rate, so the lunar high tide permanently faces us.

When that happened, about 4 billion years ago, the moon became “tidally locked,” and it has presented us the same visage ever since.

### MIT Course 8.02 :Electricity and Magnetism( Study Material)

Course Highlights

This course features a complete set of videotaped lectures. The 36 video lectures on Electricity and Magnetism, by Professor Lewin, were recorded on the MIT campus during the Spring of 2002. Prof. Lewin is well-known at MIT and beyond for his dynamic and engaging lecture style.

Course Description
In addition to the basic concepts of Electromagnetism, a vast variety of interesting topics are covered in this course: Lightning, Pacemakers, Electric Shock Treatment, Electrocardiograms, Metal Detectors, Musical Instruments, Magnetic Levitation, Bullet Trains, Electric Motors, Radios, TV, Car Coils, Superconductivity, Aurora Borealis, Rainbows, Radio Telescopes, Interferometers, Particle Accelerators (a.k.a. Atom Smashers or Colliders), Mass Spectrometers, Red Sunsets, Blue Skies, Haloes around Sun and Moon, Color Perception, Doppler Effect, Big-Bang Cosmology.

### What is Elastic Potential Energy?

Elastic Potential Energy

Any object than can be deformed (have its shaped changed) and then return to its original shape can store elastic potential energy.
• We’re still talking about potential energy, since it is stored energy until the object is allowed to
bounce back.
• “Elastic” does not refer to just things like elastic bands…other materials that would be referred
to as elastic would be
• pole vaulter’s pole
• springs

You learned in Physics that Hooke’s Law is…
F = kx
F = force (N)
k = spring constant for that object (N/m)
x = amount of expansion or compression (m)
We can use this formula to figure out a formula for the energy stored in the spring.
• Remember that W = F d

• We might be tempted to just shove the formula for Hooke’s Law into this formula to get
W = kxd = kx2
, but this is wrong!
• You have to take into account that the force is not constant as the object returns to its original
shape… it’s at a maximum when it is deformed the most, and is zero when the object is not
deformed.
• Let’s graph Force vs Distance of Expansion for a spring that was stretched and we are now
letting go of it…

But this is really just a Force vs Displacement Graph like the ones we just looked at a couple of
sections back! To figure out the energy of the spring we can just figure out the work it does by looking at the area under the graph.

Area = ½ bh
= ½ F x
= ½ (kx) x
Area = ½ kx2 = W
So the work done by the spring (and then energy it stored) can be calculated using…
Ee = ½ kx2
Ee = eleastic potential energy (J)
k = spring constant (N/m)
x = amount of expansion or compression [deformation] (m)

Example 1: Determine how much energy a spring with a spring constant of 15 N/m stores if it is
stretched by 1.6m.
Ee = ½ kx2
= ½ (15N/m) (1.6 m)
2
Ee = 19 J

### If scientists can't see dark matter, how do they know it exists?

Q.If scientists can't see dark matter, how do they know it exists?

Ans. Scientists calculate the mass of large objects in space by studying their motion. Astronomers examining spiral galaxies in the 1950s expected to see material in the center moving faster than on the outer edges. Instead, they found the stars in both locations traveled at the same velocity, indicating the galaxies contained more mass than could be seen. Studies of the gas within elliptical galaxies also indicated a need for more mass than found in visible objects. Clusters of galaxies would fly apart if the only mass they contained were visible to conventional astronomical measurements.

Albert Einstein showed that massive objects in the universe bend and distort light, allowing them to be used as lenses. By studying how light is distorted by galaxy clusters, astronomers have been able to create a map of dark matter in the universe.

Although dark matter makes up most of the matter of the universe, it only makes up about a quarter of the composition. The universe is dominated by dark energy.

After the Big Bang, the universe began expanding outward. Scientists once thought that it would eventually run out of the energy, slowing down as gravity pulled the objects inside it together. But studies of distant supernovae revealed that the universe today is expanding faster than it was in the past, not slower, indicating that the expansion is accelerating. This would only be possible if the universe contained enough energy to overcome gravity — dark energy.

All of these methods provide a strong indication that the most of the matter in the universe is something yet unseen.

### What is the Physics Behind Vacuum Cleaner?

When you sip soda through a straw, you are utilizing the simplest of all suction mechanisms. Sucking the soda up causes a pressure drop between the bottom of the straw and the top of the straw. With greater fluid pressure at the bottom than the top, the soda is pushed up to your mouth. ­
This is the same basic mechanism at work in a vacuum cleaner, though the execution is a bit more complicated. In this article, we'll look inside a vacuum cleaner to find out how it puts suction to work when cleaning up the dust and debris in your house. As we'll see, the standard vacuum cleaner design is exceedingly simple, but it relies on a host of physical principles to clean effectively.

It may look like a complicated machine, but the conventional vacuum cleaner is actually made up of only six essential components:

An intake port, which may include a variety of cleaning accessories
An exhaust port
An electric motor
A fan
A porous bag
A housing that contains all the other components

When you plug the vacuum cleaner in and turn it on, this is what happens:
The electric current operates the motor. The motor is attached to the fan, which has angled blades (like an airplane propeller).
As the fan blades turn, they force air forward, toward the exhaust port (check out How Airplanes Work to find out what causes this).
When air particles are driven forward, the density of particles (and therefore the air pressure) increases in front of the fan and decreases behind the fan.
This pressure drop behind the fan is just like the pressure drop in the straw when you sip from your drink. The pressure level in the area behind the fan drops below the pressure level outside the vacuum cleaner (the ambient air pressure). This creates suction, a partial vacuum, inside the vacuum cleaner. The ambient air pushes itself into the vacuum cleaner through the intake port because the air pressure inside the vacuum cleaner is lower than the pressure outside.
As long as the fan is running and the passageway through the vacuum cleaner remains open, there is a constant stream of air moving through the intake port and out the exhaust port. But how does a flowing stream of air collect the dirt and debris from your carpet? The key principle is friction.

### Kaniz e zehra asked What is Heat?

11/13/2014 10:06:13 Kaniz e zehra asked What is Heat?

Consider a very hot mug of coffee on the countertop of your kitchen. For discussion purposes, we will say that the cup of coffee has a temperature of 80°C and that the surroundings (countertop, air in the kitchen, etc.) has a temperature of 26°C. What do you suppose will happen in this situation? I suspect that you know that the cup of coffee will gradually cool down over time. At 80°C, you wouldn't dare drink the coffee. Even the coffee mug will likely be too hot to touch. But over time, both the coffee mug and the coffee will cool down. Soon it will be at a drinkable temperature. And if you resist the temptation to drink the coffee, it will eventually reach room temperature. The coffee cools from 80°C to about 26°C. So what is happening over the course of time to cause the coffee to cool down? The answer to this question can be both macroscopic and particulate in nature.

On the macroscopic level, we would say that the coffee and the mug are transferring heat to the surroundings. This transfer of heat occurs from the hot coffee and hot mug to the surrounding air. The fact that the coffee lowers its temperature is a sign that the average kinetic energy of its particles is decreasing. The coffee is losing energy. The mug is also lowering its temperature; the average kinetic energy of its particles is also decreasing. The mug is also losing energy. The energy that is lost by the coffee and the mug is being transferred to the colder surroundings. We refer to this transfer of energy from the coffee and the mug to the surrounding air and countertop as heat. In this sense, heat is simply the transfer of energy from a hot object to a colder object.

Now let's consider a different scenario - that of a cold can of pop placed on the same kitchen counter. For discussion purposes, we will say that the pop and the can which contains it has a temperature of 5°C and that the surroundings (countertop, air in the kitchen, etc.) has a temperature of 26°C. What will happen to the cold can of pop over the course of time? Once more, I suspect that you know the answer. The cold pop and the container will both warm up to room temperature. But what is happening to cause these colder-than-room-temperature objects to increase their temperature? Is the cold escaping from the pop and its container? No! There is no such thing as the cold escaping or leaking. Rather, our explanation is very similar to the explanation used to explain why the coffee cools down. There is a heat transfer.

Over time, the pop and the container increase their temperature. The temperature rises from 5°C to nearly 26°C. This increase in temperature is a sign that the average kinetic energy of the particles within the pop and the container is increasing. In order for the particles within the pop and the container to increase their kinetic energy, they must be gaining energy from somewhere. But from where? Energy is being transferred from the surroundings (countertop, air in the kitchen, etc.) in the form of heat. Just as in the case of the cooling coffee mug, energy is being transferred from the higher temperature objects to the lower temperature object. Once more, this is known as heat - the transfer of energy from the higher temperature object to a lower temperature object.

### Kaniz e zehra asked Two unlike capacitor is charged to a certain potential difference it is then immersed in oil what happen to it s capacitance ,charge and potential?

11/13/2014 9:49:27 Kaniz e zehra asked Two unlike capacitor is charged to a certain potential difference it is then immersed in oil what happen to it s capacitance ,charge and potential?

Ans.If you mean that an air capacitor is charged to a certain potential difference it is then immersed in oil what happen to it’s a)charge b)Potential and c) capacitance
Then the answer is :The dielectric constant Єr of the oil is grater than that of air.Whan an air capacitor is immersed in oil then
a)Its charge remain constant b)PD b/w the plates decreases c)the Capacitance increase.

But if the questions inlove two capacitors then the question will be same what like :Two unlike capacitors of different potentials and charges are joined in parallel.what happens to their pD?How are their charges distributed ?Is the energy of system affected? then answer will be :i)The potential difference will remain same.(ii) The charge is distributed (iii)The energy of the system decrease