Hot objects have a high temperature, which means they have a lot of thermal energy (heat). The more heat an object has, the more quickly it loses it into the surroundings. The unit of temperature is °C (degrees Celsius) and that of heat energy is joule (J).
Ice cubes, for example, melt once they are out of the freezer. This is because they lose their heat energy to the surrounding environment. If you hold an ice cube, your hand gets colder, because heat from your hand goes to the ice cube. Remember: Thermal energy moves from hot to cold, causing the temperature of the hot object to decrease.
When thermal energy flows from object to object, we may have a change of state. The graph shows how temperature changes during changes of state:
While a solid object is exposed to a higher temperature, its temperature increases and, when it reaches the melting point, it melts to become a liquid. While it melts, the temperature remains constant. The temperature then increases again until the boiling point is reached, when the temperature stabilises and the liquid now evaporates to become a gas.
The amount of energy needed to change the temperature of an object depends on its material. This property is called specific heat capacity. Its exact definition is: the amount of energy needed to change the temperature of one kilogram of a substance by one degree Celsius. The unit is joule per kilogram degree Celsius (J/kg °C). The formula is as follows:
energy transferred = mass × specific heat capacity × temperature change
The specific heat capacity of a material is a constant number, which will be given to you in an exam if you need it.
Another property substances have is specific latent heat. This is the energy needed to change the state of one kilogram of a substance without a change in temperature. Its unit is joule per kilogram (J/kg). Have a look at this formula:
energy transferred = mass × specific latent heat
The specific latent heat of a material is a constant number, which will be given to you in an exam if you need it.