Samacheer Class 9 Science - Heat

• (a) heat
• (b) work
• (c) temperature
• (d) food

Answer:
(a) heat

• (a) Fahrenheit
• (b) joule
• (c) Celsius
• (d) kelvin

Answer:
(d) kelvin

• (a) Both rods
• (b) Rod-2
• (c) Rod-1
• (d) None

Answer:
(c) Rod-1

Greater cross-sectional area allows faster heat conduction.

• (a) Radiation
• (b) Conduction
• (c) Convection
• (d) Both B and C

Answer:
(b) Conduction

Correction made:
Original answer “Radiation” was incorrect.

Both the assertion and reason are true, and the reason correctly explains the assertion, making the answer (a). Food cooks faster in vessels with copper bottoms because copper is an excellent conductor of heat. Copper has high thermal conductivity, meaning it efficiently transfers heat from the heat source to the food inside the vessel. When a vessel has a copper bottom, heat from the stove is rapidly conducted through the copper layer to the food, ensuring faster and more uniform heating. This efficient heat transfer reduces cooking time compared to vessels made of materials with lower thermal conductivity. This is why copper-bottomed cookware is preferred in kitchens for faster and more efficient cooking.

Both the assertion and reason are true, but the reason does not correctly explain the assertion, making the answer (b). Maximum sunlight does reach Earth during noon when the sun is at its highest position in the sky. However, the reason given is incomplete. While it is true that heat from the sun reaches Earth by radiation, this alone does not explain why maximum sunlight reaches Earth at noon. The correct explanation is that at noon, the sun is directly overhead or nearly overhead, and sunlight falls more directly and perpendicularly on Earth's surface. When sunlight strikes the surface at a more perpendicular angle, it covers a smaller area and delivers more concentrated energy per unit area compared to when it strikes at an angle during morning or evening hours. This more direct incidence of sunlight at noon results in maximum solar radiation reaching Earth's surface.

The assertion is true. When water reaches 100°C, its temperature remains constant until all the water has completely changed into steam. This occurs because during the phase change from liquid to gas, all the heat energy supplied is used to break the intermolecular bonds and change the state, rather than increasing the temperature. The reason provided is false because the boiling point of water is 100°C, not 10°C. The boiling point is the temperature at which the vapor pressure of a liquid equals the atmospheric pressure, and for water at standard atmospheric pressure, this temperature is exactly 100°C.

Answer:

Double walls contain insulating material or vacuum that reduces heat transfer by:

• conduction
• convection
• radiation

Hence ice melts slowly.

Water remains cool in an earthen pot because the pot has tiny pores in its walls through which water gradually seeps out. When water seeps through these pores and evaporates from the surface of the pot, it absorbs latent heat of vaporization from the water inside the pot. This absorption of heat energy during evaporation causes the temperature of the remaining water to decrease, keeping it cool. The process is continuous as long as there is water in the pot and the surrounding air is not saturated with moisture. This cooling effect is more pronounced in dry climates where evaporation occurs more rapidly.

People wear white clothes in summer because white color reflects most of the heat radiation falling on it and absorbs very little heat. When sunlight falls on white fabric, a large portion of the radiation is reflected back into the surroundings rather than being absorbed by the cloth. This means less heat is transferred to the body, helping to keep the person cool and comfortable. In contrast, dark colors absorb most of the heat radiation, which increases the temperature of the cloth and makes the body feel hotter. Therefore, white clothes are more suitable for summer wear as they minimize heat absorption and help maintain a lower body temperature.

Answer:

Specific heat capacity is amount of heat required to raise temperature of 1 kg of substance by (1^\circ C) or (1K).

Answer:

Thermal capacity is amount of heat required to raise temperature of an entire body by (1^\circ C).

Correction made:
Original definition incorrectly described specific heat capacity.

Specific latent heat capacity is defined as the amount of heat energy required to change the state of unit mass of a substance without any change in its temperature. It is expressed in joules per kilogram (J/kg). When a substance undergoes a phase change, such as from solid to liquid (melting) or from liquid to gas (vaporization), the heat supplied is used entirely to break the intermolecular bonds and rearrange the particles, rather than increasing the kinetic energy and temperature of the particles. Different substances have different specific latent heat capacities. For example, the specific latent heat of fusion of ice is 336,000 J/kg, and the specific latent heat of vaporization of water is 2,260,000 J/kg.

Melting

Ice melts into water.

Freezing

Water freezes into ice.

Vaporization

Water changes into steam.

Condensation

Steam changes into water.

Sublimation

Without becoming liquid.

Experiment 1: When a test tube containing ice at the bottom and water above it is heated from the top, the water at the top boils while the ice at the bottom remains unmelted. This observation shows that water is a poor conductor of heat because the heat supplied at the top does not conduct efficiently downward to melt the ice. Experiment 2: When the same test tube is heated from below, the ice melts quickly. This is because heat transfer occurs through convection currents in water. When water is heated from below, it becomes less dense and rises, while cooler water sinks, creating convection currents that distribute heat throughout the liquid efficiently. These two experiments together demonstrate that water conducts heat poorly through conduction but transfers heat effectively through convection.

Given:

Using:

In Calories

(approximately)

Given:

Using heat balance:

Solving: