Physics

One Joule is equal to 10^7 ergs because 1 erg is 10^-7 Joules, so converting Joules to ergs involves multiplying by 10^7.

The strongest parts of a magnet are its poles, both the north and south poles, where the magnetic field is most concentrated. The middle or median area of the magnet has a weaker magnetic field compared to the poles.

Paul Villard discovered gamma rays in 1900 while studying radiation emitted by radium. This discovery is well-documented in physics history.

Wilhelm Conrad Roentgen discovered X-rays in 1895, making option A the correct answer. The other scientists listed are known for different discoveries.

James Prescott Joule was the scientist who demonstrated that heat is a form of energy through his experiments, establishing the principle of conservation of energy and the mechanical equivalent of heat.

One British Thermal Unit (BTU) is defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit, which is approximately equal to 1055 joules. This makes option C the correct answer.

Gamma rays have the highest penetrating power among the listed rays because they are electromagnetic waves with very high energy and no mass or charge, allowing them to pass through most materials more effectively than alpha, beta, or X-rays.

Heavy water (D2O) is commonly used as a neutron moderator in nuclear reactors because it slows down neutrons effectively without capturing them, allowing the nuclear chain reaction to be sustained.

A steradian is the SI unit used to measure solid angles, analogous to how radians measure plane angles. It quantifies the two-dimensional angle in three-dimensional space.

Black holes are regions in space where a massive object has collapsed under its own gravity to form an extremely dense point, making option C the correct description.