Samacheer Class 8 Science - Heat

Conduction has numerous practical applications in our daily life. We cook food in vessels made up of metals such as aluminum or steel because metals are excellent conductors of heat. When the vessel is heated on a stove, heat is efficiently transferred from the metal to the food inside through conduction. When we iron clothes, heat is transferred from the hot iron plate to the fabric through conduction, allowing us to remove wrinkles. The handles of cooking utensils such as pots and pans are deliberately made up of plastic or wood because these materials are poor conductors of heat, which prevents our hands from getting burned while holding the utensils. The temperature inside an igloo, which is a snow house built by Arctic peoples, remains relatively warm because snow is a poor conductor of heat and provides excellent insulation against the cold external environment. Additionally, the soles of shoes are often made of rubber or leather, which are poor conductors, to protect our feet from cold ground. Heat sinks used in electronic devices are made of metals with high thermal conductivity to conduct heat away from sensitive components.

Convection is the form of heat transfer from places of high temperature to places of low temperature by the actual movement of molecules. In this process, the molecules themselves move from one location to another, carrying thermal energy with them. Convection takes place in liquids and gases because the molecules in these fluids are free to move about, unlike in solids where molecules are fixed in their positions. When a liquid or gas is heated, the molecules at the heated region gain energy and move faster, becoming less dense and rising upward. Meanwhile, the cooler, denser molecules from above sink downward to replace the rising warm molecules. This creates a continuous circular motion called a convection current, which efficiently transfers heat throughout the fluid. Convection is responsible for phenomena such as the boiling of water, the circulation of air in rooms, and the movement of ocean currents.

Specific heat capacity of a substance is defined as the amount of heat energy required to raise the temperature of one kilogram of a substance by one degree Celsius or one Kelvin. It is denoted by the symbol C and is expressed in units of joules per kilogram per degree Celsius (J/kg°C) or joules per kilogram per Kelvin (J/kg·K). Different substances have different specific heat capacities, which means they require different amounts of heat energy to produce the same temperature change in the same mass. For example, water has a high specific heat capacity, meaning it requires a large amount of heat to raise its temperature, which is why water is used as a coolant in many applications. The specific heat capacity of a substance is an important property that helps us understand how much energy is needed to heat or cool a particular material.

1. A calorimeter is a device used to measure the amount of heat gained or lost by a substance.
2. It consists of a vessel made up of metals like copper or aluminium which are good conductors of heat and electricity.
3. The metallic vessel is kept in an insulating jacket to prevent heat loss to the environment.4. There are two holes in it. Through one hole a thermometer is inserted to measure the temperature Of the Contents.
5. A stirrer is inserted through another hole for stirring the content in the vessel.
6. The vessel is filled with liquid which is heated by passing current through the hating element.
7. Using this device we can measure the heat capacity of the liquid in the container.

1. A thermostat is a device which maintains the temperature of a place or an object constant.
2. The word thermostat is derived from two Greek words, ‘thermo’ meaning heat and ‘static’ meaning staying the same.
3. Thermostats are used in any device or system that gets heated or cools down – to a pre – set temperature. It turns an appliance or a circuit on or off when a particular temperature is reached.
4. Devices which use thermostat include building heater, central heater in a room, air conditioner, water heater, as well as kitchen equipment including oven and refrigerators.
5. Sometimes, a thermostat functions both as the sensor and the controller of a thermal 6 system.

A thermos flask has double walls that are evacuated to create a vacuum between them. The inner surfaces of these walls are silvered. The vacuum between the two walls is highly effective at preventing heat transfer by conduction and convection because there are very few air molecules present to conduct heat or create convection currents. Since conduction requires matter to transfer heat through direct contact, the absence of air in the vacuum space eliminates this pathway almost entirely. Heat can only be conducted at the points where the two walls physically meet, which occurs at the top of the flask and through a small insulated support at the bottom. These contact points are minimal, so conduction losses are negligible. Additionally, the silvered surfaces on the inner walls act as reflectors that bounce radiated heat back toward the liquid inside the flask, preventing heat loss through radiation. By minimizing all three modes of heat transfer—conduction, convection, and radiation—the thermos flask effectively maintains the temperature of the liquid inside for extended periods.

Lakes do not completely freeze during severe winters because the ice and snow that form on the surface act as an insulating layer. Ice is a poor conductor of heat, so it prevents heat from being easily withdrawn from the water below. To freeze water into ice, a large quantity of heat must be removed from the water. However, the insulating layer of ice on the surface blocks the cold air above from directly extracting heat from the water beneath it. Additionally, water has a unique property: it reaches its maximum density at 4°C, which is above its freezing point of 0°C. This means that as water cools, it becomes denser until it reaches 4°C, after which it becomes less dense and rises to the surface. Consequently, the coldest water (at 0°C) remains at the surface and freezes, while the denser water at 4°C sinks to the bottom. The ice layer then insulates the water below, preventing further heat loss and keeping the bottom of the lake from freezing even in extremely cold conditions.

Heat capacity C’ = \(\frac{Q}{∆T}\)
Here, A = 1000 J
T = 20°C – 0°C = 20°C = 20 k
C = \(\frac{1000}{20}\) = 50 JK -1
The heat capacity of the ball = 50 JK -1

Specific heat capacity, C = \(\frac{Q}{mx∆T}\)
Here, m = 100 kg
Heat capacity = \(\frac{Q}{∆T}\) = 8000 J/°C = 8000 J/K
C = \(\frac{Q}{mx∆T}\) = 100 x 8000 J = 8,00,000 JKg -1 K -1

When the water is heated, water molecules receives heat energy, increases the kinetic energy of the molecules.
Question (ii)
What caused the temperature change?
When the molecules receive more energy, the temperature of the water increases.
Heat energy causes increase in temperature.

When ice cubes are heated, they melt and become water because heat energy weakens the forces of attraction between water molecules. In solid ice, the molecules are held very closely together by strong intermolecular forces. As heat is supplied, the molecules gain kinetic energy and vibrate more vigorously, causing the force of attraction between them to decrease. This allows the molecules to move farther apart, and the ice transforms into liquid water. When the water is heated further, the force of attraction between the molecules decreases even more. The molecules gain sufficient energy to overcome the intermolecular forces entirely and escape from the liquid phase into the gaseous phase as water vapor. Since water vapor is a gas, it escapes into the surrounding air, which is why the volume and level of water in the vessel decreases over time. This activity demonstrates that heat energy is responsible for causing changes in the state of matter. Heat provides the energy needed to break intermolecular bonds and allow substances to transition from solid to liquid to gas.

Yes, we feel heat at the end of the spoon. This occurs because heat from the hot water is transferred along the length of the silver spoon from the end immersed in the water to the free end that we touch. In solid substances like a silver spoon, atoms and molecules are arranged very closely together in a fixed structure. When heat is supplied to one end of the spoon, the atoms at that end vibrate more vigorously due to increased kinetic energy. These vibrating atoms collide with neighboring atoms, transferring some of their energy to them. This process continues along the entire length of the spoon, with heat energy being passed from one atom to the next in a chain-like manner. The heat gradually reaches the free end of the spoon, which is why we feel it becoming hot when we touch it. This mode of heat transfer, where heat is conducted through a substance from a region of higher temperature to a region of lower temperature without the substance itself moving, is called conduction. Metals like silver are excellent conductors of heat because their atoms are closely packed and can efficiently transfer vibrational energy to one another.

When water in a vessel is heated from below, the water molecules at the bottom receive heat energy and their kinetic energy increases. These heated molecules move faster and spread out, becoming less dense than the cooler water above them. Because they are less dense, the heated water at the bottom rises upward toward the surface. As the hot water rises, the cooler and denser water from the top sinks downward to replace it. This cooler water then comes into contact with the heat source at the bottom and gets heated in turn. The cooler water then rises, and the cycle repeats continuously. This circular motion of the fluid, where hot material rises and cool material sinks, creates convection currents throughout the vessel. Through these convection currents, heat is transported from the bottom of the vessel to the top. This mode of heat transfer, which involves the actual movement of the heated substance itself, is called convection. Convection occurs in fluids such as liquids and gases because their molecules are free to move and can circulate throughout the medium.

When water and cooking oil are heated under identical conditions, water takes significantly more time to reach the same temperature than cooking oil. This difference occurs because water and cooking oil have different specific heat capacities. Specific heat capacity is the amount of heat energy required to raise the temperature of one kilogram of a substance by one degree Celsius. Water has a much higher specific heat capacity than cooking oil, which means that water requires a larger quantity of heat energy to increase its temperature by the same amount. Because water absorbs more heat energy before its temperature rises, it takes longer to heat up compared to cooking oil. Conversely, cooking oil has a lower specific heat capacity, so it requires less heat energy to achieve the same temperature increase and therefore heats up more quickly. This demonstrates that heat transfer and the rate of temperature change depend on the nature and properties of the substance being heated. Different substances have different abilities to absorb and store heat energy, which is why some materials warm up faster than others when exposed to the same heat source.