The answers to all these questions lie in the concept of heat transfer and the three modes of heat transfer. But before we discuss heat transfer, let us take a brief look at the branch of physics known as 'thermodynamics'.
Thermodynamics is the study of relationship between heat and other forms of energy. Thermal equilibrium occurs when macroscopic changes in the system cease to occur with time. If you put ice in a glass of water, the water being at a higher temperature will transfer heat energy to the ice cube. This will continue till the ice cube melts into water which is at the same temperature as the water in the glass. Thus, a state of thermal equilibrium is reached. Thermodynamics is applicable to systems that are in thermal equilibrium. The phenomenon of heat transfer deals with systems that are not in thermal equilibrium.
Heat transfer analyzes the rate of transfer of heat. Temperature difference or temperature gradient between the systems in consideration is a prerequisite for heat transfer. Temperature gradient is defined as the temperature difference per unit length or the rate of change of temperature. Heat transfer in any direction depends on the magnitude of temperature gradient in that direction. Heat transfer is based on the following fundamental laws
Radiation
Radiation refers to the emission of energy in rays or waves. Heat moves through space as energy waves. It is the type of heat one feels when sitting in front of a fireplace or around a campfire. It travels in straight lines at the speed of light. This is the reason that when facing the fire, only the front is warmed. The backside is not warmed until the person turns around. The earth is heated by the sun through radiation. Most of the preheating of fuels ahead of a fire is by radiation of heat from the fire. As the fire front gets closer, the amount of radiant heat received is increased.
When we sit near a bucket of water cold or at normal temperature, you do not feel yourself getting warm, because there is no heat transfer.