General Science MCQs – Page 14 (Q131-140)

Sound waves with frequencies above the upper limit of human hearing, roughly above 20,000 hertz, are called ultrasonic waves, used naturally by animals such as bats and dolphins for echolocation to navigate and hunt in darkness or murky water. In technology and medicine, ultrasonic waves are used in medical ultrasound imaging, which is safe and non-invasive because it uses sound rather than ionizing radiation, as well as in industrial applications like cleaning delicate equipment and detecting flaws in metal. This contrasts with infrasonic waves, which fall below 20 hertz, and with subsonic and supersonic, which describe speeds relative to the speed of sound rather than frequency ranges.

An echo is produced due to the reflection of sound waves off a hard, large surface such as a wall, cliff, or building, with the reflected sound reaching the listener's ear as a distinct, delayed repetition of the original sound. For a human ear to perceive an echo as separate from the original sound, the reflecting surface generally needs to be at least about 17 meters away, since the brain can only distinguish two sounds arriving less than roughly a tenth of a second apart as separate events. This same reflection principle underlies SONAR and ultrasound imaging technologies, which measure the time delay of reflected sound to calculate distance.

SONAR, an acronym for Sound Navigation and Ranging, relies on sound waves to detect underwater objects, since sound travels much farther and more reliably through water than light or radio waves, which are quickly absorbed or scattered in seawater. Active SONAR systems emit a pulse of sound and measure the time it takes for the echo to return after reflecting off an object like a submarine, seabed, or school of fish, allowing distance to be calculated from the known speed of sound in water, about 1,480 meters per second. SONAR technology, refined significantly during both World Wars, remains essential for naval navigation, fishing, and oceanographic mapping.

Sound generally travels fastest through solids, more slowly through liquids, and slowest through gases, because sound propagation depends on how tightly and rigidly particles are bonded together, allowing vibrations to transfer more efficiently between closely packed, strongly interacting molecules. In steel, sound can travel at roughly 5,000 meters per second, compared to about 1,480 meters per second in water and only about 343 meters per second in air at room temperature. This is why placing an ear against a railway track can reveal an approaching train's sound long before it becomes audible through the air. Sound cannot travel at all through a vacuum, since it requires particles to propagate.

The apparent change in frequency, and correspondingly pitch, of a wave due to relative motion between the source and the observer is called the Doppler effect, named after Austrian physicist Christian Doppler, who described it in 1842. This effect explains why an ambulance siren sounds higher-pitched as it approaches and lower-pitched as it moves away, since sound waves bunch together in front of an approaching source and stretch out behind a receding one. The Doppler effect applies to all wave types, including light, and is used by astronomers to determine whether distant galaxies are moving toward or away from Earth through redshift and blueshift, as well as in police radar speed guns and weather radar.

The SI unit of electric charge is the coulomb (C), named after French physicist Charles-Augustin de Coulomb, who formulated the inverse-square law describing the electrostatic force between charged particles. One coulomb represents the charge carried by approximately 6.24 quintillion electrons, and the coulomb is directly related to the ampere, since one ampere is defined as a flow of one coulomb of charge per second. Charge can be positive, carried by protons, or negative, carried by electrons, and the fundamental charge of a single electron is an extremely small fraction of a coulomb, roughly 1.6 times ten to the negative 19th coulombs.

The SI unit of electric potential difference, or voltage, is the volt (V), named after Italian physicist Alessandro Volta, inventor of the first true electric battery, known as the voltaic pile, in 1800. One volt represents the potential difference that drives one joule of energy per coulomb of charge through a circuit, essentially describing how much energy is available to push electric charge through a conductor. Voltage acts like electrical pressure, with higher voltage generally able to push more current through a given resistance, following Ohm's law, and common examples range from 1.5 volts in a household battery to 220 or 240 volts in mains electricity in Pakistan.

The SI unit of electrical resistance is the ohm, represented by the Greek letter omega, named after German physicist Georg Simon Ohm, who formulated Ohm's law relating voltage, current, and resistance in 1827. Resistance describes how strongly a material opposes the flow of electric current, with good conductors like copper having very low resistance and insulators like rubber having extremely high resistance. Resistance depends on a material's resistivity, its length, and its cross-sectional area, with longer, thinner wires offering more resistance than shorter, thicker ones. Resistors are components specifically designed to introduce a controlled amount of resistance into electronic circuits.

Ohm's law states that the voltage across a conductor equals the current flowing through it multiplied by its resistance, expressed as V = IR, a foundational relationship in electrical circuit analysis formulated by Georg Simon Ohm in 1827. This means that for a fixed resistance, increasing voltage proportionally increases current, while for a fixed voltage, increasing resistance proportionally decreases current. Ohm's law allows engineers to calculate any one of the three quantities, voltage, current, or resistance, if the other two are known, and it underlies the design of nearly all electrical and electronic circuits, though it applies strictly only to ohmic materials that maintain constant resistance.

Materials that allow electric current to pass through them easily are called conductors, a property arising from having many loosely bound, freely moving electrons within their atomic structure, as seen in metals like copper, silver, gold, and aluminum. Silver is technically the best electrical conductor, but copper is far more widely used in wiring due to its much lower cost combined with excellent conductivity. Conductors contrast with insulators, which strongly resist current flow, and semiconductors, such as silicon, which have conductivity between the two extremes and can be precisely controlled, forming the basis of transistors and modern electronics.