Revision: Heat and Energy Physics (English Medium) ICSE Class 9 CISCE

Heat is that form of energy which flows from a hot body to a cold body when they are kept in contact.

The measurement of the quantity of heat is called calorimetry.

One kilo-calorie of heat is the heat energy required to raise the temperature of 1 kg of water from 14.5°C to 15.5°C.

The kinetic energy due to random motion of the molecules of a substance is known as its heat energy.

The sum of the potential energy and kinetic energy of a molecule is called its internal energy.

The total internal energy of all the molecules of a substance is called its thermal energy.

The temperature at which the pressure and volume of a gas theoretically reach zero is called absolute zero.

Temperature is a parameter which tells the thermal state of a body (i.e., the degree of hotness or coldness of the body). It determines the direction of flow of heat when two bodies at different temperatures are placed in contact.

Coefficient of Linear expansion is equal to the change in length of a rod of length 1m when its temperature rises by 1°C.

The change in area per unit original surface area of a two-dimensional body (at 0°C) per unit rise in temperature is called the coefficient of superficial expansion.

The increase in volume of a body per unit original volume (at 0°C) per unit rise in temperature is called the coefficient of cubical expansion.

The increase in the dimensions (length, area, or volume) of a body due to an increase in its temperature is called thermal expansion. Conversely, a decrease in temperature causes contraction.

OR

The increase in the dimensions of a body due to an increase in its temperature is called thermal expansion.

OR

When matter changes its shape, area and volume in response to a change in temperature (i.e., an object expands and becomes larger due to a change in its temperature), this is called thermal expansion.

1. Consider a metallic rod of length l₀ fixed between two rigid supports at T °C.
2. If the temperature of rod is increased by ΔT, length of the rod would become, l = l₀ (1 + αΔT) Where, α is the coefficient of linear expansion of the material of the rod.
3. But the supports prevent the expansion of the rod. As a result, rod exerts stress on the supports. Such stress is termed as thermal stress.

The increase in length per unit original length of a rod (at 0°C) per unit rise in temperature is called the coefficient of linear expansion.

The coefficient of superficial expansion is equal to the change in the area of a rod of area 1m² when its temperature rises by 1°c.

The coefficient of volume expansion is equal to the change in volume of a rod of volume 1m³ when its temperature rises by 1°c.

If the Coefficient of Linear expansion is denoted by α
Coefficient of superficial expansion is denoted by β
And Coefficient of volume expansion is denoted by γ
Then the relation between α, β and γ is stated as
β = 2 α and γ = 3 α

α : β : γ : : 1 : 2 : 3

When a solid is heated and its length increases, the increase in length proportional to the original length and temperature change is called linear expansion.

where α is called the coefficient of linear expansion.

When the area of an object changes with increase in temperature, it is called areal expansion (or superficial expansion).

where β is called the coefficient of areal expansion.

When a solid is heated and its volume increases, the expansion is called volume expansion.

where γ is called the coefficient of volume expansion.

The energy released in nuclear disintegrations in the interior of Earth gets stored deep inside the Earth and is called geo thermal energy.

In nuclear fission, the atom of a heavy element is bombarded by a neutron to form an unstable isotope which further splits into two or more smaller nuclei releasing extra neutrons and large amount of energy and heat.

The heating-up of the earth’s atmosphere due to trapped infrared rays reflected from the earth's surface by atmospheric gases is called the greenhouse effect.

• Heat is the energy that flows from a hot body to a cold body when they are kept in contact.
• The S.I. unit of heat is joule (J), while calorie and kilocalorie are commonly used units.
• One calorie is approximately equal to 4.2 joule, and kilocalorie is used to measure the energy value of foods.

• If no heat flows between two bodies in contact, they are said to be at the same temperature, even though their thermal energies may be different.
• The S.I. unit of temperature is kelvin (K), and absolute zero (0 K) is the temperature at which molecular motion ceases.

• Meaning of Energy Resources: Energy is essential for all activities like heating, running machines and generating electricity.
• Types of Energy Resources: Energy resources are classified into renewable (can be used repeatedly) and non-renewable (get exhausted after continuous use).
• Conventional Sources of Energy: These are traditional sources used for a long time, such as coal, petroleum and natural gas. They are non-renewable, exhaustible and polluting. Hydel power is an exception.
• Non-Conventional Sources of Energy: These are renewable and inexhaustible sources like solar, wind, tidal, geothermal, nuclear and biogas energy.
• Importance for India: Energy resources play a crucial role in India’s economic development, and their proper and efficient use is very important.

1. Working Principle: Wind energy uses the kinetic energy of moving air to rotate the blades of a wind turbine.
2. Electricity Generation: The rotating turbine drives a generator, converting kinetic energy into electrical energy.
3. Energy Transformation:
   Kinetic energy in wind → Kinetic energy in turbine → Electrical energy.
4. Clean Energy: Wind energy is a clean and renewable source with no fuel combustion or air pollution.
5. Limitations: Wind turbines require sufficient wind velocity, which is not available everywhere, limiting widespread use.

1. Working Principle: Hydroelectric power plants use the potential energy of water stored in a reservoir, which converts into kinetic energy as water flows down.
2. Turbine and Generator: The flowing water (kinetic energy) drives a water turbine, which in turn rotates a generator to produce electrical energy.
3. Energy Conversion:
   Potential energy of water → Kinetic energy of flowing water → Kinetic energy in turbine → Electrical energy
4. No Fuel Pollution: Since no fuel is burned in hydroelectric plants, there is no air pollution from combustion.
5. Advantages & Concerns: Hydroelectric power gives uninterrupted generation if water is available, but reservoir storage may submerge land and affect living things in the river ecosystem.

1. Working Principle: Nuclear fission of Uranium-235 or Plutonium releases heat, which is used to convert water into steam.
2. Electricity Generation: The high-pressure steam rotates the turbine, which drives the generator to produce electricity.
3. Energy Transformation: Nuclear energy → Thermal energy → Kinetic energy in steam → Kinetic energy in turbine → Electrical energy.
4. Advantages: No air pollution, efficient source of energy, and less fuel required compared to thermal power plants.
5. Problems: Produces radioactive nuclear waste and any accident can release harmful radiation.

• The greenhouse effect is a naturally occurring phenomenon that heats Earth's surface. Without it, Earth's temperature would be -18°C instead of 15°C.
• Greenhouse gases are transparent to solar radiation but retain and reflect back long-wave heat radiation. Main gases — CO₂ (60%), CH₄ (20%), CFCs (14%), N₂O (6%).
• Earth's surface re-emits heat as infrared radiation. Greenhouse gases like CO₂ and CH₄ absorb this and return heat to Earth's surface — causing the greenhouse effect.
• Rising CO₂ due to the burning of fossil fuels and deforestation intensifies the greenhouse effect, causing global warming.
• Global warming leads to melting of polar ice, rising sea levels, changes in rainfall patterns and loss of biodiversity.