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# Monthly Archives: July 2011

## A Problem from kinematics

The magnitude of the velocity of a particle changes uniformly from 14 m/s downward at point A to 4 m/s at point B. As the particle moves alongthe path from A to B determine the acceleration of the partice when it is at point C.

## Download CBSE Class IX and X Science Sample Question Paper for Term I (SA1)

CBSE has published sample papers in all subjects for Class IX and X. You can download Science Sample Papers for Class IX and X here. The question paper contains BluPrint and Marking scheme too. Practising this with a view to score maximum marks as per the marking scheme will be a great help for the students to perform well in exams.

CBSE Class X Science Sample Paper

CBSE Class IX Science Sample Paper

## Explanation For Empirical Horopter: Density Of Photoreceptors

Karolis  asks: ” External SiteLink -In this website (and in many more) in page 5, it’s written that photoreceptors are more densely packed in nasal areas of retina than temporal. That’s one of the explanations why empirical horopter (that’s the unit of all points in visual field, that are seen in the same position monocularly) deviates from theoretical. In page 6, in graphs we see that the same segment in visual field has longer arc in nasal retina than temporal (binocular vision). I don’t see how these two things agree: density of photoreceptors in nasal and temporal retinas and the magnitude of arcs. If compensation would be the case, then I think it should be the reverse situation. Given there are less amount of photoreceptors in temporal area, arc should be longer than that of nasal – so that the quantity of photoreceptors in both areas would be the same.
What’s wrong with my thinking?”

## A body covers a distance of 20 m in 7th sec and 24 m in the 9th sec. How much shall it cover in 15s ?

“A body covers a distance of 20 m in 7th s and 24 m in the 9th s. How much shall it cover in 15s ?”  – Raghav posted this question.

Answer: From the eqn for displacement in the nth second,

$S_{n}=u + a\left ( n-\frac{1}{2} \right )$

For the displacement in the 7th sec, we get,

$20= u+a(7-\frac{1}{2})=u+a(\frac{13}{2})$ …………………….. (1)

For the displacement in the 9th sec,

$24= u+a(9-\frac{1}{2})=u+a(\frac{17}{2})$ ……………………..(2)

(2)-(1) gives

24-20 = 2a

or a = 2 m/s2

Substituting for a in (1),

u = 7 m/s

Substituting, u= 7 m/s, a = 2 m/s2 and t = 15 sec in the eqn,
$S = ut + \frac{1}{2} a t^{2}$

we get S= 7 x 15 + 0.5 x 2x 15 x 15 = 105+225=330m

## Problem based on electromagnetic induction – motional emf

A horizontal straight wire 10 m long extending from east to west is falling with a speed of 5.0 m s–1, at right angles to the horizontal component of the earth’s magnetic field, 0.30 × 10–4 Wb m–2.
(a) What is the instantaneous value of the emf induced in the wire?
(b) What is the direction of the emf?
(c) Which end of the wire is at the higher electrical potential?

## A Few Questions from Current Electricity.

1.Why is copper wire more preferable than silver wire?
2.Why should the melting point of a fuse wire be low?
3.”The resistivity of a wire 1 ohm meter.”Explain.
4.What is the resistivity of(a)copper(b)nichrome?
5.What will happen if nichrome wire is used in transmission lines?

## Possibility of a nuclear fusion reactor

JED asks: ” Why hasn’t anyone built a nuclear fusion reactor using polodial (?) magnetic fields?

When 2 parallel wires have current running in the same direction, they attract each other

When those same 2 wires have current running in opposite directions, they repel each other

Why hasnt anyone built one where positively ionised deuterium is forced through to doughnut shaped apparatus that contains ionised hydrogen travelling in the opposite direction

This should create a strong repulsion between the ionised deuterium in the centre and the ionised hydrogen in the containment field apparatus (like a coiled up phone cord made into a doughnut shape)

With enough hydrogen running through the containment field apparatus at a high enough speed (relativistic most likely), there would be sufficient force compressing the ions to cause them to fuse, if the fuel was forced through at a rate of 0.8 – 1 gram per second with an average ion speed between 200,000,000 m/s and 300,000,000 m/s

I know about Zpinch instability, but the way the reactor is designed (it also needs a powerful current travelling in the same direction as the fuel, this will induce it to accelerate more, and will allow energy extraction from the fused ions passing out of the pinch point

The size of the reactor depends on the difficulty of containing hydrogen ions moving at relativistic speeds

It is meant to bypass the disadvantages of a Z-pinch reactor (by having a containment field generated by heavy, moving ions, through a path of no resistance

the reactor vessel would have to be a tube, as small as possible while still able to keep a vacuum and keep the fuel path clear, the containment field apparatus would also be made of tubes, designed to contain the ions and direct them around the pinch point with the highest possible current

my main question is
“would it work?(pretty much the same as “why hasn’t anyone built one yet?”)” and
“will you give me credit for design/concept if you decide to build it?” (not so much for the money for the actual reactor, but the scientific recognition so I can test out my other ideas and be taken seriously)