Tamil Nadu
Matriculation Board
Class IX: Physics |
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Chapter 1: Measurement
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Answers
for Essay Questions:
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Ans1.
Principle: The screw gauge is based on the principle of a screw, which
states that when a screw is rotated about its axis it moves through a linear distance
proportional to the amount of rotation given to the screw. This linear distance through
which the tip of the screw moves for one complete rotation of the screw is equal to the
distance between two consecutive threads of the screw.
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Construction: The screw gauge is used to measure very small distances like
the diameter of a wire. It consists of a U-shaped steel frame. One arm of this carries a
stud called the anvil. The face of the anvil A is plane. The other end of the arm is
hollow and has a groove through which the screw S is moved by a head H. The end B of the
screw S is also plane. The screw moves inside a hollow cylinder called a sleeve, which has
a linear scale in mm marked on it. This is the pitch scale. The horizontal line drawn on
the main scale is called the reference line or the axial line. The head H carries a cap C
called the thimble to which the cylinder carrying the screw is related. The bevelled edge
of the thimble is graduated into either 50 or 100 divisions. This is called the circular
or the head scale. Some screw gauges have a ratchet that slips when the end of the screw B
is in contact with the object to be measured and prevents undue pressure on it.
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Back- lash error :-
On account of wear and tear or loose fitting there is
some space left for the play between the screw and the nut. In such instruments if the
screw is adjusted by turning it in one direction and rotated back the screw may not move
along the axis for an appreciable rotation of the head. The error due to it is called
backlash error of the screw. To reduce the error the screw must always be rotated in the
same direction for a particular set of observations.
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Ans2.
The physical balance consists of a beam supported at the center by a knife-edge
made of steel or agate. The knife-edge rests on top
of a metal rod, which is inside a pillar. The metal rod is fixed to a wooden base resting
on three leveling screws and moving up and down inside the hollow pillar. The rod can be
made to move up and down by turning a handle. There are two knife-edges pointing upwards
at the two ends of the beam and at equal distances from the central knife–edge. Two
stirrups are suspended from these two knife-edges and two scale pans of equal weights are
suspended from the stirrups. There is a pointer fixed to the center of the horizontal
beam. It moves on an ivory scale fixed at the bottom of the pillar. There are two
adjusting nuts at the ends of the horizontal beam. They are adjusted to make the pointer
swing equally on both sides from the center of the scale. A plumb line is suspended from a
rigid support at the top of the pillar. When the balance is arrested the beam rests on
metal supports. The balance is kept inside a wooden box with glass windows to protect it
from the dust and air currents.
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Precautions:
The following precautions must be
observed while taking weighings in a physical balance:
i) The leveling screws must be adjusted until the plumb
line comes exactly over the index so that the pillar is made vertical.
ii) By gently turning the handle note if the pointer
swings equally on either side of the central division of the ivory scale. If not carefully
adjust the small screw nuts at the ends of the beam.
iii) The object to be weighed should be placed in the
left hand pan and the weights in the right hand pan.
iv) The beam of the balance should be arrested before
adding weights or removing it.
v) The body should not be weighed when it is hot.
vi) Weights should be handled only with forceps.
vii) Weights should be added in descending order of
magnitude.
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Ans3. (a) The zero resting point is
first determined by noting five consecutive turning points. The body whose mass is to be
determined is kept on the left pan. The weights are added to the right pan till the
pointer oscillates equally on either side of the central scale. The resting point is
determined .If this resting point is greater than the zero resting point, a weight of 10
mg is added to the right hand pan and the resting point is determined again. Of the two
resting points determined the one closer to the zero resting point is considered and the
corresponding weight gives the mass of the body correct to a centigram.
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Ans3. (b) The physical balance measures mass of an object.
Turning point- the pointer while oscillating travels up
to a division to the left and turns to the right and goes up to a division to the right.
The extreme points on the left and right are called turning points.
Resting point: When the beam is released the pointer
swings to and fro in front of the ivory scale and finally comes to rest. The division of
the ivory scale in front of which it comes to rest is known as the resting point of the
balance.
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Ans4. Zero resting point: When the scales are empty the resting
the resting point of the balance is called zero resting point.
(a) If the zeros of the head
scale and the pitch scale coincide when the studs are brought together then there is no
zero error and the readings taken will be correct. If
the instrument has zero error zero correction has to be made to every reading taken with
it. If the zero error is positive the correction is negative and if the zero error is
negative the zero correction is positive. The zero correction should be added
algebraically to the reading
Positive zero error: If the zero of the head scale is
below the axial line by 4 divisions then the head scale reading would read 4 divisions
instead of zero. The screw gauge has positive error and any reading taken with the screw
gauge will read 4 divisions more than the actual reading. Hence 4 divisions have to be
subtracted from the final reading.
Negative zero error: If the zero of the head scale is
above the reference line by say 4 divisions then the head scale reads 96 divisions instead
of zero. This screw gauge has negative zero error. To correct this counts the number of
divisions by which the zero of the head scale deviates from the reference line. This
number multiplied by the least count gives required zero error. Thus the errors in the
above cases are + 0.04mm and –0.04mm (assuming L.C = 0.01mm) respectively.
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(b)Measurement of diameter of a wire:
a) First we
will determine the pitch of the screw gauge and calculate the least count of the
instrument.
b) Next we will find the zero error and note down the
correction.
c) Now the wire is placed in the space between the plane
surfaces .The thimble is rotated till the object is gently gripped between the two faces
and the ratchet starts slipping.
d) The pitch scale reading is noted and the division on
the head scale coinciding with the reference line is noted. The head scale reading is
corrected for zero error and the readings are tabulated as under:
| Object |
S.no |
P.S.R |
H.SC |
H.S.C
´
L.C |
TOTAL
READING
P.S.R + (H.S.C ´
L.C)
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TOTAL
READING P.S.R + (H.S.C ´
L.C ) ± Zero corrections |
| DIAMETER
OF WIRE
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