In a hydel power station, the rotational motion produced in the turbines is primarily due to which physical process acting on the flowing water?

Introduction / Context:
Hydel power stations, also called hydroelectric power plants, generate electricity using the potential and kinetic energy of water. Unlike thermal power plants, which burn fuels, hydel stations rely on the natural water cycle and gravity. The question tests whether you clearly understand what actually drives the turbines in a hydel power station. Recognising the correct physical process is important for distinguishing between different types of power generation, such as thermal, hydro, and diesel power plants, which often appear together in general science exams.

Given Data / Assumptions:

• The setup is a hydel (hydroelectric) power station.
• Water is stored at a higher level in a reservoir behind a dam.
• Turbines are located at a lower level.
• The question is about the motion of the turbines, not directly about electricity itself.

Concept / Approach:
In a hydel power station, water stored at a height has gravitational potential energy. When this water is released, it flows down through large pipes called penstocks. As it falls, its potential energy converts to kinetic energy. High speed jets of water strike the blades of turbines, causing them to rotate. This rotational mechanical energy is then converted to electrical energy by generators. No coal or diesel is burned in a pure hydroelectric plant, and steam is not the primary working fluid in this type of installation. Therefore, the correct physical process that produces turbine motion is the flow of water from a height under gravity.

Step-by-Step Solution:
Step 1: Identify that the power station mentioned is hydel, which implies the use of water and gravity.Step 2: Recall that water is stored in a reservoir at a higher level behind a dam.Step 3: When water is allowed to flow downward through penstocks, its potential energy converts into kinetic energy.Step 4: The fast flowing water jets strike turbine blades and transfer momentum, causing the turbines to rotate rapidly.Step 5: The rotating turbines drive generators, which convert mechanical energy into electrical energy, confirming that the key cause of turbine motion is the flow of water.

Verification / Alternative check:
Compare hydel plants with thermal power plants and diesel power stations. Thermal power plants burn coal to heat water, produce steam, and use that steam to rotate turbines. Diesel power plants burn diesel in an internal combustion engine that directly drives an alternator. In both cases, the working principle is based on burning fuel, not on the flow of water from a height. Since the question clearly states hydel power station, we must choose the option that involves flowing water rather than combustion, confirming that the flow of water from a height is correct.

Why Other Options Are Wrong:
Burning of coal in a furnace is typical of thermal power plants and is not a feature of pure hydroelectric stations, so option B is incorrect. Burning of diesel in an engine is associated with diesel generating sets and small diesel power stations, making option C wrong. Production of steam in a boiler is again a thermal power concept where water is converted to steam by heating, not the main mechanism in typical hydel systems, so option D is also wrong. Only option A correctly describes the process in hydel power stations.

Common Pitfalls:
Students sometimes confuse general power plant concepts and think that all large power stations use steam or burning of fuel. It is important to associate the term hydel or hydroelectric directly with flowing water. Another pitfall is to think too much about the final form of energy and ignore the immediate mechanical cause of turbine motion. Always break such questions into two parts: what is the energy source and what directly drives the turbine. In hydel plants, both answers point to water flowing from a height under the influence of gravity.

Final Answer:
Flow of water from a height