Heat

Temperature is a measure of the degree of hotness of an object. Thermometer is a device used for measuring temperatures. Clinical thermometer is used to measure our body temperature. The range of this thermometer is from 35°C to 42°C. For other purposes, we use the laboratory thermometers. The range of these thermometers is usually from -10°C to 110°C. The normal temperature of the human body is 37°C.

Kelvin or Absolute Scale

On this scale the freezing point of water is 273K and boiling point of water is 373K. The difference of 100° between these temperatures is divided in 100 equal parts. 1K is equal to 1°C. Therefore,

K = 273 + °C

Effects of Heat

When a body is heated changes may occur in some of its properties. These changes are the effects of heat.

Rise in temperature

When a body is heated its temperature increases, that is why, it appears warmer when touched.

Change of state

When heat is supplied to a substance in solid state its temperature rises till at a particular temperature it may change into its liquid state without any further change in its temperature. This characteristic constant temperature at which a solid changes into its liquid state is called melting point of the solid.

Conversion of a solid into its liquid state at its melting point is called change of state from solid to liquid (fusion) and the heat that is transferred to the substance during melting is called Latent Heat of Fusion. Latent heat of fusion of a solid substance is defined as the amount of heat (in joules) required to convert 1 kg of the substance from solid to liquid state at its melting point.

When heat is supplied to a substance in liquid state its temperature rises but there is a possibility that it changes into its vapour state at a constant temperature. The heat supplied in this case is called Latent Heat of Vaporization. Latent heat of vaporization of a liquid is defined as the amount of heat (in joules) required to convert 1 kg of the substance from its liquid to gaseous state at a constant temperature.

Thermal Expansion

Every material (except water which contracts on heating from 0°C to 4°C) expands on heating. The increase in the size of a body on heating is called thermal expansion. 

The expansivity of different materials is normally different.

Consider a metallic bar of length L0 at temperature 0° C. Increase in its length ΔL at a temperature Δt is given by:

ΔL ∝ L0Δt

ΔL = αL0Δt

α = ΔL/L0Δt

Here, α is a constant for the material of the bar and is called as the Linear expansivity of the bar.

The Linear expansivity (or Coefficient of Linear expansion) of a material is defined as the change in length per unit original length per degree celsius rise in temperature. The SI Unit of coefficient of expansion is per kelvin (which is same as per degree celsius in magnitude).

A piece of solid may expand along length, breadth and height simultaneously hence there will be an increase in its volume with temperature.

The Volume expansivity of a material is defined as change in volume per unit original volume per degree celsius rise in temperature.

γ = ΔV/VΔt

Anomalous Expansion of Water

Generally all the liquids expand in volume when they are heated but water decreases in volume when heated from 0°C to 4°C and then after 4°C, it increases in volume on further heating. The volume of a fixed mass of water is minimum at 4°C, i.e. its density is maximum at 4°C with further rise in temperature its density decreases.

Specific Thermal Capacity

When two bodies at different temperatures are kept in contact, heat is transferred from the hot body to the cold body till both of them acquire the same temperature. The two bodies then are called in thermal equilibrium. In acquiring thermal equilibrium the hot body loses heat and the cold body acquires an equal amount of heat.

Heat transferred from a hot body to a cold body is directly proportional to their temperature difference.

Q ∝ Δθ

Similarly, if the mass of cold body is more it will absorb more heat from the hot body.

Q ∝ m

Q ∝ mΔθ

Q = msΔθ

Where s is a constant of proportionality which depends on the nature of the material of the body. This is also called as the specific heat capacity of the material.

The specific heat capacity of a material is defined as the amount of heat (in Joule) required to raise the temperature of 1 kg mass of that material through 1 K. The SI Unit of specific heat capacity is J kg-1 K-1.

Higher the value of specific heat of a substance lower will be the rate at which it is heated or cooled as compared to the substance of lower specific heat under identical conditions.

Latent Heat

Latent Heat of Fusion

The amount of heat required to change unit mass of a solid into the liquid at a constant temperature is called latent heat of fusion.

Latent Heat of Vaporization

The amount of heat required to change unit mass of a liquid into vapour at a constant temperature is called the latent heat of vaporization of that liquid.

Transfer of Heat

The heat flows from a body at a higher temperature to a body at a lower temperature. There are three ways in which heat can flow from one object to another:

  1. Conduction
  2. Convection
  3. Radiation

In solids, the heat is transferred by conduction. In liquids and gases the heat is transferred by convection. No medium is required for transfer of heat by radiation.

Conduction

This is the mode of transmission of heat in which although the particles of medium take part, but they do not leave their positions. In conduction, heat is transmitted from the hotter part of the medium to the colder part through one particle to the other neighboring particle. In solids and mercury, heat is transmitted only by conduction.

Convection

In this mode, the particles of a medium actually move to the source of heat energy, and on absorbing heat energy move away from it, thereby making space for other particles to move to the source of heat. This is called convection. Convection is possible only in case of liquids and gases, but not in case of solids.

Radiation

The transference of heat energy from a hot body to a cold body directly, without heating the space in between the two bodies, is called radiation. Solar energy reaches earth by radiation.

Conductors and Insulators

Conductors

The materials which allow heat to pass through them easily are conductors of heat. For example, aluminum, iron and copper.

Insulators

The materials which do not allow heat to pass through them easily are called insulators. For example, plastic and wood.