Grade 7 Chapter 9: Heat

Introduction to Heat and Temperature

We experience 'hot' and 'cold' in our daily lives. From boiling water to ice cubes, these sensations are related to a form of energy called heat. This chapter will delve into what heat is, how it travels, how we measure it, and its various effects on matter.

What is Heat?

Heat is a form of energy that flows from a hotter object to a colder object. It is the total kinetic energy of the molecules in a substance. When an object absorbs heat, its molecules move faster, and when it loses heat, its molecules slow down.

What is Temperature?

Temperature is a measure of the degree of hotness or coldness of an object. It indicates the average kinetic energy of the molecules in a substance. Temperature determines the direction of heat flow; heat always flows from a region of higher temperature to a region of lower temperature.

• SI Unit of Temperature: Kelvin (K)
• Common Units: Degree Celsius (°C) and Degree Fahrenheit (°F)

Measurement of Temperature: Thermometers

A thermometer is a device used to measure temperature. It works on the principle that substances expand on heating and contract on cooling. Mercury or alcohol are commonly used in thermometers because they expand uniformly over a wide range of temperatures.

1. Clinical Thermometer

• Purpose: Used to measure human body temperature.
• Range: Typically from 35°C to 42°C (or 94°F to 108°F). This narrow range is suitable because human body temperature does not go below or above these limits.
• Kink/Constriction: It has a small bend or 'kink' near the bulb. This kink prevents the mercury level from falling back into the bulb on its own after being taken out of the mouth, allowing the temperature to be read accurately.
• Before Use: The mercury level must be brought down below 35°C (or 94°F) by giving it a few jerks.
• Precautions:
  • Wash the thermometer before and after use, preferably with an antiseptic solution.
  • Ensure the mercury level is below 35°C before use.
  • Do not hold the thermometer by the bulb while reading.
  • Do not place it in hot flames or direct sunlight as it might break.

2. Laboratory Thermometer

• Purpose: Used to measure temperatures in laboratories for experiments, such as measuring the temperature of water, chemicals, etc.
• Range: Typically from -10°C to 110°C. This wider range is needed for various scientific applications.
• No Kink: It does not have a kink. The mercury level drops as soon as it is removed from the substance being measured. Therefore, it must be read while the bulb is still in contact with the substance.
• Precautions:
  • Hold the thermometer vertically while taking a reading.
  • The bulb should be surrounded by the substance from all sides.
  • The bulb should not touch the bottom or sides of the container.

Transfer of Heat

Heat can be transferred from one place to another by three main methods: Conduction, Convection, and Radiation.

1. Conduction

• Definition: The transfer of heat in solids, from a hotter part to a colder part, without any actual movement of the particles of the medium. Heat is transferred through vibrations and collisions of adjacent particles.
• Medium Required: Yes (solids).
• Examples:
  • Heating a metal rod: One end gets hot, and gradually the heat travels to the other end.
  • A metal spoon left in a hot cup of tea becomes hot.
  • Cooking food in a metal pan (heat transfers from the stove to the pan, then to the food).
• Conductors of Heat: Materials that allow heat to pass through them easily (e.g., metals like copper, aluminum, iron).
• Insulators (Bad Conductors) of Heat: Materials that do not allow heat to pass through them easily (e.g., wood, plastic, air, wool, glass). These are used to keep things hot or cold.

2. Convection

• Definition: The transfer of heat in liquids and gases (fluids) from a hotter region to a colder region due to the actual movement of the fluid particles.
• Medium Required: Yes (fluids - liquids or gases).
• Examples:
  • Boiling water: Water at the bottom heats up, becomes less dense, and rises. Colder, denser water from the top moves down to take its place, creating a convection current.
  • Land Breeze and Sea Breeze:
    • Sea Breeze (during the day): Land heats up faster than the sea. The hot air over the land rises, and cooler air from over the sea moves in to take its place, creating a sea breeze.
    • Land Breeze (during the night): Land cools down faster than the sea. The relatively warmer air over the sea rises, and cooler air from over the land moves towards the sea, creating a land breeze.
  • Hot air balloons rise because the air inside the balloon is heated, becomes less dense, and creates an upward buoyant force.
  • Placement of air conditioners (near the ceiling, as cold air is dense and sinks) and heaters (near the floor, as hot air is less dense and rises).

3. Radiation

• Definition: The transfer of heat without any medium, through electromagnetic waves. It can occur even in a vacuum.
• Medium Required: No.
• Examples:
  • Heat from the sun reaching the Earth.
  • Heat from a campfire or a burning candle felt even without touching it.
  • Heat from a heater warming a room.
• Effect of Surface Color on Radiation:
  • Dark-colored surfaces: Good absorbers and good emitters of heat radiation. This is why we prefer wearing light-colored clothes in summer and dark-colored clothes in winter.
  • Light-colored surfaces: Poor absorbers and poor emitters of heat radiation.

Specific Heat Capacity

Specific heat capacity is the amount of heat energy required to raise the temperature of a unit mass of a substance by one degree Celsius (or one Kelvin). Different materials have different specific heat capacities.

• Water has a very high specific heat capacity compared to most other substances (e.g., sand). This means water takes a long time to heat up and cool down.
• Applications: This property of water is why large water bodies (like oceans) moderate the climate of coastal areas, making them less extreme than inland areas. It also makes water an excellent coolant.

Effects of Heat

1. Thermal Expansion

Most substances expand (increase in size) when heated and contract (decrease in size) when cooled. This is due to the increased kinetic energy of molecules, causing them to move further apart.

• Expansion in Solids:
  • Railway tracks have small gaps left between sections to allow for expansion in summer, preventing buckling.
  • Rivets used in construction are heated before being inserted into holes and then allowed to cool, causing them to contract and hold the plates tightly.
  • Bimetallic Strip: Made of two different metals (e.g., iron and brass) bonded together. Since different metals expand at different rates, a bimetallic strip bends when heated or cooled. This property is used in thermostats, fire alarms, and automatic switches.
• Expansion in Liquids: Liquids expand more than solids for the same rise in temperature (e.g., mercury or alcohol in a thermometer).
• Expansion in Gases: Gases expand the most for the same rise in temperature (e.g., hot air balloons).

2. Change of State

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

• Melting: Solid to liquid (e.g., ice to water).
• Boiling/Evaporation: Liquid to gas (e.g., water to steam).
• Condensation: Gas to liquid (e.g., steam to water droplets).
• Freezing: Liquid to solid (e.g., water to ice).

Summary of Key Concepts

• Heat: Form of energy flowing from hotter to colder objects.
• Temperature: Degree of hotness or coldness, measured by a thermometer.
• Thermometers: Clinical (35°C-42°C, has kink) and Laboratory (-10°C-110°C, no kink).
• Heat Transfer Methods:
  • Conduction: In solids, no particle movement, via vibration (e.g., metals).
  • Convection: In fluids (liquids/gases), with particle movement (e.g., boiling water, land/sea breeze).
  • Radiation: Without medium, via electromagnetic waves (e.g., sun's heat).
• Conductors: Allow heat to pass (metals).
• Insulators: Do not allow heat to pass (wood, air, plastic).
• Specific Heat Capacity: Amount of heat to raise 1 unit mass by 1°C. Water has high specific heat.
• Effects of Heat:
  • Thermal Expansion: Most substances expand on heating (solids < liquids < gases).
  • Change of State: Melting, boiling, condensation, freezing.