CBSE Class 7 Science Advanced Physics Sound Short Answer Questions Set 01

Short Answer Questions

Question 1. Describe an experiment to show that sound travels through liquids.
Answer:
1. Fill a metal container with water.
2. Hold two coins under the water and knock one against the other; a click is heard.
3. Now press your ear against the outside wall of the container and strike the coins together again.
4. The sound now heard is louder and clearer.
Conclusion: sound travels through liquids as well as air, and a liquid carries it better than air does.
In simple words: Hit two coins in water and listen from the side of the container; the sound is clear, so water carries sound.

Teacher's Note:
1. Mention the apparatus, the steps and the conclusion in that order.
2. The louder sound through the container wall shows that water and metal conduct sound well.

Question 2. What are stringed instruments? Give examples.
Answer:
Stringed instruments have stretched strings fixed on a wooden frame. The strings are plucked, struck or bowed and their vibrations give musical sound.
The pitch can be changed by changing the length, tension and thickness of the string.
Examples: veena, sitar, guitar and violin.
In simple words: Instruments in which vibrating strings make the sound.

Teacher's Note:
1. A shorter, tighter or thinner string gives a higher pitch.
2. Do not confuse them with wind or percussion instruments.

Question 3. Explain the formation of thunder.
Answer:
During lightning, about 75% of the electrical energy heats the air in and around the flash.
In a few seconds the temperature of that region rises to about 10 000 K.
The sudden heating makes the air expand violently and creates a pressure wave of compressions and rarefactions.
This wave is heard as a violent sound called thunder.
In simple words: Lightning heats the air very quickly, the air expands with a bang and we hear thunder.

Teacher's Note:
1. Link the three steps: heating, pressure wave, sound.
2. Light is seen first because it is far faster than sound.

Question 4. A thunder is heard 2 s after a flash of lightning is seen. If the velocity of sound in air is 330 m s⁻¹, how far away from you has lightning occurred?
Answer:
Light reaches us almost instantly, so the 2 s is the travel time of sound.
Distance = speed × time = 330 × 2 = 660 m.
In simple words: Multiply the speed of sound by the time gap.

Teacher's Note:
1. Ignore the travel time of light; it is extremely small.
2. Do not halve the answer; this is not an echo.

Teacher's Note:
1. The book defines them through the effect on the listener.
2. The regular and irregular vibration points are extra explanation.

Question 6. Why is the sound produced at one end of a metallic pipe heard twice at the other end?
Answer:
The sound travels along the metal pipe and also through the air inside or outside it.
Sound is faster in a solid than in air, so the sound through the metal arrives first.
The sound through the air arrives a little later, so two separate sounds are heard.
In simple words: The sound comes by two routes at two speeds.

Teacher's Note:
1. This is the rod activity from the notes.
2. The sound through air is usually louder.

Question 7. State any three uses of ultrasonic sound.
Answer:
1. In sonar to find the depth of the sea.
2. In sonography (ultrasound scanning) to see internal organs and the condition of a baby in the womb.
3. To detect cracks and flaws inside metals.
(Other uses: homogenising milk, and the Galton whistle for dogs.)
In simple words: Ultrasound is used for measuring, scanning and testing.

Teacher's Note:
1. Choose three clear uses and give one line for each.
2. Bats also use it to catch insects.

Question 8. State some sources of noise pollution.
Answer:
Television, radio and music players played loudly, vacuum cleaners, exploding crackers, announcements from loudspeakers, factory machines and vehicles are sources of noise pollution.
In simple words: Loud machines, speakers, crackers and traffic all add noise.

Teacher's Note:
1. Think of places such as roads, factories and festivals.
2. Loudness and duration decide how harmful it is.

Question 9. What are the factors which affect the loudness of sound?
Answer:
1. Amplitude: more amplitude (more energy) gives a louder sound.
2. Surface area of the vibrating body: a larger surface gives a louder sound.
3. Distance from the source: loudness decreases as we move away from the source.
In simple words: Loudness depends on amplitude, area and distance.

Teacher's Note:
1. Remember the three: amplitude, area, distance.
2. Pitch is not one of them.

Question 10. What are the factors which affect the pitch of sound?
Answer:
Pitch depends on the frequency of vibration; the higher the frequency, the higher the pitch.
The frequency of a vibrating string changes with its length, tension and thickness: a shorter, tighter or thinner string gives a higher pitch.
For an air column, a shorter column gives a higher pitch.
Pitch does not depend on amplitude or energy.
In simple words: Pitch is set by frequency, and frequency changes with length, tension and thickness.

Teacher's Note:
1. Never link pitch with loudness.
2. Violinists change pitch by pressing the string to change its length.

Question 11. Describe an experiment to prove that sound is produced by vibrating bodies.
Answer:
1. Hang a small pith ball from a stand.
2. Strike a tuning fork with a rubber hammer so that it vibrates.
3. Touch a prong of the vibrating fork to the pith ball; the ball is thrown away repeatedly.
4. Hold the fork near your ear; a note is heard.
Conclusion: the vibrating fork produces sound, so sound is produced by vibrating bodies.
In simple words: The pith ball jumps because the sounding fork is vibrating.

Teacher's Note:
1. A plucked rubber band gives the same result.
2. Say that the ball leaps away, which proves vibration.

Question 12. Give two practical applications of reflection of sound.
Answer:
1. Megaphone: the horn-shaped tube reflects the sound repeatedly and sends it out as a beam, so it travels farther.
2. Sonar: ultrasound sent from a ship is reflected from the sea bed and the depth is found from d = vt/2.
In simple words: Megaphone and sonar both use bounced sound.

Teacher's Note:
1. Echo ranging and hearing aids are other examples.
2. Name the application and say how reflection is used.

Question 13. What is the minimum distance required to hear an echo, when the speed of sound in air is 340 m s⁻¹?
Answer:
The echo should return after at least 0.1 s.
2d = 340 × 0.1 = 34 m
d = 34 ÷ 2 = 17 m
In simple words: Use d = vt/2 with t = 0.1 s.

Teacher's Note:
1. With 330 m s⁻¹ the answer is 16.5 m.
2. Half the total path gives the distance to the wall.

Question 14. Describe an experiment to prove that the pitch of the sound is dependent on the length of a vibrating air column.
Answer:
1. Fill a burette to about three-quarters with water and blow across its mouth: a high-pitched note is heard.
2. Release some water through the tap so that the air column becomes longer and blow again: the note is lower.
3. Repeat with less and less water; the pitch keeps falling.
Conclusion: as the length of the air column increases, its frequency decreases and so the pitch decreases.
In simple words: A longer air column gives a lower note.

Teacher's Note:
1. Link length, frequency and pitch in the conclusion.
2. A shorter column gives a higher pitch.