Page 136 - Physics - XI

the water level to fall gradually till the intensity of the sound heard is maximum. At this position, close
the pinch cock at once and note the position of water level with the help of a set-square. Record this
reading of fi rst resonant length l when water level is falling.
1
6. Now, lower the water level in resonance tube by a few cm, close the pinch cock, and raise the reservoir
to the highest position. Now, place the vibrating tuning fork over the open end of the tube and release
the pinch cock. Allow the water in the tube to rise slowly till the intensity of sound heard is again
maximum. Note this position of water level with the help of a set-square and record this reading of fi rst
resonant length l when water level is rising.
1
7. Find the fi rst resonant length of air column (falling and rising) with the same tuning fork for two times
more and hence fi nd the mean fi rst resonant length l .
1
8. To fi nd the second resonant length l with the same tuning fork, lower the position of water level so that
2
the length of air column is increased about three times the length l . Repeat steps 4, 5, 6, and 7 to get
1
the second position of resonance with the same tuning fork. Record this length l of air column.
2
9. Now, take the second tuning fork and repeat steps 4, 5, 6, 7, and 8.
10. Note all the observations in the observation table.

Observations
Room temperature t = ______ °C
Frequency of the fi rst tuning fork = n = ______ Hz
1
Frequency of the second tuning fork = n = ______ Hz
2
Table for calculation of the resonant length l and l
1 2
Frequency of Resonance Position of water at resonance Mean resonant length
tuning fork Water level falling Water level rising of air column

(Hz) (cm) (cm) (cm)

First l =
1
n 1 Second l =
2
First l =
n 1
2 Second l =
2

Calculations
1. Calculate the velocity of sound v in air by putting the values of n , l , and l in the formula,
1 1 1 2
v = 2n (l – l ) = ______ cms = ______ ms –1
–1
1
2
1
1
2. In the same manner, calculate velocity of sound in air v by putting the values of n , l , and l in the
2
1
2
2
formula,
v = 2n (l – l ) = ______ cms = ______ ms –1
–1
2 2 2 1
Mean value of velocity of sound at room temperature,
v + v
v = 1 2 = ... ms –1
t
2
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