General Science MCQs – Page 15 (Q141-150)

Materials that strongly resist the flow of electric current are called insulators, a property arising from having tightly bound electrons that are not free to move and carry charge, as seen in materials like rubber, glass, plastic, and dry wood. Insulators are essential for electrical safety, used to coat wires and cables to prevent dangerous current leakage, shocks, or short circuits, and to support high-voltage power lines on utility poles. Insulators contrast with conductors, which allow current to flow freely, and semiconductors, which fall between the two and can be manipulated to conduct or insulate as needed in electronic devices like diodes and transistors.

Copper is widely used as an electrical conductor in household wiring because it combines excellent electrical conductivity, second only to silver among common metals, with relatively low cost, good ductility for drawing into wires, and strong resistance to corrosion. Aluminum is sometimes used as an alternative for large-scale power transmission lines because it is lighter and cheaper, despite having somewhat lower conductivity than copper, requiring thicker cables to carry the same current. Copper's reliability and safety record have made it the standard choice for building wiring worldwide, including throughout Pakistan's residential and commercial electrical installations.

Rubber is commonly used as an electrical insulator to cover wires because its molecular structure lacks freely moving electrons, giving it very high electrical resistance and preventing dangerous current leakage or shocks to anyone handling the wire. Modern wiring often uses PVC, polyvinyl chloride, plastic insulation instead of natural rubber, since it is cheaper, more durable, and resistant to heat and weathering, though the insulating principle remains the same. Insulation thickness and quality are critical safety factors in electrical wiring standards, since damaged or degraded insulation is a leading cause of short circuits, electrical fires, and shock hazards in homes and industry.

In a series electrical circuit, where components are connected end to end along a single path, the same value of current flows through every component, since there is only one route for charge to travel and no branching points where current could divide. This means if one component in a series circuit fails or is removed, the entire circuit is broken and no current flows at all, which is why old-style Christmas lights wired in series would all go dark if a single bulb burned out. Voltage, however, divides across the components in a series circuit, with the total voltage equal to the sum of the voltage drops across each component.

In a parallel electrical circuit, where components are connected across common points providing multiple paths for current, the voltage across each branch is generally the same, equal to the total voltage supplied by the source, since each branch connects directly across the same two points. Current, however, divides among the branches according to each branch's resistance, with lower-resistance branches carrying more current, following Ohm's law. This is why household wiring uses parallel circuits, allowing each appliance to receive full mains voltage independently, and ensuring that if one appliance or bulb fails, the others continue operating normally on their own separate branches.

An electrical fuse protects a circuit by melting and breaking the circuit when current becomes too high, since a fuse contains a thin strip of metal with a low melting point that heats up and burns through when excessive current flows, due to the resistive heating effect described by Joule's law. This interrupts the circuit before dangerously high current can overheat wires, damage appliances, or start an electrical fire. Fuses are rated by the maximum current they can safely carry before melting, and once blown, a fuse must be physically replaced, unlike a circuit breaker, which performs a similar protective function but can simply be reset and reused after tripping.

Electric power in a circuit can be calculated using the formula power equals voltage multiplied by current (P = VI), reflecting how power represents the rate at which electrical energy is delivered or consumed, measured in watts. Combined with Ohm's law, this formula can be rearranged into other useful forms, such as power equals current squared times resistance, or power equals voltage squared divided by resistance, allowing engineers to calculate power dissipation in different circuit configurations. This formula explains why household appliances are rated in watts, and why using a higher-voltage supply with the same current delivers proportionally more power.

A battery works by converting stored chemical energy into electrical energy through spontaneous oxidation-reduction, or redox, reactions occurring between two different electrode materials separated by an electrolyte, generating a flow of electrons through an external circuit. Alessandro Volta built the first true battery, the voltaic pile, in 1800 using alternating discs of zinc and copper separated by brine-soaked cloth. Modern batteries, from disposable alkaline cells to rechargeable lithium-ion batteries used in smartphones and electric vehicles, all rely on this same fundamental principle of harnessing chemical reactions to produce a continuous, usable electric current.

An electric generator mainly converts mechanical energy into electrical energy, operating on the principle of electromagnetic induction discovered by Michael Faraday in 1831, in which a changing magnetic field near a conductor induces an electric current within it. Power plants use generators driven by mechanical energy from various sources, such as spinning steam turbines heated by burning coal or nuclear fission, falling water in hydroelectric dams, or wind turning turbine blades, to rotate coils of wire within magnetic fields and produce electricity on a massive scale. This is the reverse process of an electric motor, which converts electrical energy back into mechanical motion.

An electric motor mainly converts electrical energy into mechanical energy, operating on the principle that a current-carrying conductor placed within a magnetic field experiences a force, described by the motor effect, causing it to rotate. This is essentially the reverse process of an electric generator, which converts mechanical energy into electrical energy through electromagnetic induction, and the same basic device can often function as either a motor or a generator depending on how it is used. Electric motors power countless devices, from small fans and electric toothbrushes to washing machines, elevators, and electric vehicles, making them among the most widely used machines in modern life.