Heat Transfer: Conduction, Convection and Radiation

Conduction:

  • Conduction is the transfer of thermal energy between particles of matter that are touching.
  • Transfer of thermal energy is called heat
  • Conduction is one of three ways that thermal energy can be transferred.
  • The particles of all matter are in constant random motion
  • Particles of warmer matter have more energy and move more quickly than the particles of cooler matter
  • Thermal energy moves through matter
  • In the example of freshly baked cookies, the cookie sheet transfers thermal energy to the cookies that make them bake
  • Some examples of conduction: Cup of hot drink, a hot iron, camp stove

 

Convection:

  • Convection is the transfer of thermal energy by particles moving through a fluid (either a gas or a liquid)
  • Convection is one of three ways that thermal energy can be transferred
  • When particles in one area of a fluid gain heat, they more more quickly, have more collisions, and spread farther apart
  • Result: decrease in the density of the particles
  • Particles rise up through the fluid, when they transfer their thermal energy to other particles of the fluid and cool off in the process
  • With cooler temperature, the particles sink back to the bottom because of increased density
  • The cycle repeats after, and this cycle is called the convection current
  • Examples of convection: the water cycle

 

Thermal Radiation:

  • Thermal radiation is the transfer of thermal energy by waves that can travel through air or even through empty space
  • Thermal radiation helps the sun’s heat reach earth
  • From a fire, radiation goes through the air and onto your hands, warming your hands
  • Thermal radiation is one of three ways that thermal energy can be transferred
  • Radiation is the only way of transferring thermal energy that doesn’t require matter
  • Everything radiates thermal energy, even objects that aren’t very warm.
Conclusion Questions:
 
Why are metals good conductors of heat?
All types of metal have running free electrons. Electrons are the tiny stuff that make up atoms, that make up most of the world. Metals have molecules and electrons running free within them. When heat reaches them, the heat makes both the molecules and the electrons run wilder. When the electron gets infected by the heat, and it touches a molecule, the molecule would also be start to heat up. With the electrons to speed the heating process up, metals make good conductors of heat.
Why does hot water and air rise?
When water or air is heated, the molecules of the fluid  start moving around much faster, and so the density of the fluid decreases until it becomes less dense then water around them or the air around them. When things are less dense then water, they float. It all follows the Principle of Buoyancy, which is: “Objects immersed in a liquid are buoyed up by a forced equal to the weight of liquid displaced.” This means that when the buoyant force is greater then the force of gravity, then the object would float. Same with heavier objects. When the force of gravity is greater then the buoyant force, then the object would sink. In this case, because of the Principle of Buoyancy, when water or air is heated, it rises or evaporates.
Describe how thermal radiation is transferred from a campfire to you.
Since heat itself is a form of energy, and energy can move through any distance, then it means that heat can also move through any distance. Heat or energy have different ways of releasing themselves into the world, and one way is the thermal radiation. The heat becomes heat waves (if it’s hot enough) or just radiation waves and reach out to anything around it. As soon as it separates far enough from the heat source, then the waves (if it’s not hot enough) would dissolve into the air. If it does hit the object around it, then the object would feel warm because of the heat that the heat source is providing. In this case, the heat source is the campfire and the object that is in it’s range would be you.
Conduction = Solids.
Conduction is the process where molecules inside solids are heated up so that they release heat when something touches it. For example, when you’re stirring something over a stove with a metal spoon, and you leave it there for a few minutes, then you would realise that when you come back, the metal spoon is a lot more hotter. This is because of conduction. Conduction not only can happen to solids’ molecules, but also to liquids or gases. Metal especially is good conductor of heat because of the free electrons that are running all around. Conduction can happen to liquid or gas because when you conduct liquid, you’re warming the particles inside the liquid into gas, on which liquid becomes gas and gas dissolves into the atmosphere.
Convection = Liquid
Convection is mostly to do with the Principle of Buoyancy. When the liquid is heated at the bottom, it causes the hot fluids to rise up because the buoyant forces would be greater then the gravity force (since the density of the fluid has decreased). Then when the fluid starts to go away from the heat, then the gravity force would take over and then dragging it back to the bottom where the cycle starts over again. Convection is best for only liquids and gases, since both of those have rather loose molecules, much more free then solids. So when the hot fluids go up, they don’t get stuck like solids, or rather the molecules cannot exactly move in solid mode. Both liquid and gas are fluids, and convection only really happens for fluids.
Radiation = Gases
‘Radiation’ comes from the Latin word ‘radius’ meaning ‘spoke of a wheel’. Radiation can be strongly seen in deserts, when it’s hot enough to make heat waves ripple in the air. Each source of heat radiates heat, even humans. Radiation only happens mostly through the air. There’s a common question: How does the sun’s heat reach us if the sun is so far away? As we know, heat itself is a form of energy, and energy can move through any distance. So that means heat can move through any distance, which is really how the sun’s heat reaches us. The sun radiates heat just like any other heat source so that the waves reach us. For the heat waves to reach that far, the heat source has to be really strong like the sun. Radiation waves are commonly seen in the air around us, where the energy from one heat source travels in waves to another object.

 

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