Waves & Sound

Chapter- 7

Physics

1. What is wave?

Ans: The disturbance or variation that transfers energy progressively from point to point in a medium is called a wave. Wave is an oscillatory motion that transports energy from one location to another but does not move the particles in the medium permanently.

2. What is sound?

Ans: Sound is an energy. Sound is a vibration that typically propagates as an audible wave of pressure, through a transmission medium.

3. What is oscillation?

Ans: Oscillation is defined as the to and fro motion of an object from its position.

4. Show that a wave can bring the energy from one place to another places.

Ans: When a boat passes through the middle of a river, the waves created by it travel to the bank of the river. So, definitely energy is transferred through the wave. Besides, when we speak, the sound wave transfers sound energy from the speaker to the listener. From these examples, it can be said that a wave can bring energy from one place to another place.

a) A sound or sounds caused by the reflection of sound waves from a distant surface back to the listener is called echo.

b) When we hear something the feeling persist for around 0.1 s, so to hear two sounds separately, it needs separation of 0.1 s difference between two sounds. As the velocity of sound at 0⁰C is 332 ms—₁, to create the difference of 0.1 s, sound has to travel (332 x 0.1) = 33.2 meter.

So, the minimum distance of the source from the reflecting surface = (33.2/2) = 16.6 m Ans.

C. Here given,

Velocity of sound, v = 332msㅡ¹

Frequency, f = 1328

We know,

V = f λ

λ = v/f

= 332/1328

= 0.25 m Ans.

d. We know, the minimum distance required between the source and the reflecting surface to hear echo is 16.6 m if the velocity of sound is 332 ms ¯¹.

In order to hear echo after 0.3 s, let the distance required is d.

So, v = 332ms¯¹

t = 0.3 s

We know, v = 2d/t

d = vt/2

= 332 x 0.3 / 2

= 49.8 m

So, the distance Nusrat needs to move backward

= (49.8 - 16.6 ) m

= 33.2m Ans.

5. Why is sound produced in whistle?

Ans: The main reason of the production of sound is vibration when the whistle is blown, it produces vibration in the air. If the frequency of vibration is more than 20 Hz, sound can be heard.

6. “When a wave propagates, the medium does not flow. It executes simple harmonic oscillation about its position”, true or false?

Ans: True. When a boat passes through a river, the waves created by it travel to the bank of the river. But water hyacinth or floating objects of the river just vibrate remaining in the same position. From this, it can be understood that the medium does not flow when a wave propagates. It executes simple harmonic oscillation about its position.

a) Periodic motion refers to the motion of a particle in such a way that it passes through a definite point along the path of its motion in the same direction in a definite interval of time.

b) Transverse wave indicates that the direction of motion of the particles is perpendicular to the direction of propagation of the wave.

Water wave is transverse wave because the direction of motion (vibration) of the particles of water is perpendicular to the direction of propagation of the waves of water.

c) Given,

Frequency of sound , f = 1200 Hz

Temperature of air = 3o०С

Let us suppose, velocity of sound in air at a temperature of 30 is v.

We know, the velocity of sound in air at a temperature of 0०С is 332 ms¯¹ and it increases by 0.6 ms¯¹ with an increase in temperature by 1०С.

Velocity of sound in air at a temperature of 30०С

= v = 332ms¯¹ + 0.6 ms¯¹ x 30

= 350ms¯¹

We also know that, v = fλ

or, λ = v/f

= 350 ms¯¹/1200Hz

= 0.2917 m

= 29.17 cm Ans.

d) Whether it is possible to hear echo at position S depends on the minimum distance between S and the reflector. If the distance (d) exceeds 18 m, echo cannot be heard; but if it (d) ranges up to 18m, echo can be heard. We have already found out that velocity of sound in air at a temperature of 30 ०С is 350 ms¯¹.

Since an echo implies twice the distance between the source of the sound and the reflector,

v = 2d/t

2d = vt

d = vt/2

Here,

v = 350 ms¯¹

t = 0.1 s ( the minimum time for taking place of an echo)

So, d = 350 ms¯¹ x 0.1 s / 2

= 35 m /2

= 17.5 m

This distance is less than the distance between the source and the reflector. It is therefore, possible to hear echo at position S.

7. Why is sound produced in a thunderstorm?

Ans: The main reason of production of sound is vibration. During thunderstorm, collisions between clouds occur. As a result, nearby layers of air vibrate which produces sound.

8. What problem would arise for a bat if it creates infra-sound instead of ultra-sound when flying?

Ans: Firstly, bat cannot hear infra-sound. A bat would face problems in detecting objects even if it could hear infra-sound. Because, the wavelength of infra-sound is very large. As a result, sound would not be reflected from thin or tiny objects. For these reasons, a bat would face problems if it creates infra-sound instead of ultra-sound when flying.

Ans: 1. a. Longitudinal wave

2. b) solid

3. b) i, ii , iii

4. b) 13.40

5. b) 0.12 s

9. What is phase?

Ans: The overall condition of motion of a wave at any moment in time is known as its phase.

10. What is intensity of sound?

Ans: Intensity is the amount of sound energy flowing per second per unit areas perpendicular to the direction of propagation of sound waves.

Q. Hang a stone of mass 10gm from a string 1 m long. What is its time period?

Ans: T = 2 π√l∕ｇ

= 2 π√l∕9.8 S

= 2.0 S

The time period remains the same whether the mass of the stone is 10gm or not.

Q. What is equilibrium condition?

Ans: If a mass is attached to the lower end of a spring, the mass will extend the spring end. This extended length of the spring is called the equilibrium condition.

Characteristics of Waves:

i) For mechanical waves a medium is necessary. Waves can be created in the water, a wave can be transmitted in the spring, a wave can be created in a rope. The sound we hear is also a wave and its medium is air.

ii) When a wave propagates through a medium, the particles of the medium oscillate about its own position but are not displaced permanently with the wave.

iii) Energy can be transferred from one place to another through waves. The more the energy, the more the amplitude of the wave. Energy is proportional to the amplitude is doubled, energy is increased four times.

iv) Every wave has a velocity; this velocity depends on the nature of the medium.

v) Reflection or refraction occurs for waves.

11. What does velocity of sound wave depend on?

Ans: Sound wave length depends on -

I) Type of medium

ii) temperature of the medium

iii) humidity of the medium

The intensity of sound is proportional to the square of the amplitude of the wave.

Types of Waves:

12. Why do bats not fly during the daytime?

Ans: Bats can produce and hear sound of frequencies ranging from 1kHz – 100 kHz (ultrasound). They fly using the echo of sound as they cannot see; this is why bats do not during the daytime.

Electricity and Magnetism

PTK Exams

Electricity and magnetism are interrelated concepts. Wrap a wire around a nail and connect the wire to a battery. The nail will be able to pick up small metal objects because it has become an electromagnet. The moving electric charges making up the electric current cause a magnetic field. Magnetic fields are caused by moving charges or some other change in the electric forces. In a bar magnet, the electrons in the iron atoms tend to orbit in the same direction, causing net electric currents in the piece of iron. Electric currents in Earth’s molten-iron core create the Earth’s magnetic field, which is similar to the magnetic field of a giant bar magnet. Similarly, a changing magnetic field can cause an electric current. Electric generators use this principle to generate electricity. A rotating turbine in a magnetic field will be affected by the changing magnetic field and produce an electric current. Just as there are two types of electric charges (positive and negative), there are two types of magnetic poles (north and south). The difference is that the two types of charges can exist in isolation, but the north and south magnetic poles must always come in pairs. The rule that like charges repel and unlike charges attract also applies to magnetic poles.

Question

1. You have two bar magnets. The magnets are laid end to end in such a way that the two ends are attracted to each other. What will happen if you turn one magnet around but not the other?

a) The ends will still stick together because magnets always attract each other.

b) The ends will still stick together because one of the magnets is a north pole and the other is a south pole.

c) The ends will repel because you are trying to bring like poles together.

d) The ends will repel because both magnets have become either north (or south) poles.

2. What Are the Basic Types of Circuits?

For an electric current to flow, there must be a complete circuit. For a battery to light a bulb, the wires must connect from the battery to the bulb and then from the bulb to the other terminal of the battery.

This type of circuit is called a series circuit. If two bulbs are connected in series, the wire goes from the battery to one bulb, then to the other bulb, and finally back to the battery.

The other type of circuit is a parallel circuit. If the two bulbs are connected in a parallel circuit, the wire has a junction with a wire going to each bulb. After the bulbs, the wires then join again at another junction. The current is split between the two bulbs in this type of circuit.

Work and Energy

One of the most fundamental laws of science is the conservation of energy:

Energy can change into a different form, but it can be neither created nor destroyed.

It can change forms, but the total amount of energy in a closed system must remain constant.

Mechanical energy can take the form of either potential energy or kinetic energy. Potential energy is the energy that exists due to an object’s position. For example, a weight held above the ground has more gravitational potential energy than the same weight lying on the floor. Kinetic energy is the energy that an object has because it is moving. An object with either more mass or more speed will have a greater kinetic energy.

When the temperature of an object increases, the random motions of the individual atoms and molecules increase; so heat energy is just kinetic energy that is randomized at the molecular level. Energy can also be stored in either electric or magnetic fields, hence light and other forms of electromagnetic radiation are also forms of energy. The chemical energy that is released in a chemical reaction is the potential energy stored by the electrical forces within the atoms and molecules involved in the reaction.

Energy can be exchanged among any of these forms. Einstein’s famous equation E=mc2 means that energy can also take the form of mass. Mass and energy are interchangeable; the equation gives a formula for finding the amount of energy equivalent to a certain amount of mass. In nuclear reactions, energy is released because some of the mass in the atomic nuclei undergoing the reaction is converted into energy.

Question

3. A couple paragraphs up, you were holding a weight above the ground. If you let go of the weight and allow it to fall to the floor, which of the following statements best describes the energy conversions from the time the weight is overhead until just before it hits the floor?

a) Potential energy is converted to kinetic energy.

b) Kinetic energy is converted to potential energy.

c) Potential energy is converted to heat.

d) Kinetic energy is converted to heat.

4. Who is doing more work, a weightlifter holding 500 pounds perfectly motionless over his head for ten hours or a woman flicking a mosquito off her arm?

The weightlifter is certainly exerting himself more, but the woman flicking the mosquito is actually doing more work. This doesn’t make much sense until you realize that work has a specific meaning in science that is different from its everyday meaning. In science, work is defined as the force times the distance over which the force is applied. The weightlifter is exerting 500 pounds of force, but the distance is zero. Hence his total work is also zero, despite his extreme fatigue. The woman is exerting a very small force, but the mosquito moves as a result of this force. Hence her work is slightly greater than zero.

Simple machines such as levers and pulleys don’t reduce the amount of work required to move something. They reduce the amount of force required, but the force must be applied over a larger distance. For example, if you use a lever to lift a large rock, the force you need to apply on the lever is less than the weight of the rock. However, you must move the lever arm a greater distance than the rock moves. Hence, as required by the conservation of energy, the total work required to move the rock is the same. It’s only the force that is less.

Heat

Even though we use temperature to measure how hot something is, heat and temperature are not the same thing. Heat is a form of energy that is related to both the temperature and the amount of material present.

For example, a fluorescent light bulb does not feel hot to touch, but the temperature of the gas inside is extremely hot (thousands of degrees). The temperature is high because the atoms are moving very fast. The total heat energy is, however, still low because the gas is very thin and there are very few atoms at this high temperature.

The three most commonly used temperature scales are Fahrenheit, Celsius (also called centigrade), and Kelvin. The Fahrenheit scale is most common in everyday life in the United States, while many other countries use the Celsius scale. The Celsius and Kelvin scales are used in scientific work.

The Kelvin scale is based on absolute zero. Zero kelvins are the temperatures where the random molecular motions are at the lowest amount. The other scales are based on the boiling and freezing points of water.

How is Heat Transferred?

Normally, heat energy will flow from hot to cold. There are three ways to transfer heat energy: conduction, convection, and radiation. Conduction requires direct contact. The fast-moving molecules in the hot object collide with the molecules in the cold object, thereby increasing the speed of the latter. The temperature of the hot object decreases while the temperature of the cold object increases. When you burn your finger on a hot stove, the heat energy is transferred by conduction.

When you feel the warm sun on a nice day, heat energy is being transferred from the sun to you by radiation. Light, infrared, and other forms of electromagnetic radiation transfer heat energy.

A radiator with no fan heats a room by convection currents. The air just above the radiator rises as it warms up and then moves to the other side of the room and drops as it cools. The resulting convection currents transfer heat energy to the other side of the room. Convection currents in the earth’s mantle cause plate-tectonic motions. Most ocean currents and atmospheric wind patterns result from convection currents.

Light and Waves

A good start to this discussion is to ask: what creates a rainbow? White light, which is a mixture of all the different colors of light, is broken into its component colors by small water droplets in the atmosphere. The droplets are essentially acting as a prism, which will also divide light into its component colors.

Visible light is a form of electromagnetic radiation or electromagnetic waves. Other forms of electromagnetic radiation include:

radio waves

infrared light

ultraviolet light

x-rays

gamma rays

All of these forms of electromagnetic radiation are oscillating waves in the electric and magnetic fields. These fields oscillate from a negative value to zero, to a positive value, and then back through zero to negative again, and so on. Different colors of light and different forms of electromagnetic waves have different values of wavelength and frequency.

We often think of light and other electromagnetic radiation as having wavelike properties. Light also displays particle-like properties. Light comes in discrete packets called photons, which can be thought of as particles of light. When a photon of the correct amount of energy strikes an electron in an atom, the electron will absorb the energy and jump to a higher energy level. When the electron jumps back down to a lower energy level, it will emit a photon with the appropriate amount of energy.

These absorbed and emitted photons are the basis for spectroscopy as a way of finding the chemical composition of something. The absorbed and emitted photons cause spectral lines when the light passes through a prism and is broken into its component colors. Because each type of atom has its own unique set of energy levels, each type of atom has its own unique set of spectral lines. Scientists use these spectral lines to identify the chemical composition.

What Happens When Light Interacts with Matter?

Unlike most other types of waves, light and other electromagnetic waves propagate in a vacuum. When a light wave encounters some medium, several things can happen. Depending on the type of medium, the possibilities include:

Refraction

Refracted basically means bent. When light strikes a transparent medium, it can pass through the medium. However, it will not continue on its straight-line path. The light ray will travel in a straight line, make a sharp bend at the surface of the medium, and then travel in a straight line through the medium. It might be refracted again when it leaves the medium or encounters a new medium. When light is traveling through the medium, its speed is slower than its speed in a vacuum. Both the amount of slowing and the amount the light rays refracted depend on the optical properties of the medium. The light path being bent by a magnifying lens is an example of refraction.

Reflection

When light is reflected, it bounces off the surface of the medium. If we measure the angles of the incident light ray and the reflected light ray from a line perpendicular to the surface of the medium, the incident angle equals the reflected angle. Pool players use this principle when they bounce a shot off the wall of the pool table. Using a mirror to groom yourself is an example of light reflection.

Absorption

Sometimes light is absorbed when it strikes a medium. The amount of absorption usually depends on the color, or wavelength, of the light. For example, the dye in your favorite pair of blue jeans will reflect blue light and absorb other colors of light.

Scattering

When light encounters a medium consisting of a large number of small particles rather than a single large object, it will often be scattered. Essentially, individual light rays are reflected off individual particles. The effect is more random, however than normal reflection because the individual small particles will have different shapes and orientations. When you see a beautiful red sunset or Carolina blue sky, you see the effects of scattering. Particles in our atmosphere scatter blue light more effectively than red light. So when the sun is low in the sky even more blue light than normal is scattered, and the sun appears red. When the sun is overhead, blue light is scattered. Some of the blue light will be scattered a second time off a different part of the sky and into our eyes. Hence the entire sky will appear blue.

Diffraction

When a light wave encounters a very sharp boundary, it will often seem to bend around the boundary. This effect is called diffraction. Diffraction effects in sound waves are what allow us to hear a person talking when he is behind an obstacle. The sound waves diffract around the boundary of the obstacle. A diffraction grating is a flat piece of glass or plastic with hundreds to thousands of small straight grooves (like a plowed field) on the surface. The diffraction effects of these grooves depend on the wavelength. So the diffraction grating will, like a prism, split white light into its component colors and produce a groovy rainbow effect.

Sometimes more than one of these will occur. For example, a beam of light might strike a glass lens, and some of the light will be refracted while some is reflected.

Question

5. When you look at a spoon in a glass of water, the part of the spoon above the surface of the water is slightly displaced from the part below the surface of the water. This effect occurs because the light waves are

a) refracted.

b) reflected.

c) scattered.

d) diffracted.

What are Some Other Types of Waves?

Other types of waves besides electromagnetic waves must have a medium to propagate. When these waves are traveling through the medium and encounter obstacles or changes in the medium, they can display the same effects that were just described for light.

Some examples follow. Surfers ride waves on the top surface of the ocean. The wave propagates along the surface of the water; the individual water molecules travel in a small circular path. A sound wave travels through air or another medium as individual molecules in the medium oscillate. When tectonic plates slip and vibrate the earth, seismic, or earthquake waves are generated. When these seismic waves travel along the surface, they can cause damage.

When seismic waves travel through the earth’s interior, scientists can study the effects, such as refraction, that occur and learn about the interior structure of the earth. An oscillating slinky also produces waves that will be easily seen by your students.

Oscillating the slinky sideways, perpendicular to the slinky, will produce a transverse wave traveling through the slinky. Oscillating the slinky by gathering the coils together and then releasing them, parallel to the slinky, will produce a longitudinal wave traveling through the slinky.

All these types of waves propagating through matter will display the same sorts of behavior that light waves display when they interact with matter.

Waves & Sound

Chapter- 7