Physics Notes for UPSC CSE Prelims
A seismograph,
developed in 1890, is an instrument used to detect and record earthquakes. It comprises a mass affixed to a stationary base, with the base shifting during seismic activity while the mass remains still. This relative motion is typically converted into an electrical voltage, which is then recorded on various mediums such as paper or magnetic tape. The seismic record can be translated mathematically to represent the absolute ground motion. Earthquakes, stemming from sudden shifts in the earth's crust, generate seismic waves. They occur naturally or due to human activities, with tectonic earthquakes arising from rock displacement along fault planes, and volcanic earthquakes stemming from violent volcanic eruptions. Richter scale used to measure the magnitude of an earthquake. National Earthquake Science Data Centre is located in New Delhi.
Polyvinyl Chloride (PVC)
Electric wires are indeed coated with polyvinyl chloride (PVC), which serves several essential purposes.
- Conductivity and Protection: Electric wires are typically made of materials like copper, which is an excellent conductor of electricity. However, bare copper wires cannot be safely used within a system. The PVC coating provides insulation, protecting the wires from external factors and preventing electrical shorts or shocks.
- Enhanced Productivity: The PVC coating enhances the overall productivity of the wires. It ensures that the electrical current flows smoothly without interference, reducing energy loss and maintaining efficiency.
- Safety for Users: PVC-coated wires protect users from accidental shocks. When wires are color-coded, it becomes easier for electricians and users to identify their purpose (e.g., live, neutral, ground) within a circuit.
- Why PVC?: PVC (polyvinyl chloride) is chosen as the coating material because it is:
- Insulating: PVC effectively insulates against electrical current.
- Durable: It withstands wear, tear, and environmental conditions.
- Cost-Effective: PVC is affordable and widely available.
- Flame-Resistant: It has fire-retardant properties.
- Flexible: PVC-coated wires can be bent and routed as needed.
Power of a Lens
The power of a lens is expressed in terms of diopters (D). It signifies the lens’s ability to either converge or diverge light rays passing through it.
Concave Lenses: A convex lens is thicker at the center and thinner at the edges. Its power is greater when light rays converge more strongly toward its optical center.
The power of a convex lens is positive because its focal length is positive.
Dependence on Focal Length: The lens power depends on its focal length. Specifically, the power (P) is defined as the reciprocal of the focal length (f) in meters: [ P = {1}/{f} ]
The SI unit of lens power is the diopter (D). One diopter corresponds to a lens with a focal length of one meter.
Concave Lenses: A concave lens is thinner at the center and thicker at the edges. Its focal length is negative, resulting in a negative power.
Calorific Value (CV) of a Fuel:
The calorific value of a fuel represents the amount of heat produced when a specific quantity of that fuel is completely burned. It provides crucial information about the energy content of various fuels. The unit of calorific value is typically expressed in kilojoules per kilogram (kJ/kg).
Calculation and Conditions: Calorific value is determined by maintaining constant pressure and operating under normal temperature conditions. When we burn one gram of a fuel completely, the heat generated is measured, and this value is used to calculate the calorific value.
Significance: Understanding the calorific value helps us assess the efficiency of different fuels for various applications. It guides decisions related to fuel selection, combustion processes, and energy production.
Let’s consider wood as an example. If burning one kilogram of wood produces 17 kilojoules of heat, then the calorific value of wood is 17 kJ/kg.
Cathode rays, vacuum tubes, and some fundamental particles:
Cathode Rays:
Cathode rays are beams of electrons generated within a vacuum tube.
These rays travel from the negatively charged electrode (cathode) to the positively charged electrode (anode).
The voltage difference between the electrodes propels the electrons.
Vacuum Tubes:
A vacuum tube is a device containing sealed electrodes within a glass or metal-ceramic enclosure.
It plays a crucial role in electronic circuits by controlling the flow of electrons.
Vacuum tubes are also known as electron tubes.
Examples of Vacuum Tubes:
Cathode-ray tubes (used in old TVs and monitors), phototubes, magnetrons, klystrons, gyrotrons, and fluorescent lamps are all examples of vacuum tubes.
Some fundamental particles:
Electron:
- Discovered by J. J. Thomson in 1897.
- Charge: -1.6 × 10⁻¹⁹ C.
- Mass: 9.1 × 10⁻³¹ kg.
Proton:
- Discovered by Eugen Goldstein in 1917.
- Charge: +1.8 × 10⁻¹⁹ C.
- Mass: 1.67 × 10⁻²⁷ kg.
Neutron:
- Discovered by James Chadwick in 1932.
- No charge (neutral).
- Mass: 1.67 × 10⁻²⁷ kg.
Positron:
- Discovered by Carl D. Anderson in 1932.
- Charge: +1.6 × 10⁻¹⁹ C.
- Mass: 9.1 × 10⁻³¹ kg.
International System of Units (SI):
Nuclear reactions
- Nuclear fission is a process in which the nucleus of an atom (usually uranium or plutonium) splits into two smaller nuclei.
- By-Products:
- Free Neutrons: These are released during fission and play a crucial role in sustaining the chain reaction.
- Photons (Gamma Rays): High-energy photons are emitted, carrying away energy.
- Atomic Fragments:
- Beta Particles: These are high-energy electrons or positrons.
- Alpha Particles: These are helium nuclei (two protons and two neutrons).
- Applications:
- Nuclear fission produces energy for nuclear power plants and drives atomic weapon explosions.
- Nuclear fusion is a reaction in which two or more atomic nuclei combine to form a different nucleus.
- Example: When four hydrogen nuclei come together, they form a helium nucleus.
- Subatomic Particles: Fusion also produces subatomic particles like neutrons or protons.
- Energy Source: Fusion powers the sun and other stars, where hydrogen nuclei combine to form helium.
- Principle: A hydrogen bomb relies on uncontrollable nuclear fusion.
- Process: It involves the fusion of isotopes of hydrogen (such as deuterium and tritium) to release immense energy.
- Destructive Power: Hydrogen bombs are significantly more powerful than atomic bombs due to the energy released from fusion reactions.