Revision: Measurement and Effects of Heat General Science SSC (English Medium) 8th Standard Maharashtra State Board

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

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

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

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 measurement of the quantity of heat is called calorimetry.

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

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

"Heat is energy in transit. When two bodies at different temperatures are brought in contact, they exchange heat."

OR

The form of energy which is exchanged among various bodies or a system on account of temperature difference is called heat.

• Units: joule (J), calorie (cal), BTU (British Thermal Unit)

One mole of any substance is the amount of that substance which contains the Avogadro number (NA) of particles (such as atoms or molecules).

"Temperature is a physical quantity that defines the thermodynamic state of a system."

OR

The degree of hotness or coldness of a body, whose natural flow is from higher temperature to lower temperature, is called temperature.

• SI unit: kelvin (K) | Scalar quantity

The specific heat capacity of a substance is the amount of heat energy required to raise the temperature of unit mass of that substance through 1°C (or 1 K).

OR

Heat capacity of a body when expressed for the unit mass is called the specific heat capacity of the substance of that body.

OR

The amount of heat energy required to raise the temperature of a unit mass of an object by 1 °C is called the specific heat of that object.

OR

The amount of heat per unit mass absorbed or given out by a substance to change its temperature by one unit (one degree), i.e., 1°C or 1 K, is called specific heat capacity.

OR

The quantity of heat required to raise the temperature of a unit mass of a gas by one degree, whose exact value depends upon the mode of heating the gas and can range from zero to infinity or even be negative, is called the specific heat capacity of a gas.

The quantity of heat needed to raise the temperature of the whole body by 1°C (or 1 K) is called heat capacity.

OR

The amount of heat ΔQΔQ supplied to a substance to change its temperature from T to T + ΔT, per unit mass per unit degree change in temperature, is called specific heat:

The amount of heat required to raise the temperature of one mole of a substance through a unit degree Celsius or Kelvin is called molar heat capacity.

The heat capacity of a body is the quantity of heat required to raise its temperature by 1°C. It depends upon the mass and the nature of the body.

A calorimeter is a cylindrical vessel which is used to measure the amount of heat gained (or lost) by a body when it is mixed with another body or substance.

Calorimetry is the science of measuring heat exchange during physical or chemical processes. The word comes from the Latin calor (heat) + Greek metron (measure).

OR

An experimental technique for the quantitative measurement of heat exchange is called calorimetry.

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

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

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

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.

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.

\[E_k=\frac{3}{2}k_BT\]

Where:

• E_k = Average kinetic energy of the molecules (in joules)
• k_B = Boltzmann constant = 1.380649 × 10⁻²³ J/K
• T = Absolute temperature (in kelvin)

Q = mcΔT

Where:

• Q = Heat absorbed or released (in joules)
• m = Mass of the substance (in kg)
• c = Specific heat capacity (J/kg·K)
• ΔT = Change in temperature (T_final−T_initial)

\[Q=mc\Delta T\]

Specific heat capacity c = \[\frac{\text{Heat capacity of body } C'}{\text{Mass of the body } m}\]

or

Specific heat capacity c = \[\frac{Q}{m\times\Delta t}\]

C = M × c = Q/(nΔT)

Unit: J/mol · K

Statement: When different parts of an isolated system are at different temperatures, heat transfers from the part at higher temperature to the part at lower temperature. The heat lost by the hot object is equal to the heat gained by the cold object, provided no heat is allowed to escape to the surroundings.

(For liquid in calorimeter: m₁c₁Δθ + m_cc_cΔθ)

Key Points:

• A system is said to be isolated if no exchange of heat occurs between the system and its surroundings.
• Calorimetry literally means measurement of heat.
• Energy supplied by heater = VIt (voltage × current × time).
• This principle is based on the Law of Conservation of Energy.

• 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.

• Heat energy absorbed (Q) depends on: mass (m), rise in temperature (Δt), and specific heat capacity (c), i.e., Q ∝ m × Δt × c.
• Heat capacity (C') and specific heat capacity (c) are related by: C′ = m × c.

• A calorimeter is an insulated device used to measure heat transfer; measurement of specific heat of a substance is carried out using it.
• Principle of Calorimetry: Heat lost by hot body = Heat gained by cold body, which represents the law of conservation of heat energy.
• In the method of mixtures, a heated sample is placed in the calorimeter and the temperature change is measured to calculate specific heat using the formula Q = msΔt.
• Specific heat of a substance depends on the nature of the substance; water is preferred in calorimetry due to its high specific heat, allowing it to absorb large amounts of heat with minimal temperature change.
• For accurate results, the sample must be transferred quickly into the calorimeter and stirred well to ensure uniform heat distribution.