Heat Transfer

Here are three easy things to know about the way heat flows:

1) There has to be a temperature difference. Energy only flows as heat if there is a temperature difference.

2) Energy as heat flows from a higher temperature to a lower temperature.

3) The greater or larger the difference in temperature, the faster the energy flows.

Heat can be transferred from place to place by conduction [conduction: The transfer of heat energy through a material - without the material itself moving. ], convection [convection: The transfer of heat energy through a moving liquid or gas. ] and radiation [infrared radiation: Electromagnetic radiation emitted from a hot object. ]. Conduction and convection involve particles, but radiation involves electromagnetic waves.Dark matt surfaces are better at absorbing heat energy than light shiny surfaces. Heat energy can be lost from homes in many different ways and there are ways of reducing these heat losses.

Conduction

Heat energy can move through a substance by conduction. Metals are good conductors of heat, but non-metals and gases are usually poor conductors of heat. Poor conductors of heat are called insulators. Heat energy is conducted from the hot end of an object to the cold end.

The electrons in piece of metal can leave their atoms and move about in the metal as free electrons. The parts of the metal atoms left behind are now charged metal ions. The ions are packed closely together and they vibrate continually. The hotter the metal, the more kinetic energy these vibrations have. This kinetic energy is transferred from hot parts of the metal to cooler parts by the free electrons. These move through the structure of the metal, colliding with ions as they go.

Convection

Liquids and gases are fluids. The particles in these fluids can move from place to place. Convection occurs when particles with a lot of heat energy in a liquid or gas move and take the place of particles with less heat energy. Heat energy is transferred from hot places to cooler places by convection.

Liquids and gases expand when they are heated. This is because the particles in liquids and gases move faster when they are heated than they do when they are cold. As a result, the particles take up more volume. This is because the gap between particles widens, while the particles themselves stay the same size.

The liquid or gas in hot areas is less dense than the liquid or gas in cold areas, so it rises into the cold areas. The denser cold liquid or gas falls into the warm areas. In this way, convection currents that transfer heat from place to place are set up.

Radiation

All objects give out and take in thermal radiation, which is also called infrared radiation. The hotter an object is, the more infrared radiation it emits.

Infrared radiation is a type of electromagnetic radiation that involves waves. No particles are involved, unlike in the processes of conduction and convection, so radiation can even work through the vacuum of space. This is why we can still feel the heat of the Sun, although it is 150 million km away from the Earth.

Some surfaces are better than others at reflecting and absorbing infrared radiation. If two objects made from the same material have identical volumes, a thin, flat object will radiate heat energy faster than a fat object. This is one reason why domestic radiators are thin and flat. Radiators are often painted with white gloss paint. They would be better at radiating heat if they were painted with black matt paint, but in fact, despite their name, radiators transfer most of their heat to a room by convection.

Radiation or Conduction or Both?

When you stand near hot molten lava, the heat you feel on your skin is mostly radiant heat. This type of heat doesn't need air to travel through. Even if you were standing in a vacuum (no air) you would feel the heat (except you'd be unconscious, or worse, from lack of air).

Almost all of the energy from the sun that travels 93 million miles in 8 and 1/2 minutes through the vacuum of space is radiant energy. When you stand outside on a sunny day feeling the warm rays, remember that only 8 and 1/2 minutes ago it left the sun. Scientists call it electromagnetic radiation. Infrared, ultraviolet, and visible light are examples of electromagnetic radiation that can transfer energy from one object to another object.

Believe it or not, your body and all other objects are always giving off or absorbing heat by radiation. Heat transfer by radiation goes from a hotter object to a cooler object - like from the sun to earth, or from hot coals to you, or from your body to the cold walls of a lonely castle on a dark and stormy night.

Conduction is the type of heat flow that results when things are actually touching. Energy traveling as heat by conduction needs matter to flow through. If you touch a hot object the heat is conducted by physical contact with your skin. The energized atoms in the object transmit their energy to the atoms in your hand. If you are standing in cold air, the heat from your body flows from the molecules in your body into the cold air molecules that are touching your body. If you are floating in cold water, the heat flows from your body into the cold water molecules that are touching your skin. If you fry vegetables on a stove you are relying on conduction to cook your vegetables. Heat from the flame flows through the metal by conduction, into and through the cooking oil by conduction, and into and throughout the vegetables by conduction.

Conduction cannot travel through a vacuum because in a vacuum there are no atoms or molecules making contacting with other atoms or molecules. Something made of atoms or molecules has to touch something else made of atoms or molecules in order for there to be conduction.

Heat from a wood stove is radiated to cooler surfaces like walls, floors, ceilings, furniture, and people.

Energy as heat is also conducted into the air and circulated naturally by a process called convection from the hot metal surfaces to the air surrounding the stove.

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