What is the source of electrical energy in an artificial satellite ?

Solar cells are the correct answer. Most artificial satellites in Earth orbit generate electricity through photovoltaic cells that convert sunlight directly into electrical energy. Solar panels are mounted on the satellite body or on deployable wings, and a rechargeable battery stores power for the period when the satellite passes through Earth's shadow.

Why the other options fail:

• Thermopile: It produces electricity from a temperature difference. It is used in radioisotope thermoelectric generators (RTGs) on deep-space probes such as Voyager, where sunlight is too weak, but it is not the normal source for an Earth-orbit satellite.
• Dynamo: A mechanical generator needs a rotor driven by an engine or turbine. A satellite has no such prime mover.
• Small nuclear reactor: Reactor-powered satellites, such as the US SNAP-10A (1965) and the Soviet RORSAT series, were experimental or specialised, not the general case.

Core concept: power sources of spacecraft, a standard Science and Technology topic. Mains angle: ISRO's use of solar arrays and batteries and the power budget trade-offs in satellite design.

Trap: Students pick thermopile because RTGs appear in popular articles. Remember that RTGs are meant for deep-space missions; for ordinary satellites, the answer is solar cells.