NCERT Summary: Summary of Physics- 3 | Science & Technology for UPSC CSE PDF Download

Sound

• Sound is a form of energy that we perceive through our sense of hearing. It is produced when objects vibrate, creating disturbances in a medium, which can be solid, liquid, or gas. Sound cannot travel through a vacuum because there is no medium for it to move through.
• The medium is essential for the propagation of sound. In a longitudinal wave, like sound, the particles of the medium move in the same direction as the wave is traveling. They do not relocate; instead, they vibrate back and forth around their resting position. This movement is how sound waves travel through the medium.
• Sound travels in the form of compressions and rarefactions. Compressions are areas where the particles are closer together, while rarefactions are areas where the particles are farther apart. It is important to note that it is the energy of the sound that moves through the medium, not the particles themselves.
• There are different types of waves, such as transverse waves, where the particles move up and down, perpendicular to the direction of the wave. An example of a transverse wave is light, but unlike sound, light does not require the oscillation of medium particles to travel.
• Vibration, also known as oscillatory motion, is the back-and-forth movement of an object, and it is the basis for sound production.
• Amplitude and frequency are crucial properties of sound. The amplitude refers to the maximum displacement of particles from their resting position and determines the loudness of the sound. A larger amplitude means a louder sound.
• The wavelength (λ) is the distance between two consecutive compressions or rarefactions. The time period (T) is the time taken for one complete oscillation of density or pressure, and the frequency (f) is the number of complete oscillations per unit time, measured in hertz (Hz). Frequency can be calculated as f = 1/T.
• The frequency of a sound also affects its pitch. A higher frequency results in a higher pitch and a shriller sound, while a lower frequency produces a lower pitch.

Sound and Its Properties

• Sound travels through a medium at a specific speed, which is influenced by the medium's temperature and pressure. The speed of sound decreases from solids to gases due to the lower density and elasticity of gases.
• In any medium, increasing the temperature results in a higher speed of sound. For instance, the speed of sound in air at 0°C is about 332 meters per second.
• The speed of sound in a gas is inversely related to the square root of the gas's density.
• The law of reflection states that the angles at which sound strikes and reflects from a surface are equal, and all lie in the same plane.
• When we shout or clap near a suitable reflecting surface, such as a tall building or mountain, we hear the sound again shortly after, which is known as an echo.
• The sensation of sound remains in our brain for about 0.1 seconds. To perceive a clear echo, there must be at least a 0.1-second interval between the original sound and the reflected sound.
• If the speed of sound is taken as 344 m/s at, say, 22°C in air, the sound must travel to the reflecting surface and back to be heard as an echo within 0.1 s. Therefore, the total distance covered should be at least 34.4 meters. Thus, the minimum distance for hearing distinct echoes is 17.2 meters, which varies with air temperature.
• Multiple reflections can cause echoes to be heard more than once. Reverberation refers to the prolongation of sound due to successive reflections from nearby objects.
• A stethoscope is a medical device used to listen to sounds from inside the body, primarily the heart or lungs. In stethoscopes, the patient's heartbeat reaches the doctor's ears through multiple sound reflections.
• The human audible range is from about 20 Hz to 20,000 Hz (where 1 Hz equals one cycle per second). Children under five and some animals, like dogs, can hear frequencies up to 25 kHz (where 1 kHz equals 1000 Hz).
• Frequencies below 20 Hz are called infrasonic sounds. For example, rhinoceroses communicate using infrasonic sounds as low as 5 Hz. Whales and elephants also produce sounds in this range.
• Some animals seem to sense impending earthquakes, as they can detect low-frequency infrasound before the main shock waves, which may alert them.
• Frequencies above 20 kHz are termed ultrasonic sounds or ultrasound. Dolphins, bats, and porpoises generate ultrasound.
• Ultrasound can be used to identify cracks and flaws in metal blocks. These hidden defects can weaken structures like buildings and bridges.
• Ultrasonic waves are passed through metal blocks, and detectors identify the reflected waves. Any small defect causes the ultrasound to reflect back, indicating a flaw.
• In medical imaging, ultrasonic waves reflect from different parts of the heart to create an image, known as echocardiography.
• An ultrasound scanner uses ultrasonic waves to capture images of internal organs. This helps doctors identify issues like gallstones or tumours.
• Ultrasonic waves travel through body tissues and reflect from areas with different densities. These reflections convert into electrical signals to create organ images displayed on a monitor or printed on film, a process called ultrasonography.
• SONAR stands for Sound Navigation And Ranging. It is a device that uses ultrasonic waves to measure the distance, direction, and speed of underwater objects.
• SONAR consists of a transmitter and a detector, typically installed on boats or ships. The transmitter emits ultrasonic waves that travel through water, reflecting off seabed objects and returning to the detector.
• The detector translates the reflected ultrasonic waves into electrical signals for analysis. The distance to the object is calculated using the speed of sound in water and the timing of the ultrasound transmission and reception.
• The method of determining distance by measuring time intervals is called echo ranging. This technique is vital for finding underwater features, such as hills, valleys, submarines, icebergs, and wrecked vessels.
• If an object, especially an aircraft, travels faster than the speed of sound in air, it is said to have supersonic speed. The ratio of an object's speed to the speed of sound in air is called its Mach number.
• A Mach number greater than 1 indicates that the object is moving at supersonic speed.

Units and Measurement

Physics is a quantitative science that relies on measuring physical quantities. Some physical quantities are designated as fundamental or base quantities, including length, mass, time, electric current, thermodynamic temperature, amount of substance, and luminous intensity.

SI Units and Base Quantities

A unit is a specific standard used to define each base quantity. Some examples of units include the metre, kilogram, second, ampere, kelvin, mole, and candela. The quantities measured with these units are called fundamental units.

Derived units are used to express other physical quantities that come from base quantities. These derived units are combinations of base units. Together, fundamental and derived units create a system of units.

The most widely used system is the International System of Units (SI), which is based on seven base units. SI units are used for all physical measurements, covering both base and derived quantities. Some derived units, like joule, newton, and watt, have special names.

SI units have specific symbols, such as m for metre, kg for kilogram, s for second, and A for ampere. Measurements are often expressed using scientific notation with powers of 10, making it easier to represent and calculate numbers while showing their precision.

1. Unit of Length

• The SI unit of length is the metre. m ). Other metric units for measuring length are based on the metre using multiples or submultiples of 10:
• 1 kilometre = 1000 (or 10³) m
• 1 centimetre = 1/100 (or 10^-2) m
• 1 millimetre = 1/1000 (or 10^-3) m

Very Small Distances

• Micrometre or Microns. μm ): 1 m = 10⁶ μm
• Nanometres. nm ): 1 m = 10⁹ nm
• Angstroms. Å ): 1 m = 10¹⁰ Å
• Femtometres. fm ): 1 m = 10¹⁵ fm

Large Distances

• Light Year. 1 light year = 9.46 × 10¹⁵ m (distance light travels in a year)

2. Unit of Mass

• The SI unit of mass is the kilogram. kg ). Other metric units for measuring mass are related to the kilogram by multiples or submultiples of 10:
• 1 tonne (t) = 1000 (or 10³) kg
• 1 gram (g) = 1/1000 (or 10^-3) kg
• 1 milligram (mg) = 10^-6 kg

3. Unit of Time

The SI unit of time is the second. s ).

SI Base Quantities and Units

Important Units of Measurement

• Nautical Mile. A nautical mile is equivalent to 1852 meters (or 6080 feet). This unit was based on the idea of one minute of arc along a great circle of the Earth, which is 1/60 of 1/360 of the Earth's circumference.
• Every sixty nautical miles corresponds to one degree of latitude anywhere on Earth, and also one degree of longitude at the equator. This is why it is called a nautical mile, as it is particularly useful for navigation at sea.
• The nautical mile is still commonly used in fields like shipping, aviation, and aerospace. In contrast, the ordinary mile, known as the statute mile, is defined by law.
• In discussions about distances in near outer space, measurements are sometimes made in relation to the radius of the Earth, which is about 6.4 × 10⁶  meters. For example:
• The planet Mars has a radius that is half that of the Earth.
• A geosynchronous orbit around the Earth is approximately 6.5 times the radius of the Earth.
• The distance from the Earth to the Moon is roughly 60 times the radius of the Earth.
• Astronomical Unit (AU). The average distance from the Earth to the Sun is called an astronomical unit, which is about 1.5 × 10¹¹  meters. Distances within the solar system are often expressed in astronomical units. For instance:
• The distance from the Sun to Mars is about 1.5 AU.
• The distance from the Sun to Jupiter is approximately 5.2 AU.
• The distance from the Sun to Pluto is about 40 AU.
• The star that is closest to the Sun, Proxima Centauri, is located approximately 270,000 AU away from Earth.

Waves

• Mechanical Waves require a medium to travel through. These waves can be further classified into:
• Transverse Waves: In these waves, the medium moves perpendicular to the direction of the wave. For example, when you drop a stone in a pond, the ripples that form are transverse waves.
• Longitudinal Waves:Here, the medium moves parallel to the direction of the wave. An example of a longitudinal wave is a sound wave, where the air particles vibrate back and forth in the same direction as the wave.
• Surface Waves: These waves are a combination of transverse and longitudinal waves. An example is the waves you see on the surface of the ocean, where water moves both up and down and back and forth.
• Electromagnetic Waves: Unlike mechanical waves, electromagnetic waves do not require a medium to travel. Examples include light waves, radio waves, and microwaves. These waves can travel through a vacuum, such as space.
• Matter Waves:These waves are associated with particles of matter, such as electrons and atoms. Matter waves are a fundamental concept in quantum mechanics, where particles exhibit wave-like behavior. For instance, an electron in an atom can be described as a matter wave, with a specific wavelength and frequency.
• Crest and Trough:In a wave, the crest is the point of maximum positive displacement, while the trough is the point of maximum negative displacement. The distance between two consecutive crests or troughs is called the wavelength.
• Period and Frequency:The period (T) is the shortest time for a point on a transverse wave to return to its starting position. Frequency (f) is the number of vibrations per second, measured in hertz (Hz). The relationship between frequency and period is given by the formula: f = 1 / T.
• Wavelength (λ): Wavelength is the shortest distance between peaks (highest points) and troughs (lowest points) of a wave. It is an important parameter in determining the properties of a wave.
• Wave Speed (v):The speed of a wave can be calculated using the formula: v = λ f. The speed of a wave depends on the properties of the medium through which it is traveling. For example, sound waves travel faster in water than in air due to the differences in density and elasticity of the two mediums. Increasing the wavelength does not increase the wave speed; rather, it decreases the number of vibrations per second. For instance, in a string instrument, plucking the strings harder increases the amplitude but does not affect the speed of the wave traveling through the string.
• Amplitude: Amplitude measures the energy transferred by the wave and represents the distance from a crest to the equilibrium position. A larger amplitude indicates more energy being transferred. For example, in a sound wave, a louder sound corresponds to a higher amplitude of the sound wave.

Work, Power, and Energy

• Work is a measure of the energy transfer that occurs when a force causes an object to move. It is calculated by multiplying the force applied to an object by the distance it moves in the direction of the force. The unit of work is the joule, which is defined as one newton of force applied over one meter of distance. If there is no movement or displacement, then no work is done. For example, if you push against a wall and it doesn’t move, you are not doing any work even though you are exerting a force.
• Power is the rate at which work is done. It is calculated by dividing the work done by the time taken to do it. The SI unit of power is the watt, which is equivalent to one joule of work done per second. Power can also be expressed in terms of horsepower, which is a unit commonly used to measure the power of engines and motors. One horsepower is equal to 550 foot-pounds of work done per second or 33,000 foot-pounds per minute.
• An object that has the capacity to do work possesses energy. Energy is measured in the same units as work, such as joules. There are different types of energy, including kinetic energy, potential energy, thermal energy, and chemical energy. Kinetic energy is the energy of an object in motion and is calculated using the formula (1/2) mv², where m is the mass of the object and v is its velocity. Potential energy, on the other hand, is the energy stored in an object due to its position or shape. For example, gravitational potential energy is calculated using the formula mgh, where m is the mass of the object, g is the acceleration due to gravity, and h is the height above the ground.
• The law of conservation of energy states that energy can be transformed from one form to another, but it cannot be created or destroyed. The total amount of energy in a closed system remains constant. For instance, when you rub your hands together, the kinetic energy of your hands is transformed into thermal energy, which raises the temperature of your hands. The sum of kinetic and potential energies of an object is called mechanical energy. For example, a swinging pendulum has both kinetic energy (due to its motion) and potential energy (due to its height above the ground).
• Pressure is defined as the force exerted per unit area. It is calculated using the formula: Pressure = force/area. The SI unit of pressure is newton per meter squared, which is also known as a Pascal. Pressure can be increased by applying the same force over a smaller area. This is why nails have pointed tips and knives have sharp edges, as they concentrate the force over a smaller area to increase pressure and make it easier to penetrate materials.
• Fluids are substances that can flow, including liquids and gases. Solids exert pressure due to their weight, while fluids exert pressure in all directions. When an object is immersed in a fluid, it experiences a buoyant force equal to the weight of the fluid displaced. This is why objects with a lower density than the fluid, such as ice in water, float, while denser objects, such as rocks, sink.
• Archimedes’ Principle states that a body immersed in a fluid experiences an upward force equal to the weight of the fluid displaced. This principle is essential in designing ships and submarines, as it determines their buoyancy and stability in water. For example, a ship floats because the weight of the water it displaces is equal to the weight of the ship, creating an upward buoyant force that keeps it afloat.
• Density is the mass of a substance per unit volume, measured in kilograms per meter cubed. It is calculated using the formula: Density = mass/volume. Relative density compares the density of a substance to that of water, which has a density of 1000 kg/m³. Relative density is calculated using the formula: Relative density = Density of a substance / Density of water. For example, if the density of a substance is 800 kg/m³, its relative density is 0.8, meaning it is less dense than water and will float.