Physics: Electricity and Magnetism: UPSC Previous Year Questions (Science and Technology)

Explanation

Fluorescent tube lights are commonly rated by their color temperature, which is measured in Kelvin (K). Color temperature refers to the hue of light emitted by the tube and it is based on the concept of a black-body radiator.

Option (c) is correct: The color temperature of fluorescent tube lights is typically around 6500 K, which corresponds to "daylight" and provides a bright white light. Options (a), (b) and (d) are incorrect:

Color temperatures as low as 220 K or 273 K are not suitable for lighting as they are far too low. 273 K represents a color temperature lower than a standard incandescent bulb (around 2700 K). A 9000 K light would appear slightly blue and is not commonly used for general room lighting, but rather for specialized purposes like aquarium lighting.

Explanation

A fluorescent lamp or fluorescent tube is a gas discharge lamp that uses electricity to excite mercury vapour. The excited mercury atoms produce shortwave ultraviolet light that then causes a phosphor to fluoresce, producing visible light. The choke coil reduces current in the circuit. A choke coil, also known as an inductor, is used in fluorescent tubes to limit the current flowing through the circuit. It provides impedance to the alternating current (AC) which helps in stabilizing the current and preventing it from reaching excessively high levels. The choke coil does not step up or step down the voltage (these functions are typically performed by a transformer).

Explanation

Statement 1 is correct: The filament in a light bulb is typically made of tungsten. When the filament is not uniform, the thinner sections tend to heat up more, leading to higher temperatures and stress, which can cause premature failure.

Statement 2 is correct: Light bulbs typically contain an inert gas, like argon, to prevent oxidation of the filament and improve lifespan. Complete evacuation of the bulb is not possible. A small quantity of gases left behind which expands on heating by heat through filament. If the bulb is not evacuated properly, the filament would burn out quickly causing the bulb to have a short life.

Statement 3 is correct: The wires supporting the filament are designed to withstand high temperatures. However if they are not robust enough or if the filament heats up too much, and they can melt, causing the bulb to fail.

Explanation

Statement 1 is correct: Most car batteries are 12-volt lead-acid batteries. This is the standard voltage for automotive applications as it provides sufficient power for starting the engine and running electrical systems in vehicles.

Statement 2 is incorrect: The electrolyte used in a car battery is sulfuric acid (H₂SO₄), not hydrochloric acid. Sulfuric acid is mixed with distilled water to create the electrolyte solution, which facilitates the electrochemical reactions in the battery.

Statement 3 is incorrect: The electrodes in a car battery are made of lead (Pb) and lead dioxide (PbO), not copper. The negative electrode (anode) is made of pure lead, and the positive electrode (cathode) is made of lead dioxide.

Statement 4 is correct: The capacity of a car battery is expressed in ampere-hours (Ah). This unit measures the amount of charge a battery can deliver over time. For example, a 50 Ah battery can theoretically deliver 1 ampere of current for 50 hours or 5 amperes for 10 hours.

Explanation

A fuse is a safety device designed to protect electrical circuits from overloads or short circuits. It works by having a thin wire or filament that melts when the current exceeds a certain level, breaking the circuit and preventing damage to electrical appliances. The fuse is specifically made with a low melting point to ensure it melts quickly in the event of excessive current flow, protecting the circuit from potential damage. Options (a), (b) and (d) are incorrect:

They are wired in series with the circuit not parallelbecause this setup ensures that the full current passing through the circuit flows through the fuse. This allows the fuse to effectively interrupt the circuit and halt the electrical flow in the event of an overcurrent. Fuses are not constructed from silver alloys as they are usually made from materials such as copper, zinc, or tin. They do not need high resistance rather, they should allow normal current flow until the current exceeds a safe limit. Direction: The following item consists of two statements, one labeled as the Assertion (A) and the other as Reason (R). You have to answer this item using the code given below.

Explanation

Assertion (A) is true: The flow of electric current through a conductor causes energy dissipation in the form of heat due to the collisions between free electrons and the metal atoms. Reason (R) is true: As electrons travel through a metal as they collide with metal atoms causing the atoms to vibrate more and collide with each other which produces heat. The phenomenon is known as Joule heating. When current flows through the metal wire, the free electrons that carry the current interact with the metal atoms, transferring energy and causing the atoms to vibrate more, leading to a temperature increase due to Joule heating. The amount of heat generated depends on factors such as the wire’s resistance, the current, and the duration of the current flow. This effect is the foundation for devices like electric heaters. Thus Reason (R) explains Assertion (A)

Explanation

Paramagnetism refers to the property of a material that is weakly attracted to an external magnetic field due to the presence of unpaired electrons in its atomic or molecular structure. When placed in a magnetic field, these unpaired electrons align their magnetic moments in the direction of the field, causing a weak but positive magnetization. However, this effect disappears once the external field is removed.

Option (c) is correct: Oxygen (O₂) is paramagnetic because it has two unpaired electrons in its molecular orbital configuration, making it weakly attracted to a magnetic field. Options (a), (b) and (d) are incorrect:

Iron is ferromagnetic (not always paramagnetic), meaning it is strongly attracted to magnetic fields. Hydrogen and Nitrogen are diamagnetic, meaning they are weakly repelled by magnetic fields due to their paired electrons. Directions: The following item consists of two statements, one labeled as the Assertion (A) and the other as Reason (R). You have to answer this item using the code given below.

Explanation

Assertion (A) is true: A transformer is designed to change the voltage level of an alternating current (AC) by using electromagnetic induction, either increasing (stepping up) or decreasing (stepping down) the voltage. Reason (R) is false: A transformer only works with alternating current (AC) because it relies on the changing magnetic field created by the alternating current to induce voltage in the secondary coil. It does not function with direct current (DC).

Explanation

Domestic electrical wiring is primarily a parallel connection, meaning that appliances and devices are connected in parallel to the power supply. This ensures that each device receives the same voltage, and turning one device on or off does not affect the others. Parallel wiring also allows for individual control of appliances and prevents overloading of a single circuit. In a series connection, devices share the same current, and the total resistance adds up, which can cause voltage drops, as seen in old-fashioned Christmas lights where if one bulb fails, the entire string goes out.