Samacheer Class 6 Science - Heat

An ordinary glass bottle cracks when boiling water is poured into it because ordinary glass has a high coefficient of thermal expansion. When the hot water comes into contact with the glass, the outer surface expands rapidly while the inner surface remains relatively cool and expands more slowly. This uneven expansion creates internal stress in the glass, causing it to crack. Borosilicate glass, on the other hand, does not expand much on being heated because it has a very low coefficient of thermal expansion. This means the outer and inner surfaces expand at nearly the same rate, preventing the buildup of internal stress and allowing the glass to withstand sudden temperature changes without cracking.

During summer, the electric wire experiences thermal expansion due to the increase in temperature. The heat causes the molecules in the wire to vibrate more vigorously and move farther apart, resulting in an increase in the length of the wire. This causes the wire to sag between the poles. In winter, when the temperature decreases, the wire contracts as the molecules move closer together and vibrate less. This contraction reduces the length of the wire, making it become straight again.

Common electrical equipment at home from which we get heat include water heaters used for heating water for bathing and washing, electric iron boxes used for pressing clothes, electric kettles used for boiling water quickly, microwave ovens used for cooking and heating food, electric room heaters used for warming rooms during winter, electric stoves and cooktops used for cooking, and electric toasters used for toasting bread.

Thermal expansion is the increase in the size, volume, or length of a substance when it is heated. When heat energy is supplied to a substance, the molecules gain kinetic energy and vibrate more vigorously. This increased molecular movement causes the molecules to move farther apart from each other, resulting in an expansion of the substance. Thermal expansion occurs in solids, liquids, and gases, though the amount of expansion varies depending on the material and the temperature change.

Thermal equilibrium is the state reached when two objects in thermal contact no longer exchange heat energy and therefore no longer affect each other's temperature. When two objects at different temperatures are brought into contact, heat flows from the hotter object to the cooler object until both objects reach the same temperature. Once they reach the same temperature, there is no net flow of heat between them, and they are said to be in thermal equilibrium. At this point, both objects have the same thermal energy and will remain at the same temperature as long as they are not exposed to external heat sources or sinks.

When we heat solids, the molecules gain thermal energy and their vibrations and movement increase significantly. This increased molecular motion causes the distance between molecules to increase as they move farther apart from each other. As a result of this increased molecular movement and separation, the temperature of the object increases. The rise in temperature is a direct measure of the increased kinetic energy of the molecules. Additionally, the overall dimensions of the solid increase, a phenomenon known as thermal expansion, because the molecules are now occupying more space due to their increased movement and separation.

Heat is a form of energy transferred between bodies due to temperature difference and depends on mass and specific heat, whereas temperature is a measure of the average kinetic energy of particles in a body and indicates how hot or cold the body is.

If your normal body temperature was lower than what it actually is, the sensation of hot and cold would change significantly. Since the sensation of hot and cold is relative and depends on the difference between the surrounding temperature and your body temperature, a lower body temperature would alter these sensations. Objects and environments that currently feel warm would feel even warmer because the temperature difference between your body and the surroundings would be greater. Conversely, objects that currently feel cold would feel less cold. Your body would feel comfortable at a lower temperature than it does now, and you would require less external heat to maintain thermal comfort.

Gaps are left in between rails while laying a railway track to allow space for thermal expansion. During summer, when the temperature increases, the metal rails undergo thermal expansion and increase in length. If no gaps were provided between the rails, the expanding rails would push against each other, creating enormous internal stress and pressure. This stress could cause the rails to buckle, bend, or warp, making the railway track unsafe for trains. The gaps provide the necessary space for the rails to expand freely without causing damage to the track structure.

The main source of heat is the Sun, which provides heat energy to the Earth through radiation. Other important sources of heat include combustion, which is the burning of fuels such as wood, coal, gas, and oil that releases heat energy. Friction is another source of heat, generated when two surfaces rub against each other, such as when striking a match or rubbing hands together. Electricity is also a significant source of heat, produced when electric current flows through a conductor with resistance, as seen in electric heaters, light bulbs, and other electrical appliances. Additionally, chemical reactions and nuclear reactions can also produce heat energy.

Heat is a form of energy that flows from a hotter object to a cooler object, causing an increase in temperature by making the molecules in that object move faster and vibrate with greater intensity. Heat is transferred through conduction, convection, and radiation. The transfer of heat continues until both objects reach the same temperature, at which point thermal equilibrium is established. Heat is measured in units called joules or calories.

Two objects are said to be in thermal equilibrium when they are in thermal contact with each other and have the same temperature. In this state, there is no net flow of heat between the two objects because the kinetic energy of molecules in both objects is equal. When two objects at different temperatures are brought into contact, heat flows from the hotter object to the cooler one until both reach the same temperature and achieve thermal equilibrium.