Heat
Heat is a type of energy that moves from one object to another due to a difference in their temperatures. The amount of heat in an object depends on its mass.
Mechanical Equivalent of Heat
When a certain amount of work (W) is done and it produces a certain amount of heat (H), the ratio of work to heat is denoted by J. This ratio is constant and is called the Mechanical Equivalent of Heat. Its value is 4.186 joules per calorie. This means that if 4.186 joules of work are done, 1 calorie of heat is produced.
Units of Heat
C.G.S unit: Calorie
This is the amount of heat needed to raise the temperature of 1 gram of pure water by 1°C.International Calorie
This is the amount of heat required to raise the temperature of 1 gram of pure water from 14.5°C to 15.5°C.FPS unit: British Thermal Unit (B.Th.U)
This is the amount of heat needed to raise the temperature of 1 pound of pure water by 1°F.
Relations between different units:
- 1 British Thermal Unit (B.Th.U) = 252 calories
- 1 calorie = 4.186 joules
- 1 Therm = 100,000 British Thermal Units (B.Th.U)
- 1 pound calorie = 453.6 calories.
Temperature tells us if something is hot or cold. We measure it with a thermometer.
Thermometers have two special points: when water freezes (freezing point) and when it boils (boiling point).
Different thermometers use different scales. They count the intervals between freezing and boiling points.
| Celsius | Fahrenheit | Reaumur | Kelvin | Rankine | |
|---|---|---|---|---|---|
| UFP(Upper Fixed Point) | 100°C | 212°F | 80°F | 373.15K | 672°Ra |
| no. of fundamental interval | 100 | 180 | 80 | 100 | 180 |
| LFP | 0°C | 32°F | 0 R | 273.15K | 492°Ra |
| Absolute zero | -273.15°C | -459.6°F | -218.4°R | 0 K | 0°Ra |
C−0100=180F−32=180R−0=100K−273=180Ra−492
Conversion of temp difference from and scale to other scale
ΔC/5 = ΔF/9 = ΔR/4 = ΔK/5 = ΔRa/9
Celsius Scale: Originally called centigrade. The freezing point of mercury is -39°C, below which alcohol thermometers are used (-15°C).
Range of Thermometers:
Mercury Thermometer: -30°C to 350°C
Constant Volume Gas Thermometer:
Hydrogen (H2): -200°C to 500°C
Helium (He): below -200°C to -268°C
Nitrogen (N2) with porcelain bulb: above 100°C to 1600°C
Platinum Resistance Thermometer: -200°C to 1200°C
Thermocouple Thermometer: -200°C to 1600°C
Specific Heat Capacity: The amount of heat required to raise the temperature of a substance by 1°C. Specific heat capacity of water is 1 cal/g°C or 4200 J/kgK.
| Material | Specific Heat Capacity (J/kg°C) |
|---|---|
| Water | 4200 |
| Ice | 2100 |
| Iron | 460 |
| Kerosene Oil | 210 |
| Mercury | 140 |
| Lead | 130 |
Anomalous Expansion of Water: Water's volume decreases from 0°C to 4°C, then increases. Maximum density at 4°C.
Transmission of Heat:
Conduction: Heat transfer through vibrating particles in solids.
Convection: Heat transfer through particle movement in liquids and gases.
Radiation: Heat transfer at the speed of light without affecting the medium.
Newton's Law of Cooling: Rate of heat loss is proportional to the temperature difference with surroundings.
Kirchhoff's Law: Ratio of emissive power to absorptive power is constant for all surfaces at a given temperature, signifying good absorbers are good emitters.
Total Radiation Pyrometer: Measures body temperature by the radiation emitted, proportional to the fourth power of absolute temperature.
Stefan’s Law: Stefan’s Law states that the radiant energy emitted by a black body per unit area per unit time (emissive power) is directly proportional to the fourth power of its absolute temperature. Mathematically, this relationship is expressed as
, where represents the emissive power and represents the absolute temperature. A constant , known as Stefan’s constant, is introduced to the equation, giving us . This law is fundamental in understanding how the emission of radiation from objects depends on their temperature.
Change of state
Change of state refers to the transition of a material from one physical state (solid, liquid, or gas) to another. This transition occurs when heat is either added to or removed from the substance. Each substance undergoes these changes at specific temperatures, which remain constant during the transition. Understanding change of state is crucial in various fields, including physics, chemistry, and engineering.
| Process | Description |
|---|---|
| Solid to Liquid | Melting/Fusion |
| Liquid to Gas | Vaporization |
| Liquid to Solid | Freezing/Solidification |
| Gas to Liquid | Condensation |
| Solid to Gas | Sublimation |
| Gas to Solid | Deposition |
Freezing: Freezing is when a substance changes from a liquid to a solid. It occurs at a fixed temperature known as the freezing point (F.P.). For most substances, the melting point is the same as the freezing point.
Changes in Melting Point: The melting point of a substance can change with pressure. Some substances, like ice, contract when they melt, so their melting point decreases with increased pressure. Others, like wax, expand when they melt, so their melting point increases with increased pressure. Adding impurities, such as salt to ice, lowers the melting point.
Vaporization: Vaporization is when a substance changes from a liquid to a vapor (gas). It happens through two methods: evaporation and boiling.
Evaporation: Evaporation is vaporization that occurs only from the surface of a liquid, and it happens at all temperatures. Evaporation causes cooling, which is why water in an open container cools down in summer.
Boiling: Boiling is when vaporization happens throughout the liquid, not just at the surface. It occurs at a specific temperature called the boiling point. The temperature at which boiling takes place is called the boiling point.
Condensation: This is when a substance changes from a gas to a liquid.
Latent heat: This is the energy needed to change the state of a substance without changing its temperature. For example, turning a solid into a liquid or a liquid into a gas.
Unit of latent heat: It's measured in Joules per kilogram.
Types of latent heat:
Latent heat of fusion: This is the energy needed to turn a solid into a liquid, like when ice melts into water. It's also the energy released when a liquid turns back into a solid.
Latent heat of vaporization: This is the energy needed to turn a liquid into a gas, like when water boils. It's also the energy released when a gas turns back into a liquid.
Latent Heat of Water
| Latent Heat | In Cal/g | In J/kg |
|---|---|---|
| of fusion | 80 | 336 x 103 |
| of vaporisation | 540 | 2256 x 103 |
Sublimation: Sublimation is the process in which a solid transforms directly into vapor without passing through the liquid phase. It occurs when the boiling point of a substance is lower than its melting point. Examples include camphor and ice in a vacuum.
Hoar frost: Hoar frost is the reverse of sublimation, where vapor directly turns into solid ice.
Steam can cause more severe burns compared to water at the same temperature due to steam having higher internal energy than water at the same temperature.
Relative humidity: Relative humidity measures the amount of water vapor in the air compared to the amount needed to saturate the air at the same temperature. It's expressed as a percentage and can be measured with a hygrometer. Relative humidity increases with temperature.
For a healthy environment for humans, ideal conditions include a temperature range of 23°C to 25°C, relative humidity between 60% to 65%, and airspeed between 0.75 to 2.5 meters per minute.
Thermodynamics
First Law of Thermodynamics: When you give heat to a system, it uses that energy in two main ways: (i) to increase its temperature and internal energy, and (ii) to do work. Mathematically, this is stated as ΔQ (the heat energy given to the system) equals ΔU (the increase in internal energy) plus ΔW (the work done by the system). This law is just another way of saying that energy is always conserved.
Isothermal Process: This happens when a system changes, but its temperature stays the same throughout the process.
Adiabatic Process: This happens when a system changes, but no heat is exchanged between the system and its surroundings. For example, when carbon dioxide expands suddenly, it turns into dry ice. This is an adiabatic process.
Second Law of Thermodynamics: While the first law ensures energy conservation, the second law tells us if a process can happen or not. Two important statements of this law are: (i) Kelvin's statement, which says that we can't convert all heat into work, and (ii) Clausius statement, which says that heat can't flow by itself from a colder object to a hotter one.
Heat Engine: This is a device that turns heat energy into mechanical work through a cyclic process. A heat engine has three main parts: a source (a hot body), a sink (a cold body), and a working substance. There are two types: (i) Internal Combustion Engine, like a petrol or diesel engine, where heat is produced inside the engine, and (ii) External Combustion Engine, like a steam engine, where heat is produced outside the engine.
Refrigerator or Heat Pump: A refrigerator moves heat energy from a cold object to a hot one using energy supplied by an external agent. The substance it uses, called refrigerant, is often freon (CCL2F2) in real refrigerators.
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