The sun is the primary source of heat. It is defined as the energy in transit. The rate of its transfer is variable from substance to substance. It can be conducted rapidly through the conductors. Metals such as silver, copper are good conductors. Cloth, wood, air, glass, wool, distilled water, etc are bad conductors. It is conducted very slowly through the bad conductors.
The conductivity of substances is directly proportional to the magnitude of free electrons. When any substance is heated, the free electrons acquire the kinetic energy and thus move towards the remaining cooler locations. The heat energy would be transferred from one atom to the other atom due to their vibrations in the shape of waves. Hence, the atoms do take an active role in the process of transfer of heat. This method of transmission is known as conduction.
The woolen textile materials do contain very fine pore spaces. Both the air and the wool are very bad conductors of heat. Hence, the woollen clothes are being used to maintain the warm during cold seasons. The bricks are bad conductors of heat. Hence, thick walls are constructed in building the cold storage rooms. Metal cookers are fitted with ebonite handles to avoid the heat transfer.
Medium is required to transfer heat by convection. The fluids are heated mainly by this process. The heated particles themselves transfer heat. The liquid or gas particles do move to other locations after acquiring heat. The remaining particles move to their place to acquire heat. Thus the convection currents are set up.
The air which is exhaled does become lighter and thus rises up. It moves out through the ventilators. Similarly, the colder and the fresh air from outside does enter the room to take its place. Thus a convection current would be set up.
The heat does transfer in the form of electromagnetic waves in radiation. Heat radiation can also be termed as infrared rays. Hence, the properties of radiated heat are similar to light. But the wavelength of heat radiation is higher than the light radiation. The radiant heat does travel with the speed of light. Medium is not required in this process of heat transfer. In other words, heat radiation can travel through vacuum.
The heat radiations can travel in all directions, but in straight lines. When a person sits around any fire, he would feel heat all around equally. Heat transfer is the fastest in the case of radiation.
The radiation rate from a body does depend up on the nature, area, and also the temperature of its surface. Different types of surfaces maintained at the same temperature, do emit variable amounts of heat energy.
In a thermos flask, the vacuum between its walls does prevent the transfer of heat either by conduction or by convection. Silver lining does prevent the heat transfer by radiation. It also does reflect back the hot radiations from both outside and inside. A meagre amount of heat would be transferred by conduction through a thin glass wall at the neck and also through the bad conducting cork.