General Science MCQs – Page 16 (Q151-160)

Alternating current, or AC, is characterized by periodically reversing direction, with the current's magnitude and polarity oscillating in a regular pattern, typically a sine wave, rather than flowing steadily in one direction. In Pakistan and most of the world, mains electricity is supplied as AC at 50 hertz, meaning the current reverses direction 100 times per second, chosen because AC voltage can be easily stepped up or down using transformers, making long-distance power transmission far more efficient than with direct current. Nikola Tesla was a key figure in developing practical AC power systems, famously competing with Thomas Edison's DC-based approach during the so-called War of Currents in the late nineteenth century.

Direct current, or DC, is characterized by flowing in only one direction, with a steady polarity that does not reverse over time, unlike alternating current which periodically switches direction. Batteries, solar cells, and fuel cells all produce DC power, which is why devices like smartphones, laptops, and electric vehicles that rely on internal batteries use DC internally, requiring an adapter to convert AC mains power into DC for charging. While AC dominates power grid transmission due to easy voltage transformation, DC is often preferred within electronic circuits and for long-distance high-voltage transmission lines, called HVDC, which can reduce certain transmission losses over very long distances.

A transformer is mainly used in electrical systems to change the voltage level of alternating current, operating on the principle of electromagnetic induction through two coils, primary and secondary, wound around a shared iron core, without any moving parts or direct electrical connection between the coils. Transformers can only work with AC, since they rely on a continuously changing magnetic field to induce voltage in the secondary coil, which is one major reason power grids use alternating rather than direct current. Power stations use step-up transformers to raise voltage for efficient long-distance transmission, and step-down transformers near homes to reduce it to safe usable levels.

A step-up transformer increases an alternating voltage by using a secondary coil with more turns of wire than the primary coil, since the voltage ratio between the coils equals the ratio of their number of turns. This higher voltage output comes with a proportionally lower current, since a transformer conserves overall power, ideally without loss. Step-up transformers are used at power stations to boost voltage to extremely high levels, sometimes hundreds of thousands of volts, before transmission over long-distance power lines, since higher voltage transmission dramatically reduces energy lost to resistive heating in the wires.

A step-down transformer decreases an alternating voltage by using a secondary coil with fewer turns of wire than the primary coil, following the same turns-ratio principle that governs all transformers. As voltage decreases, current correspondingly increases, since power is conserved between the primary and secondary sides in an ideal transformer. Step-down transformers are essential in local electrical distribution networks, reducing the extremely high transmission voltages used on long-distance power lines down to safer levels, such as the 220 to 240 volts commonly supplied to homes in Pakistan, before further reduction inside devices like phone chargers.

Magnetic field lines around a bar magnet are conventionally described as emerging from the north pole, curving through the surrounding space, and re-entering the magnet at the south pole, forming closed loops that continue inside the magnet from south back to north. This convention, established to give a consistent direction to magnetic fields, means field lines are always denser near the poles, where the magnetic force is strongest, and never actually cross one another. Iron filings sprinkled around a bar magnet visibly trace out this characteristic pattern, a classic demonstration used in physics classrooms to visualize otherwise invisible magnetic fields.

When two magnets are brought close together, like magnetic poles, north near north or south near south, repel each other, while unlike poles, north near south, attract, following the same fundamental pattern seen with electric charges, where like charges repel and opposite charges attract. This behavior arises from the interaction of the magnets' surrounding magnetic fields, and the strength of the attractive or repulsive force decreases rapidly with increasing distance between the magnets. Unlike electric charges, which can exist singly as isolated positive or negative charges, magnetic poles always occur in inseparable north-south pairs, since cutting a magnet in half simply creates two smaller magnets, each with its own north and south pole.

Earth behaves like a giant magnet because of convective currents of molten iron and nickel in its outer core, which generate a magnetic field through a process called the geodynamo, with the magnetic poles located roughly near, but not exactly at, the geographic poles. This offset, along with the fact that Earth's magnetic north pole is actually a magnetic south pole in physics terms, since it attracts the north-seeking end of a compass, are common sources of confusion in exam questions. Earth's magnetic field also slowly shifts over time and has even reversed polarity entirely multiple times throughout geological history, evidenced by magnetic patterns preserved in ancient ocean floor rock.

A magnetic compass needle generally aligns itself to point toward magnetic north, since the needle is itself a small magnet whose north-seeking pole is attracted toward Earth's magnetic north region, which is technically a magnetic south pole in physics terms, located in the Canadian Arctic and slowly drifting over time. Magnetic north differs slightly from true geographic north, a discrepancy called magnetic declination, which varies by location and must be accounted for in precise navigation, surveying, and orienteering. Compasses have been used for navigation since ancient China, and remain a reliable low-tech backup even in the age of GPS.

An electromagnet is typically created by passing electric current through a coil of wire wound around a soft iron core, since the iron core dramatically strengthens the resulting magnetic field compared to an air-core coil alone, because iron becomes strongly magnetized while the current flows. Unlike a permanent magnet, an electromagnet's magnetic field can be switched on and off simply by controlling the current, and its strength can be adjusted, making electromagnets essential in devices like electric bells, relays, MRI machines, industrial cranes for lifting scrap metal, and the electric motors and generators found throughout modern technology.