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## Doubts from Faraday’s Experiments on electromagnetic induction

I wanna ask that in faradays’s first experiment with coil and magnet,when the magnet is moved away from coil the deflection in galvanometer becomes opposite as compared to first case…..what leads to this and if that means direction of current becomes opposite so HOW and WHY the direction of this changes…..plz can you tell me the practical reason of this…..its really confusing me!!!

Answer will be posted soon

(Visitors are welcome to post their response via comments)

## How light maintain its speed after refraction?

How light maintain its speed after refraction? -(Naveen Saxena asked)

The speed of light in a medium is constant. When light enters from one medium to another, there is a change in the speed of light and the change is almost instantaneous. The speed of light in a medium depends on the electric and magnetic properties of the medium, more specifically, the electric permitivity and magnetic permeability of the medium.

The speed of light in a medium is given by

$v=\frac{1}{\sqrt{\mu \varepsilon }}$

Wikipedia says

“At the microscale, an electromagnetic wave’s phase speed is slowed in a material because the electric field creates a disturbance in the charges of each atom (primarily the electrons) proportional to the electric susceptibility of the medium. (Similarly, the magnetic field creates a disturbance proportional to the magnetic susceptibility.) As the electromagnetic fields oscillate in the wave, the charges in the material will be “shaken” back and forth at the same frequency. The charges thus radiate their own electromagnetic wave that is at the same frequency, but usually with a phase delay, as the charges may move out of phase with the force driving them . The light wave traveling in the medium is the macroscopic superposition (sum) of all such contributions in the material: The original wave plus the waves radiated by all the moving charges. This wave is typically a wave with the same frequency but shorter wavelength than the original, leading to a slowing of the wave’s phase speed. Most of the radiation from oscillating material charges will modify the incoming wave, changing its velocity. However, some net energy will be radiated in other directions or even at other frequencies .

Depending on the relative phase of the original driving wave and the waves radiated by the charge motion, there are several possibilities:

• If the electrons emit a light wave which is 90° out of phase with the light wave shaking them, it will cause the total light wave to travel more slowly. This is the normal refraction of transparent materials like glass or water, and corresponds to a refractive index which is real and greater than 1.
• If the electrons emit a light wave which is 270° out of phase with the light wave shaking them, it will cause the total light wave to travel more quickly. This is called “anomalous refraction”, and is observed close to absorption lines, with X-rays, and in some microwave systems. It corresponds to a refractive index less than 1. (Even though the phase velocity of light is greater than the speed of light in vacuum c, the signal velocity is not, as discussed above). If the response is sufficiently strong and out-of-phase, the result is negative refractive index discussed below.
• If the electrons emit a light wave which is 180° out of phase with the light wave shaking them, it will destructively interfere with the original light to reduce the total light intensity. This is light absorption in opaque materials and corresponds to an imaginary refractive index.
• If the electrons emit a light wave which is in phase with the light wave shaking them, it will amplify the light wave. This is rare, but occurs in lasers due to stimulated emission. It corresponds to an imaginary index of refraction, with the opposite sign as absorption.

For most materials at visible-light frequencies, the phase is somewhere between 90° and 180°, corresponding to a combination of both refraction and absorption.”

## Potential Difference

“What is meant by potential difference?”

“Potential Difference between two points is the difference in Potential between the two points and is equal to the work done per unit charge ion carrying a positive test charge from one point to another against the electric field and without any acceleration”

## Electric Flux

From: Nancy James

Subject: Electric flux
Message Body:
Could some one please explain what is written in the ncert book of PHYSICs class XII about ‘electric flux’?

Save the images, open and zoomin to read

## A Question from Electromagnetic Waves

we know that electric and magnetic field are associated with charged particle but when light travells,it produces electric and magnetic field both but it consests of photons,which is a uncharged particle.why it did so?

Master will soon answer this.

By the time, if any visitor wants to answer, please use the commenting system

## Force on a charged particle

What is the force experienced by a charged particle moving perpendicular to an electric field? I understand that when it is moving parallel to the electric field, it accelerates.

When a charged particle is at rest in an electric field, it experiences a force in the direction of electric field (if it is a positive charge)

If it is moving in the direction of electric field, it will be accelerated.

If it is moving opposite to the electric field, it will be decelerated.

If it is moving perpendicular to the electric field, its trajectory will be a parabola, similar to that of a horizontal projectile

## A few selected questions fro CBSE Class 10 Physics Summative Assessment 1

Dear students,

If you are preparing for the CBSE Summative Assessment I for Class X, Give the following questions a try.

If you are not able to solve any of it, please post your doubts as comments to this post. The questions can be discussed here with due explanations.

Make it a practice to solve as much Sample papers and Guess papers as you can before writing the Board Exam.

All the best

1. An electric geyser has the ratings 2000W, 220V marked on it. What should be the minimum rating in whole number of a fuse wire, that may be required for safe use with this geyser? (1Marks)
2. Consider a circular loop of wire lying in the plane of a table. Let the current pass through the loop clockwise. Draw the direction of magnetic field inside and outside the loop. (2Marks)
3. A wire is stretched so as to make its length double. What is the effect on its resistivity and why? (2Marks)
4. Define electric power. Write its SI unit. What is the value of 1 kWh in joules. (2Marks)
5. A torch bulb is rated 3.0V, and 600mA. Calculate its (i) power (ii) resistance (iii) energy consumed if this bulb is lighted for 4 hours.  (3Marks)
6. (i) Differentiate between AC and DC giving two points of difference. (ii) What is the frequency of AC and DC in India? (iii) Draw a graph of current – time for AC and DC. (3Marks)
7. Name and state the law which relates the potential difference in a circuit to the magnitude of current that flows through it. Give its mathematical expression and draw the theoretical graph too. (3Marks)
8. What is electromagnetic induction? State Fleming’s Right Hand Rule. A coil of copper wire is connected to a galvanometer. What would happen if a bar magnet is (a) pushed into the coil with north pole entering first? (b) pulled out of the bar magnet? (c) held stationary inside the coil?
9. State Joule’s law of heating. How does the electric fuse prevent fires due to shortcircuit or overloading? A household uses the following electric appliances. (i) Refrigerator of rating 400W for ten hours each day. (ii) two electric fans of rating 80W each for twelve hours each day. (iii) six electric tubes of rating 18W each for six hours each day. Calculate the electricity bill of the household for the month of June if the cost per unit of electric energy is Rs. 3.00. (5Marks)

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