What Is Heat Class 7: Complete NCERT Science Explanation

Understanding Heat: Definition and Basics

Heat is a form of energy that flows between two objects because of a temperature difference. When a hot object touches a cold object, heat energy moves from the hotter to the colder one until both reach the same temperature. This transfer of energy is what we call heat.

In Class 7 NCERT Science, heat is introduced as an important energy form that causes changes in temperature and state of matter. It is measured in joules (J) or calories (cal), but temperature is measured in degrees Celsius (°C).

Key points:

• Heat always flows from higher to lower temperature
• It is a form of energy, not a substance
• Heat causes changes in temperature and state (like melting or boiling)

Temperature vs Heat: What’s the Difference?

Many students confuse heat and temperature, but they are different concepts:

Example: A small cup of hot water and a large bucket of warm water may have different heat amounts but different temperatures. Heat depends on quantity and temperature difference, temperature is just a measure.

Methods of Heat Transfer Explained

Heat can be transferred in three main ways:

1. Conduction: Transfer of heat through a solid without the movement of the material itself. Example: Heating one end of a metal rod makes the other end hot.

2. Convection: Transfer of heat by the movement of fluids (liquids or gases). Example: Warm air rising and cool air sinking creates convection currents.

3. Radiation: Transfer of heat through electromagnetic waves without needing any medium. Example: Heat from the Sun reaches Earth by radiation.

Each method plays a role in daily life and natural processes. For example, conduction heats a spoon in hot tea, convection causes wind, and radiation warms us from sunlight.

Effects of Heat on Matter: Changes in State

Heat energy can cause matter to change its state:

• 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)

When heat is added, particles gain energy and move faster, leading to melting or boiling. When heat is removed, particles lose energy, causing freezing or condensation.

Example formula related to heat and state change:

$$ Q = m imes L $$

Where:

• $Q$ = heat energy (in joules)
• $m$ = mass of substance (in kg)
• $L$ = latent heat (specific to the substance and change of state)

This formula helps calculate the heat required for melting or boiling without changing temperature.

Measuring Temperature: Thermometers and Scales

Temperature is measured using thermometers. The most common scale in India is Celsius (°C).

• Mercury or alcohol thermometers expand when heated, showing temperature rise.
• Digital thermometers use sensors to measure temperature electronically.

Important points:

• Normal human body temperature is about 37 °C
• Water freezes at 0 °C and boils at 100 °C at standard atmospheric pressure

Thermometers are essential tools in science and daily life to measure heat intensity indirectly.

Practical Examples: Calculating Heat Transfer

Let’s solve a simple example to understand heat transfer:

Example: How much heat is required to raise the temperature of 2 kg of water from 20 °C to 80 °C? (Specific heat capacity of water = 4200 J/kg°C)

Solution:

Use the formula:

$$ Q = m imes c imes riangle T $$

Where:

• $m = 2$ kg
• $c = 4200$ J/kg°C
• $\triangle T = 80 - 20 = 60$ °C

Calculate:

$$ Q = 2 imes 4200 imes 60 = 504,000 ext{ J} $$

So, 504,000 joules of heat is needed.

This formula is useful in many practical problems involving heat and temperature changes.