Heat Energy and Transfer

Heat Energy

Heat and Temperature are things that everyone is familiar with. They are also something that in everyday language, may not match the science explanation.

The following is the science definition for:-

  1. Heat - Heat is a type of Energy, it is measured in Joules (J).

  2. Temperature - Temperature is a measure of how hot or cold an object is, it is measured in °C.

The diagram below shows an example of why temperature is not always the best way to understand heat:-

Note - Sparklers are dangerous. Even though each spark has only a small amount of heat Energy, the sparkler itself can cause serious burns or start fires. Sparklers are classed as fireworks and as such are illegal to buy if under 18 under the Fireworks Act 2003. They should only be used in direct adult supervision.


Heat Transfer

Heat Energy does not stay in one place, but will move from place with lots of heat Energy to places with less. This is why a cup of coffee will cool down over time, and why ice cream melts on a hot day.

There are three main types of heat transfer:-

  1. Conduction - Heat moving through a solid, by causing particles to vibrate more.

  2. Convection - Heat moving through a liquid moving in a convection current.

  3. Radiation - Heat moving as electromagnetic radiation (Infra-Red Radiation).

When Iron is heated in a blacksmith's furnace, the Iron starts to glow, first a dull red, then becoming a brighter yellow colour. This happens even in parts of the metal bar that is not in the furnace. Somehow, the heat Energy can travel through the metal. Eventually, the blacksmith would burn their hand!

The video below shows a blacksmith heating metal to forge a sword.

In the video above, the metal turned red in parts that had not been in the furnace. This means that the heat Energy was moving through the metal. The blacksmith must wear protective gloves as even though parts of the metal are not hot enough to glow, the whole bar will soon become very hot. This is because of the heat transfer process called Conduction.

The diagram below shows how heat Energy can be transferred by Conduction:-

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The molecules that make up a metal bar (or any solid) are tightly packed together, vibrating around a fixed point. When one end is heated, it makes the molecules vibrate more. This causes the molecules next to these to vibrate more, and so on, transferring the heat Energy along the bar. This is called Conduction.


Conductors and Insulators

Metals are good conductors of heat Energy. This means that heat Energy easily can move through a metal (as described above). Not all materials let heat Energy move through them easily, however. Materials which do not let heat Energy move through them easily are called Insulators.

Substances that allow heat Energy to pass through them easily are called Conductors.

Substances that do not allow heat Energy to pass through them easily are called Insulators.

The table below shows some examples of Conductors and Insulators:-

Convection

In a solid, the molecules are close enough together for heat Energy to be passed from atom to atom, but this is not the case in liquids and gases. As the molecules can move around, there is another way to transfer heat Energy, known as Convection.

The diagram below shows the process of Convection:-

When a fluid (a liquid or a gas) is heated, it becomes less dense. As it is now less dense than the fluid around it, the warm fluid rises. As it rises it begins to cool and become more dense, eventually sinking back down. This causes a convection current in the fluid.

Examples of Convection

Convection currents can be found anywhere there is a fluid that is being heated. Some examples include:-

  1. Central heating within the home.

  2. Air thermals.

  3. Plate tectonics.

  4. Lava lamps.

Central Heating

The central heating systems within homes use the process of convection to heat rooms.

The radiators heat the air above them,the hot air becomes less dense than the air around it, and so the hot air rises. After a short time, the air cools, becoming more dense until it sinks back down again.


Air Thermals

Birds (and glider pilots) can use the process of convection to gain height without having to do any work.

The Sun heats the ground, which causes the air at ground height to become warm. The warm air becomes less dense than the air around it, and so the warm air rises. A bird can enter this warm air as it flows upwards, and 'ride' these thermals to a greater height.


Plate Tectonics

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The surface of the Earth is not stationary, but moving all the time. This is because the plates that make up the Earth's surface are floating on molten rock called Magma. When magma is heated by the Earth's core, The hot molten rock becomes less dense than the surrounding rock and so the magma rises. When this magma reaches the surface, it pushes along the underside of the solid crust, dragging the crust along with it. This causes the plates to move apart from each other.

The video below shows a type of lamp called a Lava Lamp. A lava lamp is a lamp filled with water and a special type of wax.

In the above video, the fluid within the lamp is heated by a bulb in the base. After a short time, the wax melts. This molten wax is less dense than the water around it, and so the wax rises. After a short time, the wax cools, becoming more dense until it sinks back down again.

Radiation

The two processes above (Conduction and Convection) explain how heat Energy moves through materials. There is a way for heat Energy to move without a material, however. Heat Energy can be transferred from place to place as a type of Electromagnetic radiation called Infra-Red radiation.

Heat can be transferred as Infra-Red radiation. This Infra-Red radiation is part of the Electromagnetic spectrum and travels in straight lines at the speed of light. Unlike conduction and convection, Infra-Red radiation does not need a material to move through.

Uses of Infra-Red Radiation

Infra-Red radiation is invisible to our eyes, but we can sense it through our skin as heat. We can, however, use special cameras called thermal imaging cameras to 'see' this radiation.

The image below shows a person's face using a thermal imaging camera:-

In the image above, the colours have been added to the image to show the different amounts of Infra-Red radiation being given off by each part of the face. The cooler hair, ears and nose 'look' purple, with the warmer mouth and skin 'looking' yellow and red.

Doctors can use these Thermograms to detect infections or tumors, because as they are warmer than surrounding tissue they have a different colour on the Thermogram.

Thermal imaging cameras can also be used by the police to find people at night. It is very hard to find someone in the dark, but because people give off Infra-Red radiation, they can be easily seen through a thermal imaging camera, even in total darkness.

The video below shows the view through a thermal imaging camera on board a police helicopter:-