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# Monthly Archives: June 2011

## Image formed by a thick Plane Mirror

“A thick plane mirror forms a number of images of a point source of light . which image is brightest ?”

Answer: In the case of a thick plane mirror, reflection takes place at the two surfaces, (top and bottom surfaces) and images are formed due to both. The second surface is silvered and therefore the second image is the brightest. Further images are formed due to multiple reflections and due to absorption of light by medium the images become fainter.

If the front surface of the mirror is silvered, only one image will be formed.

## A few Questions from electricity, em waves, kinematics

Riya Posted:

1) A particle moves along a straight line parallel to the x-axis at a constant velocity, Its angular momentum about the origin?

2) Can a free electron moves from higher potential to low potential?

3) Progressive wave carries energy in a plane perpendicular to the plane wave front-is the statement true?

4) Why a heavier body doesn’t fall more quickly than a lighter body?

5) Is it possible to transfer electic charge from a body of a lower potential to a higher potential?

## A Problem from Projectile motion

“A ball  is projected vertically  upwards  with  an initial  velocity of 28m/s and is observed  to pass a  point  30m above  the  projection point,at what  2times does  the ball  pass these  points?”

ans : 1.356 sec and 1.444 sec .
sol:
from v=u+at we get t=2.8 . (time taken to reach the max height )
fm v2-u2=2aS we get S=39.2 m (max height reached)
observer at 30 m .
so , in btw distance btw observer and ball (at the top) =9.2 m
at top ball is at rest and time taken to reach the observer can be calculaed from S=ut + 1/2 at2 ,we get t=1.356s .
and next time can be calculated by 2.8 – 1.356 =1.444 sec .

(The answer has not been scrutinized. If any error, teachers and visitors can post as comment)

## Screw Gauge

Neeraja asks: “in screw guage the nut is a fixed one or not?”

## Motion of electron in Electric and Magnetic Fields

“Both magnetic and electric feilds are perpendicular in a region.If we place an electron at rest in that region.what will be the path followed by that electron ?”

When the electron is at rest, it is not affected by magnetic field, but due to electric field, it will begin to move in a direction opposite to the field. This initial direction determines the plane of motion. Then it is acted upon by magnetic field as the motion starts

(The answer is incomplete. I am leaving it for the visitors to think and respond)

Charge is placed at rest in crossed electric and magnetic fields
Let electric and magnetic fields are aligned along z and x directions and charge is placed at the origin of coordinate system. Initially, there is no magnetic force as charge is at rest.
However, there is electric force, which accelerates the charge in z-direction. As the particle acquires velocity in z-direction, the magnetic force comes into play and tries to rotate the particle in xz plane about a center on x-axis.
However, z-component of velocity keeps increasing with time due to electric force in that direction. The magnetic force though draws the charged particle away from z-axis along a curved path. This action of magnetic force is countered by electric force in z-direction. The velocity of charged particle ultimately reduces to zero at x-axis. This cycle repeats itself forming cycloid motion. The cycloid path is generated by a point on the circumference of a
rolling wheel.

## A Question from Errors in measurement

An experiment measures quantities a,b and c, and X is
calculated from

$\large x=\frac{ab^{2}}{c^{3}}$

if the percentage error in a,b,c are ±1%, ±5%, ±2%  respectively, the percentage error in X will be
a) 7%
b) 5%
c) 1%
d) 9%

Percentage error = 1x percentage error in a + 2 x percentage error in b + 3 x percentage error in c

= 1×1 +2×5+3×2

=1+10+6

= 17%

SO there could be a misprint in the question posted by you.

## A question from Scientific Method

Somanna posted the question:

Explain the discovery of comet halley based on-

• systemic observation
• logical reasoning
• model making
• theoretical prediction

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