Heat Transfer - Conduction, Convection and Radiation

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What Is Thermal Energy?
All matter is made up of tiny particles. Whether matter is in a solid, liquid or gas, these particles are always in motion. How much these particles move is called thermal energy.
The more thermal energy matter has, the higher its temperature. Temperature is how hot or cold matter is. It is a measure of the average kinetic energy of the particles in matter. The faster the particles mover, the greater the temperature.
When matter gains thermal energy, the particles that make up the matter move faster. Its thermal energy increases and its temperature also increases. When matter loses thermal energy, the particles that make up the matter move slower. Its thermal energy decreases and its temperature decreases.
So how does matter gain thermal energy? The movement of thermal energy is called heat. Heat occurs whenever there is difference in temperature.
Let’s look at an example of the heat that occurs when a hot object comes in contact with a cold object. In both objects, the particles are moving - they have kinetic energy. When the objects are touching, their particles collide with each other. As they collide, the hot object passes energy to the cold object. The hot object gets cooler and the cold object gets warmer. Heat will continue to move in this way until the temperature of both objects is equal.
When you place a cold metal spoon into a hot cup of tea, energy moves from the hot tea to the spoon. The tea loses thermal energy and gets cooler. The spoon gains thermal energy and gets hotter. The metal spoon will continue to gain thermal energy from the hot tea until the temperature of the tea and the spoon are equal.
Just as heat moves from hot to cold between objects, it also moves in this way within objects. Let’s look at an example of placing a metal rod over a fire. The heat from the fire will cause the particles at the tip of the rod to gain thermal energy and its temperature increases. These particles then collide with the particles adjacent to them. In this way, heat in transferred from the hot region to the cold region of the metal rod.
Thermal energy moves in three main ways - conduction, convection and radiation.
Think again how heat moved in the metal rod placed on a fire. The thermal energy moved from the hot part of the rod to the cooler part of the rod. This movement of heat within an object is called conduction. Conduction also occurs when objects are touching. If you were to hold the cool end of the rod, before too long it would feel warm as thermal energy moves by conduction to your hand.
Heat transfer from a hot cup to your hands and cooking a piece of fish in a pan are also examples of conduction.
Liquids and gases are similar in that they have no fixed shape. They are fluids. Thermal energy can be transferred from one region of a fluid to another due to movements within the fluid. This movement of thermal energy, called convection, is caused by hot parts of the fluid rising and the cooler parts of the fluid sinking.
Think about the water in a kettle as it is heated. The water touching the heating element is heated by conduction. The water becomes warm and rises. As it does so, the cooler water sinks and a current is created within the water. The current causes the thermal energy to spread through the water in the kettle.
The heating of air in a hot air balloon is also an example of convection.
For thermal energy to move by conduction and convection it must travel through matter. Between the Sun and the Earth is empty space. How does the heat from the Sun warm the Earth and other planets in the solar system? The answer is radiation. Radiation is the transfer of energy through electromagnetic waves.
The warmth you feel when you place your hand near an electric heater or a campfire are examples of heat in the form of radiation.