Heat Science Notes
Important Points :
- Heat is a form of energy. Particles of matter (atoms, molecules, etc.) possess potential energy and kinetic energy.
- Total energy (potential energy + kinetic energy) of all particles of matter in a given sample is called its thermal energy.
- When two bodies at different temperatures are in thermal contact with each other, there is transfer of thermal energy from a body at higher temperature to a body at lower temperature.
- This energy in transfer is called heat. It is expressed in joule, calorie and erg.
- Temperature is a quantitative measure of degree of hotness or coldness of a body.
- It is expressed in °C, °F or K (kelvin).
- Temperature determines the direction of energy transfer.
- Ways of heat transfer : conduction, convection and radiation.
[Note : heat = heat energy. In the textbook, both the terms are used.
Latent heat :
→ Latent heat : When there is a change of state of a substance, [solid ↔ liquid, liquid ↔ gas (vapour), solid ↔ gas (vapour)] heat energy is absorbed by the substance or heat energy is removed from the substance at constant temperature.
→ This heat energy is called latent heat of transition (or transformation or change of state). Latent heat per unit mass of the substance is called specific latent heat.
→ The amount of heat energy absorbed at constant temperature by unit mass of a solid to convert into liquid phase is called the specific latent heat of fusion, and that constant temperature is called the melting point of the substance.
→ The amount of heat energy absorbed at constant temperature by unit mass of a liquid to convert into gaseous phase is called the specific latent heat of vaporization and that constant temperature is called the boiling point of the substance.
→ Specific latent heat is expressed in J/kg, erg/g, cal/g, kJ/kg and kcal/kg.
Heat absorbed or given out in change of state = mL, where m is the mass of the substance.
Regelation: The phenomenon in which ice converts to liquid due to applied pressure and then re-converts to ice once the pressure is removed is called regelation.
Anomalous behaviour of water:
→ Anomalous behaviour of water : If water is heated from 0 °C to 4 °C, it contracts instead of expanding. At 4 °C its volume is minimum. If the water is heated further, it expands, i.e., its volume increases.
→ This abnormal behaviour of water in the temperature range 0 °C to 4 °C is called anomalous behaviour of water. The density of water is maximum at 4 °C. Hope’s apparatus is used to study anomalous behaviour of water.
Dew point and humidity :
→ Dew point temperature : If the temperature of unsaturated air is decreased, a temperature is reached at which the air becomes saturated with water vapour. This temperature is called the dew point temperature.
→ Absolute humidity : The mass of water vapour present in a unit volume of air is called absolute humidity. Generally it is expressed in kg/m.
→ Relative humidity : The ratio of the actual mass of water vapour content in the air for a given volume and temperature to that required to make the same volume of air saturated with water vapour at the same temperature is called the relative humidity. This ratio, multiplied by 100, gives the percentage relative humidity. At the dew point the relative humidity is 100%.
→ Take a bottle of cold water out of a refrigerator and keep it outside for a while. Observe the outer surface of the bottle.
→ Drops of water can be observed on the outer surface of bottle. In the same way, if we observe the leaves of plants/ grass or windowglass of a vehicle in the early morning we see water droplets collected on the surface.
→ Through these two observations, we sense the presence of water vapour in the atmosphere. When air cools, due to decrease in temperature it becomes saturated with water vapour. As a result, the excess water vapour gets converted into tiny droplets. The dew-point temperature is decided by the amount of vapour in the air.
→ Unit of heat : Heat can be expressed in various units, e.g. joule (J), erg, calorie (cal), kilocalorie (kcal). The amount of heat necessary to raise the temperature of 1 g of water by 1 °C from 14.5 °C to 15.5 °C is called one calorie.
→ The amount of heat necessary to raise the temperature of 1 kg of water by 1 °C from 14.5 °C to 15.5 °C is called one kilocalorie.
1 kcal = 103cal, 1 cal = 4.18 J,
1 kcal = 4.18 × 103 J.
Specific heat capacity :
→ Specific heat capacity (c): The amount of heat energy required to raise the temperature of a unit mass of an object by 1 °C is called the specific heat capacity or simply specific heat of the object. It is expressed in various units such as J/kg . °C, erg/g . °C, cal/g . °C, kcal/kg .°C.
|Substance||Specific heat (c) (cal/g.°C)|
- Heat absorbed by an object = mcΔt, where m is the mass of the object and ΔT is the increase in the temperature of the object.
- Heat lost (given out) by an object = mcΔT
- Here ΔT is the decrease in the temperature of the object.
→ Principle of heat exchange : If a system of two objects is isolated from the environment by keeping it inside a heat resistant box, then no energy can leave the box or enter the box. In this situation, heat energy lost by the hot object = heat energy gained by the cold object. In due course, the two objects attain the same temperature.
Additional Important Information :
→ Sublimation: The phenomenon in which a solid directly passes to the gaseous state without passing through the intermediate liquid state is known as sublimation. Iodine, ammonia and camphor possess the property of sublimation.
→ Effect of pressure on the melting point of a substance : At a given pressure, a given solidmelts at a fixed temperature. A change in pressure changes the melting point of the substance. In the case of the solids which expand on melting, an increase in the pressure raises the melting point of the solid, e.g., lead and wax.
→ In the case of the solids which contract on melting, an increase in pressure lowers the melting point of the solid, e.g., ice, antimony and bismuth.
→ Effect of pressure on the boiling point of a liquid : The boiling point of a liquid depends on the pressure on its surface. An increase in pressure raises the boiling point of a liquid while a decrease in pressure lowers its boiling point.