Emerging Technologies: Energy and mobility

In an intrinsic semiconductor, heat breaks more covalent bonds and lifts more electrons across the gap, creating electron-hole pairs. The rise in the number of charge carriers outweighs the slight fall in their mobility, so the conductivity rises and the resistivity falls: a semiconductor has a negative temperature coefficient of resistance. In metals the carrier number is fixed and stronger lattice vibration raises resistivity.

The answer is the second option. A conventional AGV follows a predetermined path marked by wires buried in the floor, magnetic tape, reflectors or other external guidance; the International Federation of Robotics, citing ISO 8373, describes it as a mobile platform following a path indicated by markers or external guidance commands. It is reliable on fixed routes, but if a pallet or a person blocks the path, it simply stops and waits. An AMR carries its own sensors, such as cameras and laser scanners, builds or uses a map of the building and localises itself with techniques such as SLAM; it can then plan a route to any destination and re-plan on the fly to go around obstacles. This flexibility lets AMRs work in changing layouts and mixed traffic with people, which is why they are spreading quickly in e-commerce warehouses and hospitals.

Both statements are correct and Statement-II explains Statement-I. The best lithium batteries hold roughly 0.15 to 0.3 kWh per kg, while petrol or kerosene holds about 12 kWh per kg in chemical energy; even after allowing for the low efficiency of engines, fuel carries several times more usable energy per kilogram. A multirotor also spends power continuously just to stay up. Together, the low specific energy of batteries and the cost of hovering limit most small electric multirotors to roughly 20 to 45 minutes. That is why long- endurance military UAVs use piston, turboprop or jet engines, and why hydrogen fuel cells are being tried for longer civil flights.

All three statements are correct. NCERT classifies solids by the energy gap (Eg): in insulators Eg is large, greater than about 3 eV, so almost no electron can reach the conduction band at room temperature; in metals the bands overlap or the conduction band is partly filled, so free electrons are always available; semiconductors have a small but finite gap. The gap of silicon is about 1.1 eV and that of germanium about 0.7 eV, so silicon's gap is larger.

Statements 1 and 3 are correct. Phosphorus is pentavalent: four of its electrons bond with silicon and the fifth becomes a free electron, so it is a donor and the material is n- type. Boron, aluminium or indium (trivalent) create holes and give p-type material, where holes are the majority carriers. Statement 2 is wrong: every free electron comes from a neutral donor atom that is left as a fixed positive ion, so the doped crystal stays electrically neutral.

Statements 1 and 2 are correct. A multirotor must hold its whole weight up with rotor thrust all the time, which costs a lot of power; a fixed-wing UAV gets most of its lift from air flowing over the wing, so the same energy gives much longer endurance and range. Hybrid VTOL aircraft (for example 'quadplanes' and tilt-rotors) take off and land vertically like a multirotor and then cruise on a wing. Statement 3 is wrong: in a quadcopter two rotors turn clockwise and two counter-clockwise, so that their reaction torques cancel; if all turned the same way, the frame would spin.

Both statements are correct and II explains I. The number of electrons that heat can lift across the gap falls steeply (exponentially) as the gap widens. With a gap of about 0.7 eV against silicon's 1.1 eV, germanium has an intrinsic carrier concentration more than a thousand times higher at 300 K. This is why germanium devices leak more current and are more temperature-sensitive, and why silicon became the standard material for chips.

In the Hall effect, a magnetic field perpendicular to the current pushes the moving carriers to one side of the sample. Electrons and holes moving in the same current direction are pushed to the same side, so the sign of the Hall voltage shows whether the side collects negative or positive charge, that is, whether the sample is n-type or p-type. The size of the voltage also gives the carrier density. Band gap and temperature behaviour are similar for both types.