Heat – Notes for CBSE Class 7 Science

Introduction to Heat

Heat is a form of energy that flows from one object to another due to a difference in temperature. It plays a vital role in our daily lives, from cooking food to keeping us warm. In this chapter, we explore how heat is measured, transferred, and affects different materials.

1. Heat and Temperature

• Heat: Heat is the energy transferred between objects because of a temperature difference. It flows from a hotter object to a cooler one until both reach the same temperature.
• Temperature: Temperature is a measure of how hot or cold an object is. It determines the direction of heat flow.
• Difference: Heat is the total energy, while temperature is the degree of hotness or coldness.

Units of Measurement:

• Temperature is measured in degrees Celsius (°C) or Kelvin (K) in the SI system.
• Heat energy is measured in joules (J), but in daily life, we often refer to temperature to understand heat effects.

Thermometer:

• A thermometer is an instrument used to measure temperature.
• Clinical Thermometer: Used to measure human body temperature. It has a range of 35°C to 42°C. Normal body temperature is about 37°C.
• Laboratory Thermometer: Used for experiments, with a range of -10°C to 110°C. It has no kink (unlike clinical thermometers) and is not used for body temperature.

How to Use a Clinical Thermometer:

1. Wash the thermometer with an antiseptic solution.
2. Shake it to bring the mercury level below 35°C.
3. Place it under the tongue or armpit for 1 minute.
4. Read the temperature by observing the mercury level.
5. Wash it again after use.

2. Hot and Cold Objects

• We can sense whether an object is hot or cold by touching it, but this is not always accurate.
• Example: Metal feels colder than wood at the same temperature because metal conducts heat faster from our body.
• A thermometer provides a reliable measurement of temperature.

3. Transfer of Heat

Heat flows from a region of higher temperature to a region of lower temperature. There are three main ways heat is transferred:

a. Conduction:

• Conduction is the transfer of heat through a material without the movement of the material itself.
• It occurs mainly in solids.
• Conductors: Materials that allow heat to pass through easily (e.g., metals like copper, iron).
• Insulators: Materials that do not allow heat to pass through easily (e.g., wood, plastic, wool).
• Example: When you heat one end of an iron rod, the other end becomes hot due to conduction.

Activity: Take a metal spoon and a wooden spoon. Dip them in hot water. The metal spoon becomes hot quickly (conductor), while the wooden spoon does not (insulator).

b. Convection:

• Convection is the transfer of heat by the movement of a fluid (liquid or gas).
• It occurs in liquids and gases.
• Example: When water is heated in a pan, the hot water rises, and cooler water sinks, creating a convection current.
• Natural Convection: Occurs naturally due to density differences (e.g., sea and land breezes).
• Forced Convection: Caused by external means like a fan.

Activity: Heat water with a few crystals of potassium permanganate. The colored streaks rise and spread, showing convection currents.

c. Radiation:

• Radiation is the transfer of heat without any medium (solid, liquid, or gas).
• Heat from the sun reaches Earth through radiation.
• All hot objects emit heat radiation (infrared rays).
• Example: We feel warm sitting near a fire due to radiation.
• Dark vs. Light Surfaces: Dark-colored objects absorb more heat (e.g., black clothes in summer feel hotter), while light-colored objects reflect heat (e.g., white clothes keep us cool).

Activity: Hold your hand near a hot object (not touching it). You feel the heat due to radiation.

4. Temperature Scales

• Celsius Scale: Most commonly used, where water freezes at 0°C and boils at 100°C.
• Kelvin Scale: Used in scientific research, where 0 K is absolute zero (-273°C). To convert: K = °C + 273.
• Example: 37°C (normal body temperature) = 37 + 273 = 310 K.

5. Effects of Heat

When heat is added to a substance, it can:

• Increase the temperature (e.g., water becomes hotter).
• Change the state (e.g., ice melts into water).
• Expand the substance (e.g., a metal rod becomes longer when heated).

Expansion and Contraction:

• Most materials expand when heated and contract when cooled.
• Example: Gaps are left in railway tracks to allow expansion in summer; otherwise, they could bend.

6. Clothing and Heat

• Summer: We wear light-colored, loose cotton clothes to reflect heat and allow air circulation.
• Winter: We wear dark-colored, thick woolen clothes to absorb heat and trap air (an insulator) to keep warm.

7. Experiments and Observations

Experiment 1: Conduction:

• Take a metal rod and heat one end with a candle. Attach paper clips with wax at intervals. The clips fall off one by one as heat conducts through the rod.

Experiment 2: Convection:

• Heat water in a beaker and drop a paper spiral above it. The spiral spins due to rising hot air (convection current).

Experiment 3: Radiation:

• Place two identical containers—one painted black, one white—in sunlight with thermometers inside. The black container shows a higher temperature as it absorbs more heat.

8. Applications of Heat Transfer

• Cooking: Pans (conduction), boiling water (convection), and grilling (radiation).
• Thermos Flask: Uses insulation to prevent heat loss by conduction, convection, and radiation.
• Ventilation: Hot air rises and escapes through vents, keeping rooms cool.

9. Key Terms

• Thermal Equilibrium: When two objects reach the same temperature, and no more heat flows between them.
• Absolute Zero: The lowest possible temperature (0 K or -273°C), where particles stop moving.
• Mercury: A shiny liquid metal used in thermometers (now replaced by alcohol in many due to toxicity).

10. Safety Tips

• Handle hot objects with care using insulators (e.g., cloth or mittens).
• Do not touch a thermometer’s bulb after use without washing it.
• Avoid prolonged exposure to extreme heat sources.

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Summary

Chapter 3 – Heat introduces students to the concept of heat as energy and its measurement using temperature. It explains the three modes of heat transfer—conduction (in solids), convection (in fluids), and radiation (no medium)—with practical examples like cooking and weather phenomena. The chapter also covers the use of thermometers, the effects of heat (expansion, state change), and how clothing helps regulate body temperature. Through simple experiments, students learn to observe heat transfer in action, making the concepts relatable and easy to understand.

These notes provide a comprehensive overview of the chapter, ensuring students grasp both theoretical and practical aspects of heat for their CBSE Class 7 exams.

Ch 03 Heat