Class 7 Science Notes Chapter 4 (Chapter 4) – Examplar Problems Book

Detailed Notes with MCQs of Chapter 4, 'Heat', from your Science Exemplar book. This is an important chapter, not just for your class exams but also for various government competitive exams, as the fundamental concepts of heat transfer are frequently tested. Pay close attention to the definitions, differences, and applications.

Chapter 4: Heat - Detailed Notes for Competitive Exams

1. Heat and Temperature

• Heat: A form of energy that flows from a region of higher temperature to a region of lower temperature. Its SI unit is Joule (J), though 'calorie' is also sometimes used.
• Temperature: A measure of the degree of hotness or coldness of an object. It indicates the direction in which heat energy will spontaneously flow.
  • Measured using a thermometer.
  • Common units: Degree Celsius (°C), Degree Fahrenheit (°F). (Kelvin (K) is the SI unit, but less common at this level).
• Sense of Touch vs. Temperature: Our sense of touch is unreliable for judging the exact degree of hotness or coldness. An object might feel hotter or colder depending on the temperature of our own body or what we touched previously. Temperature measurement provides an objective value.

2. Measuring Temperature: Thermometers

• Principle: Thermometers work on the principle that substances (like mercury or alcohol) expand on heating and contract on cooling.

• Types:

  • Clinical Thermometer:
    • Use: Measures human body temperature.
    • Range: 35°C to 42°C (or 94°F to 108°F).
    • Key Feature: Has a kink (constriction) near the bulb. This prevents the mercury level from falling back immediately after removal from the mouth, allowing time for an accurate reading.
    • Normal Body Temperature: Approximately 37°C (98.6°F).
    • Precautions: Wash before/after use (preferably with antiseptic), ensure mercury is below 35°C before use, read keeping the level of mercury along the line of sight, handle with care (glass!), do not hold by the bulb while reading, do not use for objects other than the human body, do not wash with hot water.
  • Laboratory Thermometer:
    • Use: Measures temperature in science experiments (e.g., temperature of boiling water, melting ice).
    • Range: Typically -10°C to 110°C (can vary).
    • Key Feature: Does not have a kink.
    • Precautions: Should be kept upright (not tilted), the bulb should be surrounded by the substance whose temperature is being measured, the bulb should not touch the surface of the container, read the temperature while the bulb is in the substance.

• Digital Thermometers: Increasingly common, easier to read, do not use mercury (safer).

3. Transfer of Heat

Heat always transfers from a hotter object to a colder object. There are three modes of heat transfer:

• a) Conduction:

  • Mechanism: Transfer of heat through direct contact, from particle to particle, without the actual movement of the particles from their position (they vibrate).
  • Medium: Primarily occurs in solids.
  • Examples: Heating a metal rod at one end, cooking utensils getting hot on a stove.
  • Conductors: Materials that allow heat to pass through them easily (e.g., metals like iron, copper, aluminium, silver). Used for making cooking utensils, boiler tanks.
  • Insulators (Poor Conductors): Materials that do not allow heat to pass through them easily (e.g., wood, plastic, glass, rubber, air, water). Used for making handles of utensils, covering electrical wires (though that's more for electricity), false ceilings.

• b) Convection:

  • Mechanism: Transfer of heat by the actual movement of the particles of the medium. Hotter, less dense parts of the fluid (liquid or gas) rise, while cooler, denser parts sink, setting up convection currents.
  • Medium: Occurs in liquids and gases (fluids). Cannot occur in solids or vacuum.
  • Examples: Boiling water (hot water rises, cold water sinks), ventilation in rooms (hot air rises and goes out), smoke rising from a chimney, sea breeze and land breeze.

• c) Radiation:

  • Mechanism: Transfer of heat in the form of electromagnetic waves. It does not require a medium and can occur through vacuum.
  • Medium: No medium required (can travel through vacuum, air, etc.).
  • Examples: Heat from the Sun reaching Earth, heat felt near a bonfire or heater, heat radiated by a hot utensil kept away from the flame.
  • Properties: All hot bodies radiate heat. Dark-coloured surfaces are better absorbers and better radiators of heat than light-coloured surfaces. Shiny surfaces are poor absorbers and poor radiators (good reflectors) of heat.

4. Practical Applications

• Sea Breeze and Land Breeze (Convection):
  • Sea Breeze (Daytime): Land heats up faster than the sea. Air above land becomes hot and rises. Cooler air from the sea blows towards the land to take its place.
  • Land Breeze (Night-time): Land cools down faster than the sea. Air above the sea is warmer and rises. Cooler air from the land blows towards the sea to take its place.
• Ventilators: Placed near the ceiling because warm air (which we breathe out) is lighter and rises. Ventilators allow this warm air to escape, facilitating circulation.
• Clothing Choices:
  • Winter: We wear dark-coloured woollen clothes.
    • Dark Colour: Absorbs more heat radiation from the surroundings.
    • Wool: Wool is a poor conductor of heat. The air trapped between wool fibres acts as an excellent insulator, preventing the loss of body heat.
  • Summer: We wear light-coloured cotton clothes.
    • Light Colour: Reflects most of the heat radiation from the sun, keeping us cool.
    • Cotton: Allows air to circulate and absorbs sweat, which evaporates and causes cooling.
• Other Examples:
  • Base of cooking pans often painted black (good absorber).
  • Outer walls of houses sometimes painted white/light (reflect heat).
  • Thermos flask uses vacuum, silvered surfaces, and insulating stopper to minimize heat transfer by all three modes.

Key Takeaways for Exams:

• Know the precise definitions of heat and temperature.
• Understand the differences between clinical and laboratory thermometers (range, kink, usage).
• Clearly distinguish between conduction, convection, and radiation (mechanism, medium, examples).
• Identify conductors and insulators and their uses.
• Explain sea breeze and land breeze based on convection.
• Reason out the choice of clothing based on colour and material properties related to heat transfer.

Multiple Choice Questions (MCQs)

Here are 10 MCQs based on Chapter 4 - Heat, keeping competitive exams in mind:

1. A wooden spoon is dipped in a cup of hot soup. Its other end:
   a) Becomes hot by the process of conduction.
   b) Becomes hot by the process of convection.
   c) Becomes hot by the process of radiation.
   d) Does not become hot because wood is an insulator.

2. Which of the following statements about a clinical thermometer is INCORRECT?
   a) It has a temperature range from 35°C to 42°C.
   b) It has a kink near the bulb to prevent mercury backflow.
   c) It can be used to measure the temperature of boiling water.
   d) It should be washed before and after use.

3. In coastal areas, during the daytime, the wind generally blows from the sea towards the land. This phenomenon is called:
   a) Land Breeze
   b) Sea Breeze
   c) Monsoon Wind
   d) Conduction Current

4. Heat transfer that occurs through the movement of fluids (liquids or gases) is known as:
   a) Conduction
   b) Convection
   c) Radiation
   d) Insulation

5. Stainless steel pans are usually provided with copper bottoms. The reason for this could be that:
   a) Copper bottom makes the pan more durable.
   b) Such pans appear colourful.
   c) Copper is a better conductor of heat than stainless steel.
   d) Copper is easier to clean than stainless steel.

6. Heat from the sun reaches the Earth primarily through which mode of transfer?
   a) Conduction only
   b) Convection only
   c) Radiation only
   d) Both conduction and convection

7. Why are ventilators in rooms usually placed near the ceiling?
   a) To allow cool air to enter easily.
   b) Hot air is lighter and rises, escaping through ventilators.
   c) It is an architectural design with no scientific reason.
   d) To prevent heat loss by conduction.

8. Two identical mugs, one painted black on the outside and the other white, are filled with the same amount of hot water and left in a room. Which mug will cool down faster?
   a) The white mug
   b) The black mug
   c) Both will cool down at the same rate
   d) Cooling depends on the material of the mug, not the colour

9. Which of the following materials is the best conductor of heat?
   a) Wood
   b) Air
   c) Silver
   d) Glass

10. Woollen clothes keep us warm in winter because:
    a) Wool fibres generate heat.
    b) Wool is a good conductor of heat, bringing heat from outside.
    c) Wool fibres trap air, which is a poor conductor of heat.
    d) Wool reflects all the coldness from the surroundings.

Answer Key for MCQs:

1. d
2. c
3. b
4. b
5. c
6. c
7. b
8. b
9. c
10. c

Study these notes thoroughly. Focus on understanding the 'why' behind each concept, as application-based questions are common in competitive exams. Good luck!