The Class 8 is an important year in a student’s life and Maharashtra State Board Science is one of the subjects that require dedication, hard work, and practice. It’s a subject where you can score well if you are well-versed with the concepts, remember the important formulas and solving methods, and have done an ample amount of practice. Worry not! Home Revise is here to make your Class 8 journey even easier. It’s essential for students to have the right study material and notes to prepare for their board examinations, and through Home Revise, you can cover all the fundamental topics in the subject and the complete Maharashtra State Board Class 8 Science Book syllabus.
Q1. Fill in the blank with appropriate word:
a. The region in a sound wave, with higher pressure and density is called …………. and that with low pressure and density is called ………..
b. Medium is ……………for generation of sound.
c. The total number of compressions and rarefactions produced per second in a sound wave is 1000. The frequency of the sound wave is …..
d. Different sound notes have different ……..
e. In loudspeaker,……..energy is converted into …………energy.
Answer a: The region in a sound wave with higher pressure and density is called compression and that with low pressure and density is called a rare fraction.
Answer b: Medium is required for generation of sound.
Answer c: The total number of compressions and rarefactions produced per second in a sound wave is 1000. The frequency of the sound wave is 500.
Answer d: Different sound notes have different frequencies.
Answer e: In a loudspeaker, electrical energy is converted into sound energy.
2. Give scientific reasons:
a. It is essential to change the tension on the vocal cords, as we produce different sound notes from our larynx.
b. Astronauts on the moon can not hear each other directly.
c. As the sound wave propagates from one place to the other in the air, the air itself is not required to move from one place to the other.
Answer a: In the humans, sound is produced in the larynx. While swallowing food, we can feel with our finger a hard bump that seems to move. This is nothing but larynx. It is at the upper end of the windpipe. Two vocal cords are stretched across the voice box or larynx in such a way that it leaves a narrow slit between them for the passage of air. When the lungs force air through the slit, the vocal cords vibrate, producing sound. Muscles attached to the vocal cords can make the cords tight or loose. When the vocal cords are tight and thin, the type or quality of voice is different.
Answer b: Two astronauts on the moon talking to each other directly, will be unable to listen to each other, even if they are very close to each other. The moon does not have an atmosphere. Since there is no medium, which is necessary for the generation and propagation of sound, between the astronauts, direct sound propagation between them is not possible. Therefore, the astronauts use some technology like the one used in our cell-phones to communicate with each other. The waves used in cell-phones do not need any medium for propagation.
Answer c: The sound wave doesn ’t move from one place to another, but it creates a disturbance in the medium particles. Suppose at first, the sound waves hit the medium particle, then that median particle hits the nearby one and then the nearby one the other. Like this the sound disturbance travels through the air with help of medium particles by creating disturbances.
Q3. How are different sound notes generated in musical instruments like guitar, which uses strings for sound generation, and flute, which uses blown air for sound generation?
Answer: Sound waves of different frequencies produce different sound notes. In the field of music, various musical instruments are used for the creation of sound notes. This includes instruments like sitar, violin, guitar, which use strings for production of sound and instruments like flute, shehnai, which use air blown into pipes for the production of sound.
In string based instruments, the frequency of vibration of the string is changed by changing the tension on the string and/or by changing the vibrating length of the string using fingers. This results in generation of different notes.
In musical instruments like flute, the holes on the flute are opened or closed to change the length of the vibrating air column in the flute. The frequency of the waves, therefore, changes and it results in the production of different notes. In a flute, different notes can be generated by changing the way of air-blowing also.
Q4. How is sound produced in a human larynx and a loudspeaker?
Answer: In the humans, sound is produced in the larynx. While swallowing food, we can feel with our finger a hard bump that seems to move. This is nothing but larynx. It is at the upper end of the windpipe. Two vocal cords are stretched across the voice box or larynx in such a way that it leaves a narrow slit between them for the passage of air. When the lungs force air through the slit, the vocal cords vibrate, producing sound. Muscles attached to the vocal cords can make the cords tight or loose. When the vocal cords are tight and thin, the type or quality of voice is different.
Sound can be produced using a loudspeaker. It consists of a permanent magnet. A coil is wound around it and if a current flows through this coil, it also generates a magnetic field. We know that if two magnets are brought near each other, they move depending on their positions. In the same way, here, depending on the magnetic field created due to the coil, the coil moves back and forth. The frequency and amplitude of the movement of the coil depend on the variables in the current flowing through the coil. As the coil moves, the conical screen of the loudspeaker, which is attached to the coil, also moves back and forth. Due to back and forth motion of the loudspeaker screen, sound waves are produced in an air. We can feel these vibrations of the loudspeaker screen, just by gently touching the screen of a loudspeaker, which is producing sound.
5. Explain the experiment, with neat diagram, to prove the following:
‘Sound needs a material medium for propagation.’
Answer: Sound needs a material medium for their propagation like solid, liquid or gas to travel because the molecules of solid, liquid and gases carry sound waves from one point to another. Sound cannot progress through the vacuum because the vacuum has no molecules which can vibrate and carry the sound waves.
Experiment:
Now think how the sound produced by drums is heard by us? How is this transferred to our ears? Let us understand this with the help of an experiment.
This still means that the sound is being produced, but now we cannot hear it. This is because sound waves always need a medium to propagate. In the bell jar, it was, hence sound waves cannot travel.
Q6. Match the following:
| Human larynx | Vibrations of metal arms |
| Loudspeaker | Vibrations in air column |
| Jal-tarang | Vibrations in vocal cords |
| Tuning fork | Vibrations in strings |
| Sitar | Vibrations of screen |
Answer:
| Human larynx | Vibrations in vocal cords |
| Loudspeaker | Vibrations in air column |
| Jal-tarang | Vibrations of screen |
| Tuning fork | Vibrations of metal arms |
| Sitar | Vibrations in strings |
Q7. What is a sound and frequency of sound?
Answer: A sound is a vibration that propagates through a medium in the form of a mechanical wave. The medium in which it propagates can either be a solid, a liquid or a gas. Sound travels faster in solids, relatively slower in liquids and slowest in gases.
The frequency is measured in Hertz. The number of rarefactions and compressions that occur per unit time is known as the frequency of a sound wave. The formula of the frequency of a wave is given as: f = 1/T
Q8. Define sound waves.
Answer: A sound wave is the pattern of disturbance caused by the energy travelling away from the source of the sound. Sound waves are longitudinal waves. This means that the propagation of vibration of particles is parallel to the direction of the energy wave propagation. When the atoms are set in vibration they move back and forth. This continuous back and forth motion results in a high-pressure and a low-pressure region in the medium. These high- pressure and low-pressure regions are termed as compressions and rarefactions, respectively. These regions are transported to the surrounding medium resulting in the sound waves to travel from one medium to another.
Q9. How is sound produced?
Answer: Sound is produced by the vibration of objects and is propagated through a medium from one location to another. The to and fro or back and forth motion of an object is termed as vibration. The motion of objects causes vibrations. In many cases, we can see the vibrations with our naked eyes. While in some cases their amplitude is so small that we cannot see them but can experience them.
Q10. Discuss how we can produce sound using a tuning fork.
Answer: A fork consists of two tines and a handle. When it is hit with a rubber hammer, the two tines start vibrating. The back and forth motion of tines produces disturbances in the surrounding air molecules. As the tines extend outward from their original position, they compress surrounding air molecules which result in the creation of high-pressure region close to the tine. When the tine moves inwards, it produces a low-pressure region near the tine. By more vibrations of time, an alternating pattern of high and low-pressure regions is created. These pressure difference regions help in the propagation of sound waves from one location to another.
Q11. An electric bell is placed inside a transparent glass jar. The bell can be turned on and off using a switch on the outside of the jar. A vacuum is created inside the jar by sucking out the air. Then the bell is rung using the switch. What will we see and hear?
Answer: In this case, we can see the bell ringing, but we cannot hear it. This is because sound needs a medium to travel and it cannot travel in a vacuum. As a result, we do not hear the bell ringing.
Q12. How is sound produced in a human ear?
Answer: A sound produced by objects like drums or alarm clocks is carried away from the object to the ears through the air which acts as the medium. The sound produced is in the form of vibration. It first enters in the ears through the funnel-shaped outer part of the ear. It moves down through a canal till the eardrum (thin membrane is stretched tightly). Eardrum plays an important role in the functioning of the ears. The vibrations produced by the objects reach till the eardrum through the air. These vibrations produced make the eardrum vibrate. And then the vibrations are passed to the inner ear through middle ear bones or ossicles. The shape of the inner ear is like a snail. The inner ear is also termed as the cochlea. There are a numerous number of tiny hair cells present in the cochlea. These hair cells convert the vibrations into electrical signals, which are then transferred to the brain through the auditory nerve. This is how humans detect sound or noise.
Q13. Explain if sound can travel in space.
Answer: A medium is essential for the propagation of sound. Sound cannot travel through a vacuum due to the fact that there are no molecules that can be compressed and expanded in space. Our voice is produced by the vibration of strings known as the vocal cords, which are inside Adam’s apple. When you make a sound, its vibration travels through the air and when it reaches your brain through your ears, it is interpreted as sound. In this case propagation of sound takes place through the air medium.
Q14. List physical factors that affect sound propagation.
Answer: Following are a few factors that affect the propagation of sound:
Atmospheric Turbulence: If the atmosphere in which the sound wave is travelling is turbulent, sound waves would scatter due to velocity fluctuations of the medium.
Wind Gradient: Sound propagating along the wind would bend downwards while sound propagating against the wind would bend upwards.
Temperature Gradient: Sound waves travel faster in a warm atmosphere near the surface of the earth. Here, there is an upward refraction of sound waves. In case of a decrease in temperature at higher altitudes, the refraction would be downwards.
Q15. Define speed of sound.
Answer: The speed of sound is defined as the dynamic propagation of sound waves. This depends on the characteristics of the medium through which the propagation takes place. Speed of sound is used for describing the speed of sound waves in an elastic medium.