Introduction:
The chapter “Heat” in Class 7 Science explores the concept of heat, temperature, and how they relate to each other. It introduces the different ways heat can be transferred between objects and the importance of understanding heat for our daily lives. Heat is a form of energy that causes changes in the state of matter, and it is essential for various processes, from cooking food to weather changes.
Key Concepts Covered:
- What is Heat?
- Heat is a form of energy that is transferred from one body to another due to a temperature difference.
- It always flows from a body at a higher temperature to one at a lower temperature.
- Heat causes particles in matter to move faster, which may result in changes in temperature or phase (solid, liquid, gas).
- Temperature and Its Measurement:
- Temperature is a measure of the hotness or coldness of an object, indicating the average kinetic energy of its particles.
- Temperature is measured in Celsius (°C) or Fahrenheit (°F).
- The thermometer is the instrument used to measure temperature. A clinical thermometer is used to measure body temperature, and a laboratory thermometer is used to measure the temperature of substances.
- Thermal Expansion:
- Thermal expansion occurs when the temperature of a substance increases, causing its particles to move faster and spread out, leading to an increase in volume.
- Solids: As solids are heated, their particles vibrate more and take up more space, which leads to expansion.
- Liquids: Liquids also expand when heated, but they expand more than solids.
- Gases: Gases expand the most when heated because their particles are far apart and move freely.
- Transfer of Heat:
Heat can be transferred in three ways:- Conduction: The process of heat transfer through direct contact between molecules. It happens in solids, where particles are closely packed.
- Example: A metal spoon gets hot when placed in a hot liquid because heat transfers from the liquid to the spoon.
- Convection: The transfer of heat in fluids (liquids and gases) by the movement of the fluid itself. Warm fluid rises, and cooler fluid sinks, creating a circulation pattern.
- Example: In a boiling pot of water, hot water rises to the top while cooler water sinks.
- Radiation: The transfer of heat in the form of electromagnetic waves. This does not require a medium, so it can occur in a vacuum (space).
- Example: The heat from the Sun reaches the Earth through radiation.
- Conduction: The process of heat transfer through direct contact between molecules. It happens in solids, where particles are closely packed.
- Heat and Its Effects:
- Change in State: Heat can cause a change in the state of matter. For example, heating water causes it to change from liquid to gas (evaporation).
- Expansion and Contraction: Heat causes materials to expand, and when cooled, they contract. This is important in the construction of bridges, railway tracks, etc.
- Applications of Heat:
- Heat is used in cooking, heating homes, industrial processes, and even in power generation.
- Thermal insulators: Materials like wood, plastic, and rubber are poor conductors of heat and are used for insulation.
- Thermal conductors: Metals like copper and aluminum are good conductors of heat and are used in cooking utensils and radiators.
- Principle of the Thermos Flask:
- A thermos flask is designed to keep liquids hot or cold by minimizing heat transfer. It uses a vacuum between two walls to prevent heat loss by conduction, convection, and radiation.
Important Questions with Answers:
- What is the difference between heat and temperature?
- Answer: Heat is the energy transferred between bodies due to a temperature difference, while temperature is the measure of the average kinetic energy of particles in a substance.
- How is heat transferred in solids?
- Answer: Heat is transferred in solids by conduction, where particles vibrate and pass on their energy to adjacent particles.
- What is thermal expansion?
- Answer: Thermal expansion is the increase in the volume of a substance when it is heated due to the faster movement of its particles.
- What are the three methods of heat transfer?
- Answer: The three methods of heat transfer are conduction, convection, and radiation.
- Explain the role of convection in the atmosphere.
- Answer: In the atmosphere, convection causes warm air to rise and cool air to sink, leading to the formation of winds and weather patterns.
- Why do metals feel colder than wood at the same temperature?
- Answer: Metals are good conductors of heat, so they absorb heat from your skin faster than wood, making them feel colder.
- How does a thermos flask prevent heat transfer?
- Answer: A thermos flask uses a vacuum between two walls to prevent heat transfer by conduction, convection, and radiation.