# Heat quantity

Heat is the energy transferred to a body during heating or taken from a body during cooling. Heat is a physical quantity that characterizes the transfer of thermal energy from one thermodynamic system to another.

$$Q=cm \Delta T$$

where,

c— specific heat of the substance;
m— mass of the body;
ΔT— temperature change.

The unit of heat in the International System of Units (SI) is the joule. The joule is named after the English physicist James Prescott Joule.

## Heat during Melting/Solidification

Melting is the phase transition from a solid state to a liquid state, and solidification is the reverse transition. Each substance has a specific temperature – melting point, at which it changes from a solid to a liquid state. The substance solidifies at the same temperature at which it melts.

The amount of energy required for melting/solidification (heat Q) is proportional to the mass of the body (m) and depends on the material of the body:

$$Q=\pm \lambda m$$

where,

λ— latent heat of fusion (J/kg);
m— mass of the substance (kg).

NOTE! Use a plus sign in the formula when describing melting and a minus sign when describing solidification.

## Heat during Vaporization/Condensation

Vaporization is the phase transition from a liquid state to a gaseous state, and condensation is the reverse transition. Different substances have different boiling points. The boiling temperature depends on the pressure exerted by the atmosphere (air pressure) – the higher the pressure, the higher the boiling temperature of the liquid.

The amount of energy required for vaporization/condensation (heat Q) is proportional to the mass of the body (m) and depends on the material of the body:

$$Q=\pm Lm$$

where,

L— latent heat of vaporization (J/kg);
m— mass of the substance (kg).

NOTE! Use a plus sign in the formula when describing vaporization and a minus sign when describing condensation.