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# Monthly Archives: January 2014

## An electric dipole placed in an external electric field experiences a torque.why?

Shaji asked “An electric dipole placed in an external electric field experiences a torque.why?”

Answer:

If the axis of electric dipole makes an angle worth the electric lines of force, it will experience a torque.

What is torque?

The turning effect of a force is called torque. When an electric dipole is placed in an electric field making an angle between the axis of the dipole and the direction of electric field, each of the charges constituting the dipole experiences a force qE in the opposite directions, but not along the same line.

Such a pair of forces of equal magnitude but opposite direction not acting along the same line constitute a couple of forces and has a turning effect. (Torque)

## Motion and Newton’s Laws

This question has been extremely irksome and I wish to be answered. Please see the figure below.

When a body is placed on a table, the force of gravity is pulling it down;but it cannot move down because the table top restricts its motion. In this case the two forces which we considered here are the force of gravity acting vertically downwards on the block  and the reaction force offered by the table on the block top in the vertically upward direction along the same line. The forces are acting on the same body and cancel each other.

Here the forces are not acting at the same point, but are collinear – acting on the same line.

So, what is the misconception here?

You ( and many others) imagined that the body exerts a force on the table top, whereas it is the force of gravity acting on the body trying to bring it downwards. But, when to downward motion is restricted by the table top, the force of normal reaction comes into play.

(This is one way of explaining it. Any other answer visitors? Please post them as comment to this post)

## Current Electricity – Drift Velocity based numerical

Two different wires X and Y of same diameter but different materials are joined together in series     across a battery . If the number density of electrons in X is  twice that in Y . Find ratio of drift velocity of electrons in two wires .

Asked Kavita

Answer:

In series combination, the current is the same.

I = nAev

=> n1v1 = n2 v2

=> v1:v2 =n2:n1

## Neutrons, Neutrons, Neutrons

I understand that  in large elements which contain many protons, there exist too many neutrons to extend the strength of strong nuclear force and to overcome the electromagnetic repulsion force of protons. Such elements are stable in this way. But, I read that element with too many neutrons are also unstable. WHY ?  What I know is that too many neutrons can’t enlarge the electromagnetic repulsion force and can increase the strong nuclear force, so they even make the nucleus more stable . What’s wrong with me ? Please!

Asked a student

## Stay Fit and Stress Free During Exams

‘Sound mind in a sound body’; this is an old adage and the adage is very relevant to students who are preparing for their examination. Remember, you have put in one year of hard work for this ‘D’ day. The marks or grade you score in the examination is not just a jugglery of figures; it is a benchmark of your effort, your performance. I am telling all these not to frighten you, but only to advise you to take some simple steps so that you keep fit to effectively take on the examination and come out successfully.

Your physical fitness:

Examination is the time when you are glued to your study table for several hours. Therefore, it is the time when you also lack physical exercises. The body requires some rejuvenation. The body cells need enough blood to function properly. So, take at least about one hour of your time for exercises. Go to the nearest park or playground or to your backyard garden. Play with your brother or sister or friend in the neighbor or at least take rigorous walk so that you sweat and in turn make the body relaxed and rejuvenated.

Your food:

Keep a tab on your food; totally give up high calorie food and junk food. Instead, switch over to food containing enough protein like fruits, nuts, vegetable salad etc. Drink enough water but reduce on beverages like tea and coffee. Remember, the food you take must supply enough protein and nutrients to your body. If you want you can even consult your physician or dietician who will guide your suitably.

Your habits:

You must have at least six hours of sound sleep. Some students are in the habit of reading during night. But according to physicians it is always advisable to sleep during night because with that the body gets maximum relaxation. If you have enough sleep you will not carry over the fatigue of the previous day and you feel fully relaxed.

Keep calm:

Last minute preparation always makes you tensed. Therefore if you want to remain calm during the period of examination, the simple remedy is to start studying from the day one of your school or college days. You can also take the help of your teachers who would perhaps even provide with better tips on the method of preparing for various subjects. At the same time, you can also practice yoga and meditation. That would certainly provide you the much needed mental relaxation. Certainly your anxiety level will come down.

Take break:

For example, after every two hours of study, take a short break of about 5 to 10 minutes. Just move out of your study table and walk in your back yard garden. Some students will not agree with this method. But according to educationists, this method of taking short breaks really does wonder. You will be able to retain what all you have studied during the last two hours.

If you follow these simple suggestions certainly you will be able to take your examination ‘head on’ and you will also come out with great success.

## Define tensor quantity with example

Define tensor quantity with example … posted Mr. Sunil Kumar

Tensors are geometric objects that describe linear relations between vectors, scalars, and other tensors. Elementary examples of such relations include the dot product, the cross product, and linear maps. Vectors and scalars themselves are also tensors.

Tensors are used to represent correspondences between sets of geometric vectors.

Tensors are important in physics because they provide a concise mathematical framework for formulating and solving physics problems in areas such as elasticity, fluid mechanics, and general relativity. Tensors were first conceived by Tullio Levi-Civita and Gregorio Ricci-Curbastro

Please read the following for more information

## Planck Temperatures

I’ve read of some incredibly large temperatures that emenate from stars. Many in the millions of degrees. How accurate are these claims,how do they tell the difference between say…9 million degrees and 9.5 million ? Is is done by measuring the brightness of the source along with distance and would not the fuel itself affect the brightness? I’ve heard that the big bang generated temps in the trillion degree range. How can that be measured or even assumed to be factual. Is it guesswork or is there proof ? There seems to be a physical limit to negative temperatures yet no limit for positive temps. Is that because all atomic activity stops @ -460 f ? I wonder too why no upper limt as I would think any and all (fuel) used to heat those high temps would disappear at some point.
Noviceily yours;
Craig
When I was told to question everything, I asked why.

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