Heat and Thermodynamics

Dark-colored clothes absorb more heat from sunlight because darker colors absorb more wavelengths of visible light rather than reflecting them. This absorbed light energy is converted to heat, making darker clothes warmer in winter.

When a liquid is heated, its molecules gain kinetic energy and move farther apart on average, causing the volume to increase while mass stays constant. Since density equals mass divided by volume, increasing volume decreases density.

This is Avogadro's Law: equal volumes of all gases at the same temperature and pressure contain equal numbers of molecules. The number of molecules (not mass, weight, or density) remains constant under these conditions.

The Joule-Thomson effect is the temperature change of a gas when it expands through a porous plug without doing external work and with no heat transfer. For an ideal gas, there are no intermolecular forces, so this effect equals zero.

Microwaves work by generating electromagnetic waves that excite water molecules in food, creating heat. This process is fundamentally based on thermal radiation - the transfer of heat through electromagnetic waves.

Setting -40°C = -40°F in the conversion formula F = (9/5)C + 32 gives -40 = (9/5)(-40) + 32 = -72 + 32 = -40, which is true. This is the only temperature where both scales give identical readings.

An isothermal process is a thermodynamic process where temperature remains constant throughout. 'Iso' means equal or same, and 'thermal' refers to temperature, making 'isothermal' the correct term.

The SI (International System of Units) unit of temperature is Kelvin, abbreviated as K. While Celsius and Fahrenheit are commonly used in everyday life, Kelvin is the scientific standard unit used in physics and chemistry.

Water reaches its maximum density at 4°C. When cooled from 7°C, it first contracts (7°C to 4°C) as it becomes denser, then expands (4°C to 1°C) as the hydrogen bonds form an open hexagonal structure - a unique property known as anomalous expansion.

Water has the highest specific heat capacity among all common substances (~4.18 J/g°C). This means water can absorb more heat energy per unit mass for a given temperature change compared to metals, wood, soil, or other materials.

The SI unit of heat is Joule (J), named after James Prescott Joule. While calorie is a common unit for heat energy (1 calorie ≈ 4.18 J), it is not an SI unit. Celsius is a unit of temperature, not heat energy.

Using the formula C = (F-32) × 5/9: (98.6-32) × 5/9 = 66.6 × 5/9 = 37°C. This is the standard normal human body temperature. Options A and B are miscalculations.

Metal rails expand when heated during summer due to thermal expansion. If no gaps are left, the expanding rails would buckle and bend, causing derailment risks. The gaps (expansion joints) provide space for this expansion.

When boiling water is poured into a glass, the inner surface heats up rapidly and tries to expand. The outer surface remains cooler and doesn't expand at the same rate, creating thermal stress that can crack the glass. This is called thermal shock.

The photosphere (visible surface of the Sun) has a temperature of approximately 6000 K (about 5700°C). This is why the Sun appears white/yellow to us - it emits light characteristic of a 6000 K black body radiator.

Earthen pots have microscopic pores through which water seeps and evaporates from the outer surface. Evaporation requires latent heat of vaporization, which is drawn from the water inside the pot, cooling it down. This is an effective natural cooling method.

Relative humidity is the ratio of actual water vapor to the maximum vapor the air can hold at that temperature. When temperature increases, the air's capacity to hold water vapor increases significantly. If moisture content stays the same, the ratio (relative humidity) decreases.

The Sun's energy travels through the vacuum of space by radiation (electromagnetic waves). Conduction requires a medium, convection requires fluid motion with gravity, and only radiation can travel through empty space. This is how we receive solar energy.

Critical temperature is the temperature above which a gas cannot be liquefied by applying pressure, no matter how high the pressure. Above this temperature, the substance exists only as a gas or supercritical fluid. This is a fundamental property of each substance.

Radiation is the only heat transfer method that does not require a material medium. It occurs through electromagnetic waves (infrared radiation). Conduction requires direct contact, convection requires bulk motion of fluids, but radiation can travel through vacuum.

Dew forms when water vapor condenses on cool surfaces. Bad conductors don't conduct heat away easily, allowing surfaces to stay cool. Good radiators efficiently radiate heat, cooling the surface. Objects near the ground cool faster through radiation to the night sky. All three conditions facilitate dew formation.

At the extreme positions of its swing (maximum amplitude), a simple pendulum momentarily comes to rest as it changes direction. At this point, its kinetic energy is zero. The kinetic energy then increases as it accelerates toward the lowest point, converting potential energy to kinetic energy.

The time period of a simple pendulum is given by T = 2π√(L/g), where L is the length of the string and g is acceleration due to gravity. The time period depends only on the length and gravitational acceleration, not on the mass, shape, size, or density of the bob.

Reverberation is the persistence of sound in an enclosed space due to multiple reflections from walls, ceiling, and other surfaces. When sound waves continue to reflect off surfaces rather than being absorbed, they create the prolongation of sound known as reverberation.

The acceleration due to gravity (g) is higher at the poles than at the equator due to Earth's shape (oblate spheroid) and rotational effects. Since the time period of a pendulum is T = 2π√(L/g), a higher g means a shorter time period. The clock completes each swing faster, so it effectively runs slow (loses time) when compared to its rate at the equator.