Grade 8 Chapter 14: Measurement and Effects of Heat

Introduction

Heat is a form of energy that flows from a hotter body to a colder body. Temperature is a measure of the degree of hotness or coldness of a body. These two concepts are fundamental to understanding many phenomena in our daily lives, from cooking food to weather patterns. This chapter will explore how temperature is measured, the various effects heat has on matter, including changes in size and state, and the different ways heat can be transferred from one place to another.

Heat and Temperature

• Heat: A form of energy that is transferred between objects or systems due to a temperature difference. It flows from a region of higher temperature to a region of lower temperature. SI unit of heat is Joule (J). Other common unit is calorie (cal).
• Temperature: A measure of the average kinetic energy of the particles within a substance. It indicates the degree of hotness or coldness of a body.

Measurement of Temperature

Temperature is measured using a device called a thermometer.

• Clinical Thermometer:
  • Used to measure human body temperature.
  • Range: 35°C to 42°C (94°F to 108°F).
  • Has a kink (constriction) near the bulb to prevent mercury from falling back immediately, allowing time for reading.
  • Should be sterilized before and after use.
• Laboratory Thermometer:
  • Used to measure temperature in laboratories for various experiments.
  • Range: -10°C to 110°C.
  • Does not have a kink, so mercury level falls quickly.
  • Should be held vertically while taking readings.
• Digital Thermometer:
  • Modern thermometers that use electronic sensors to measure temperature.
  • Do not use mercury, making them safer and environmentally friendly.
  • Provide quick and accurate readings.

Units of Temperature

The common units for measuring temperature are:

• Celsius (°C): Most commonly used unit. Freezing point of water is 0°C, boiling point is 100°C.
• Fahrenheit (°F): Used in some countries. Freezing point of water is 32°F, boiling point is 212°F.
• Kelvin (K): The SI unit of temperature. 0 K is absolute zero (-273.15°C).
  Conversion: K = °C + 273.15

Effects of Heat

When a substance is heated, it can undergo several changes:

1. Expansion and Contraction

Most substances expand (increase in size) on heating and contract (decrease in size) on cooling.

• Thermal Expansion of Solids: Solids expand in length, area, and volume on heating.
  • Example: Railway tracks have gaps to allow for expansion in summer. Metal rims are heated before fitting onto wooden wheels.
• Thermal Expansion of Liquids: Liquids expand more than solids for the same rise in temperature.
  • Example: Mercury in a thermometer expands on heating, indicating temperature.
• Thermal Expansion of Gases: Gases expand the most on heating.
  • Example: Hot air balloons rise because heated air expands and becomes less dense.
• Anomalous Expansion of Water: Water shows unusual behavior. It contracts on heating from 0°C to 4°C, and then expands on heating from 4°C to 100°C. It has maximum density at 4°C. This is crucial for aquatic life in cold regions.

2. Change of State

Heat can cause a substance to change its physical state (solid, liquid, gas).

• Melting (Fusion): Solid to liquid (e.g., ice to water). Occurs at melting point.
• Boiling (Vaporization): Liquid to gas (e.g., water to steam). Occurs at boiling point.
• Freezing (Solidification): Liquid to solid (e.g., water to ice). Occurs at freezing point.
• Condensation: Gas to liquid (e.g., steam to water droplets).
• Sublimation: Solid directly to gas without passing through liquid state (e.g., camphor, dry ice).

3. Latent Heat

Latent heat is the hidden heat energy absorbed or released by a substance during a change of state at a constant temperature. It does not cause a change in temperature but rather a change in phase.

• Latent Heat of Fusion: The amount of heat energy required to change a unit mass of a solid into a liquid at its melting point without any change in temperature. (e.g., for ice, it's 334 kJ/kg or 80 cal/g).
• Latent Heat of Vaporization: The amount of heat energy required to change a unit mass of a liquid into a gas at its boiling point without any change in temperature. (e.g., for water, it's 2260 kJ/kg or 540 cal/g).

4. Specific Heat Capacity

Specific heat capacity (c) is the amount of heat energy required to raise the temperature of a unit mass of a substance by 1°C (or 1 K).

• Unit: Joule per kilogram per degree Celsius (J/kg°C) or calorie per gram per degree Celsius (cal/g°C).
• Water has a very high specific heat capacity (4.18 J/g°C or 1 cal/g°C), meaning it absorbs a large amount of heat without a significant rise in temperature. This property is important for climate regulation and as a coolant.

Transfer of Heat

Heat can be transferred from one place to another by three main modes:

1. Conduction

• Definition: Transfer of heat through direct contact between particles, without any actual movement of the medium itself. Occurs mainly in solids.
• How it works: Vibrating particles transfer energy to adjacent particles.
• Examples: Heating a metal rod from one end, heat transfer through a cooking pan.
• Conductors: Materials that allow heat to pass through them easily (e.g., metals like copper, aluminum).
• Insulators: Materials that do not allow heat to pass through them easily (e.g., wood, plastic, air, wool).

2. Convection

• Definition: Transfer of heat by the actual movement of fluid (liquid or gas) particles. Occurs mainly in liquids and gases.
• How it works: Heated fluid becomes less dense and rises, cooler fluid sinks to take its place, creating convection currents.
• Examples: Boiling water in a pot, sea breezes and land breezes, heating of rooms by heaters, hot air balloons.

3. Radiation

• Definition: Transfer of heat in the form of electromagnetic waves (infrared radiation), without requiring any medium.
• How it works: Heat energy travels as waves and can pass through vacuum.
• Examples: Heat from the sun reaching Earth, heat from a glowing fire, heat from a radiator. Dark, dull surfaces are good absorbers and emitters of radiation, while shiny, light surfaces are poor absorbers/emitters.

Applications of Heat Effects

• Thermal Expansion:
  • Gaps in railway tracks and bridges.
  • Riveting (joining metal plates by heating rivets).
  • Fitting metal rims on wooden wheels.
  • Thermostats (bimetallic strips).
• Change of State:
  • Refrigeration and air conditioning (based on vaporization and condensation).
  • Cooking (boiling, steaming).
  • Ice used as a coolant (due to latent heat of fusion).
• Heat Transfer:
  • Cooking utensils made of metals (conduction).
  • Woolen clothes keep us warm (trapped air is an insulator, reducing convection).
  • White clothes are preferred in summer (reflect radiation).
  • Vacuum flasks (thermos) minimize heat transfer by all three modes.

Summary

• Heat: Energy transfer due to temperature difference. Temperature: Measure of hotness/coldness.
• Temperature measured by thermometers (Clinical, Laboratory, Digital) in Celsius, Fahrenheit, Kelvin.
• Effects of Heat:
  • Expansion/Contraction: Substances expand on heating, contract on cooling (Anomalous expansion of water).
  • Change of State: Melting, Boiling, Freezing, Condensation, Sublimation.
  • Latent Heat: Heat absorbed/released during phase change at constant temperature (Fusion, Vaporization).
  • Specific Heat Capacity: Heat required to raise unit mass temperature by 1°C. Water has high specific heat.
• Transfer of Heat:
  • Conduction: Through direct contact (solids, good in metals).
  • Convection: By movement of fluid particles (liquids, gases).
  • Radiation: By electromagnetic waves, no medium needed (e.g., sun's heat).
• Applications are found in daily life, engineering, and climate.

References

1. Maharashtra State Board Science and Technology Standard Eight Textbook (Specific Edition/Year) - Chapter 14: Measurement and Effects of Heat.
2. Maharashtra State Board 8th Standard Science Syllabus.
3. Balbharati Science and Technology Textbook.