Energy transfer by Heating



States of MatterInfrared radiation is a part of the electromagnetic spectrum. This part is invisible to the naked eye but we can detect it with our skin by feeling heat which makes us feel warm. Everything above absolute zero emits this radiation and the hotter the object, the more it emits. The Sun emits immense amounts and you emit a little. Infrared radiation can travel through a vacuum which is how it gets from the Sun to us through the vacuum of space.

Different surfaces emit and absorb at different rates. Dark matt surfaces are good absorbers and emitters. Light shiny surfaces are good reflectors. For example, if you wear a black t-shirt outside in the Sunshine, you'll feel very hot quickly as the dark matt surface absorbs lots of radiation from the Sun. Likewise, painting a house white in a very hot country helps to reflect more infrared radiation away. Stainless steel tea pots help to keep the tea inside hot as the shiny surface is a poor emitter of infrared radiation.

To understand how heat energy is transferred through different materials, look again at the particles in the three states of matter. In a solid, the particles are in a fixed position but vibrate, more if hot, and less if cold. In a liquid, particles continue to vibrate but also touch each other and can move around so it can flow. In a gas, they vibrate still but are now a great distance apart compared to a solid or a liquid. They are not fixed so can flow and these particles move around randomly at great speed. As the particles are further apart, the gas has a much lower density than the liquid.

Conduction of heat occurs mainly in solids. Gases and liquids are generally poor conductors of heat. If you heat one side of a solid conductor, the particles gain kinetic energy and vibrate more, in turn, they make their neighbours vibrate more and it moves along the solid. This passing of heat energy along the solid by way of passing particle kinetic energy is called conduction.

ConvectionLava lamp convectionConvection occurs in fluids. A fluid is either a liquid or a gas. Basically, when an amount of the fluid gets hotter, the particles move further apart which makes is lower in density than the surroundings. This causes it to rise through the more dense material. Eventually, it will pass on its extra energy to the surroundings causing it to cool and contract and become more dense so it will sink once again. This rising and falling current is called convection. That is exactly what is going on in a lava lamp.

The third type is, of course, radiation which you have already met at the top of the page.

Materials can transform from one state to another by gaining or energy from a heat source of losing it to the environment. Evaporation is when a liquid becomes a vapour (gas). When liquid particles gain enough kinetic energy they escape as a gas (boiling). This energy must have come from somewhere and it comes from the average energy contained in the liquid. If the energy level falls so does the temperature. Evaporation causes cooling. This is why we sweat, as the sweat evaporates, it cools the skin. The rate of evaporation is increased with a hotter liquid, more surface area or more air flow over it.

Condensation occurs when a gas turns back into a liquid. You'll see this on cold windows at night and the rate of condensation is increased by having a bigger surface area of the cold surface and reducing the temperature of that surface. This is why you get more condensation in the frosty winter nights than in summer.

Vacuum flaskThe rate of heat loss depends on differences in temperature, shape, materials and surface area. To Minimise heat loss, paint it white and shiny, use materials that are good insulators and prevent convections by trapping pockets of air like in fluffy clothing and loft insulation. Here the flask is trying to minimise heat loss.
Engine fins coolingTo maximise heat loss, use a good conductor, paint it dull and black and get as much air around it as possible. On this motorcycle engine, heat needs to be released quickly so they have created a large surface area for air to flow over by using fins while keeping it exposed to the air outside and used a metal which is a good conductor. They could further improve it by painting it matt black but that might interfere with the aesthetics.

The final area to be covered in this first section involves the amount of energy require to warm something up. In its simplest terms, we can say that the amount of energy required to warm 1g of water by 1°C is different from the amount of energy required to warm the same mass of iron by the same amount. This difference is due to a substance's Specific Heat Capacity. We can easily work out the amount of energy required to heat something or rearrange an equation to find out by how much something will warm up if we know the amount of energy put in.

Energy in (J) = mass of substance (kg) x Specific Heat Capacity (J/g°C) x temperature change (°C)

Conservation of energy pendulumThere are lots of different forms of energy such as : kinetic, light, sound, electrical, gravitational potential, elastic potential and chemical. The last three are ways in which energy can be stored. Any object above the ground has gravitational potential energy as it may fall and release that energy. Energy can be transferred from one form to another so an object falling towards the ground initially has lots of gravitational potential energy but as it starts to fall, it gains kinetic energy (as it is getting faster) and loses gravitational potential energy (as it is getting closer to the ground). This is an example of one type of energy being transferred into another.

One of the fundamental theories of energy is that it cannot be created or destroyed, it is simply transferred from one type into another as mentioned above. If we swing a pendulum, the energy is never lost despite the speed of the mass at the end changing, look at this diagram and notice that the energy is constantly changing from one type to another and in differing proportions of the two energy types.

Sankey diagrm tradiational light bulbFor another example, as the chemical energy in a battery or cell is used up, that chemical energy has not disappeared, it has been transferred into electrical energy which will be transferred into other types such as heat and light energy in a bulb.

Different types of energy are useful only in certain circumstances, for example, heat energy is really useful in a cooker but not from a conventional light bulb when you only want light energy. So in a machine which transfers energy, we have useful energy - energy in the desired form and wasted energy - energy in a form that we don't want. Wasted energy is spread out into the immediate surroundings which makes it difficult to use again.

In machines that have moving parts, lots of energy is wasted in friction. As parts rub together, they generate sound and heat energy that are unwanted and escape into the surroundings and are not used. This may be reduced by using lubricants.
We measure a machine's efficiency by taking into account how much of the energy that we put in is transferred into an energy type that we want. The formula is very simple:

Efficiency = Useful energy transferred x 100    
                      Total energy supplied

In a traditional light bulb, for every 100J of energy supplied, only 10J becomes light energy the rest is wasted as heat. So the efficiency is 10/100 x 100 = 10% efficient. As efficiency is a ratio, there are no units so the answer could also be 0.1 with nothing after it.

We also represent these calculations in the form of a Sankey Diagram. Energy depicted moving straight across is useful energy and that dropping off the bottom is wasted energy. This diagram represents the bulb that we have just calculated the efficiency of.



Key words and terms for this topic: infrared radiation, emit, absorber, emitter, reflector, solid, liquid, gas, conduction, conductor, free electrons, insulator, convection, fluid, convection current, evaporation, temperature, condensation, temperature difference, maximise, minimise, specific heat capacity, mass, energy transfer, solar panel, kinetic, electrical, gravitational potential, elastic potential, chemical, conservation of energy, machine, useful and wasted energy, friction, joule, input, oputput, efficiency, Sankey diagram.

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