Conduction
Convection
Radiation
Reflection
Heat is the form of energy transferred between two (or more) systems or a system and its surroundings by virtue of temperature difference. The SI unit of heat energy transferred is expressed in joule (J) while SI unit of temperature is Kelvin (K), and degree Celsius (°C) is a commonly used unit of temperature.
Heat transfer is defined as the movement of heat across the border of the system due to a difference in temperature between the system and its surroundings.
- Conduction is the transfer of heat from the hotter part of a material to its colder part (or from a hot material to a cold material in contact with it) without the movement of material as a whole. For example: The heat from a hot liquid makes the cup itself hot. A snowball in your hand. Using an iron to curl your hair. Melting of butter on warm pancakes.
- Convection is the transfer of heat from the hotter parts of a liquid (or gas) to its colder parts by the movement of the liquid (or gas) itself. Convection cannot take place in solids because the particles in solids are not free to move. For example: Pouring cold water in your hot tea. Baking a cake inside the oven.
- Radiation is the transfer of heat energy from a hot body to a cold body by means of heat rays, without any material medium between them. For example :
- When we stand next to a burning fire, we can feel the heat of the fire falling on our face. The heat is transferred from the fire to our face by the process of radiation.
- When we sit in front of a room-heater, we get heat directly by the process of radiation. The invisible heat rays which transfer heat by radiation are called infra-red rays.
Someone is warming their hands over a bonfire. They don’t have to touch the fire to feel its warmth. How is warmth from the fire transferred to their hands? Click here to read more about it.
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