(A) Write a note on data storage in a 'USB'. How is it different from the Hard disk? (B) Describe how a 'Solar Cell' converts light into electricity. Describe its construction, operation, and applications. (C) Write a comprehensive note on the 'Food cycle in our body'. (D) What is 'Artificial Intelligence'? How is it helpful for humanity?

(A) Data storage in a USB drive

A USB flash drive stores data in NAND flash memory, a non-volatile semiconductor memory that retains data without power. It has no moving parts. Its main parts are the memory chip, a controller, which manages reading and writing, and the USB connector, which provides data transfer and power.

Storage: the memory consists of millions of floating-gate transistors, called cells. Each cell stores data as trapped electric charge, which represents bits. Cells are organised in pages and blocks; data are written in pages, but must be erased in whole blocks. Each cell survives only a limited number of write cycles, so the controller spreads writing across cells, a process known as wear levelling.

USB drive and hard disk

Technology: a USB drive stores data electrically in flash memory. A hard disk stores data magnetically on spinning platters. Moving parts: a USB drive has none, while a hard disk has platters, heads and motors. Durability: USB drives resist shock better, whereas hard disks are more vulnerable to being dropped. Capacity and cost: hard disks generally offer more capacity and lower cost per gigabyte. Size: USB drives are small and portable, whereas hard disks are larger. Use: USB drives are suited to transferring files, and hard disks to mass storage. USB speed varies greatly by model, and a USB drive's flash cells also wear with repeated writing.

(B) Solar cell

A solar (photovoltaic) cell converts sunlight directly into electricity through the photovoltaic effect.

Construction: it is usually made from silicon, with a thin n-type layer (doped with phosphorus) placed over a p-type layer (doped with boron), forming a p-n junction. An anti-reflective coating and a protective glass layer cover the top, with metal contacts on the front and back. Cells are joined in modules, or panels.

Operation: photons of light striking the cell are absorbed by the silicon, freeing electrons and leaving holes. The built-in electric field at the p-n junction drives electrons towards the n-side and holes towards the p-side. Connecting an external circuit lets the electrons flow through it, which produces direct current (DC). An inverter converts this to AC for household use.

Applications: rooftop and utility-scale power, such as the Quaid-e-Azam Solar Park; calculators and street lights; water pumps and tube wells; remote telecom towers; satellites; and off-grid electricity in rural areas.

(C) Food cycle in our body

The path of food through the body consists of a series of stages by which it is converted into energy and body-building material, and its waste is removed.

Ingestion: food enters the mouth, and teeth and saliva begin breaking it down. Salivary amylase starts to digest starch. Swallowing: food passes down the oesophagus by muscular waves known as peristalsis. Digestion in the stomach: acid and pepsin begin protein digestion, and food is churned into chyme. Digestion in the small intestine: bile from the liver and enzymes from the pancreas and intestine complete digestion into simple sugars, amino acids and fatty acids. Absorption: nutrients pass through the villi of the small intestine into blood and lymph. Assimilation: the liver processes nutrients, and body cells use them for energy, growth and repair. Glucose is used in cellular respiration to produce ATP, and excess energy is stored as glycogen and fat. Egestion: the large intestine absorbs water, and undigested matter leaves as faeces. Excretion: metabolic wastes are removed by the kidneys, lungs and skin.

(D) Artificial intelligence

Artificial intelligence (AI) is the branch of computer science concerned with building systems able to perform tasks that normally require human intelligence, such as learning, reasoning, understanding language and recognising images.

How it helps humanity

Health: aids in diagnosis from scans, drug discovery and treatment planning. Education: provides personalised tutoring and translation. Agriculture: supports monitoring of crops, pests and water use. Climate and disasters: improves forecasts, flood warnings and analysis of satellite images. Accessibility: assists people with disabilities through speech and vision tools. Economy and safety: automates repetitive and dangerous work, and detects fraud.

These benefits depend on responsible use, with safeguards against bias, privacy loss, misinformation and job displacement.