Total Energy of a System
Energy and Thermal Physics

Energy resources and pathways - Physics narrative

Physics Narrative for 11-14

A Physics Narrative presents a storyline, showing a coherent path through a topic. The storyline developed here provides a series of coherent and rigorous explanations, while also providing insights into the teaching and learning challenges. It is aimed at teachers but at a level that could be used with students.

It is constructed from various kinds of nuggets: an introduction to the topic; sequenced expositions (comprehensive descriptions and explanations of an idea within this topic); and, sometimes optional extensions (those providing more information, and those taking you more deeply into the subject). 

The ideas outlined in this subtopic include:

  • Pathways are about power
  • Fuels and resources are both depleted
  • Avoiding representing chains

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Energy resources for society

Energy and Thermal Physics

Energy resources for society

Physics Narrative for 11-14

A variety of energy resources

Western society exploits a wide variety of energy resources such as coal, gas and oil to do useful jobs including heating and lighting your home. Historically people exploited fewer energy resources, and often these resources were less concentrated, meaning that people needed to do more hard work to keep warm and comfortable. An example of this is the development from burning wood to burning coal. Burning coal can shift more energy per kilogram than burning wood, so with coal you do not need to spend so much time collecting the fuel.

Power stations

These days, power stations are the main places where we shift energy for public consumption, using resources such as coal and gas that have easily depleted chemical stores of energy (see episode 01). The technologies that enable this shifting of energy from one store to another have become more sophisticated, allowing much more of the store to be depleted (for example by ensuring complete combustion of coal) and also channelling more of the energy into the target store. Not that this is how the inventors talked of such things: the concept of energy is quite recent, only acquiring prominence in its modern form around 1850. From the power station, electrical connections (via the national grid) then allow the energy to be shifted between stores, well away from the station, to achieve the job we want it to do. This is how most of the useful output of the power station is harnessed.

Not so long ago, warming your house involved moving the coal from the pit into the house, and then burning it. Now it can be burnt in one place, and the results enjoyed elsewhere. However, even in a well designed power station, not all of the energy makes it out via the electrical circuit: some is dissipated locally in thermal stores, warming up the surroundings. One way to make use of this is to warm houses or other working areas close to the power station.

Renewable resources

One difficulty with the greener resources such as solar or wind power is that the technologies are less developed and there is the problem of shifting significant quantities of energy from these resources. For example, both solar power stations and wind turbines require large areas of the Earth's surface to provide the same output as even a moderately sized fossil fuel powered station.

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Energy resources: facts and figures

Energy and Thermal Physics

Energy resources: facts and figures

Physics Narrative for 11-14

Energy dissipation

Power stations are a feature of western society and often a topic of social, scientific and political debate. Few would voluntarily return to a situation where there is less energy at their disposal, yet equally few welcome new power stations in their own back yard. Current lifestyles demand the shifting of large quantities of energy from one store to another and in ways that inevitably result in the energy being dissipated. This makes the energy less easily available for future generations.

Here are the energy dissipation figures for the UK over the last few years.

Resources put to use (1990–2002)

This is a comparison of the use of energy resources in electricity generating power stations in the UK in 1990 and 2002. The most striking changes are the reduction in the amount of coal used and the accompanying increase in the consumption of gas.

Resources put to use (1970–2002)

Estimates of the energy resources that are available (that is, stores that can be depleted reasonably easily) vary depending on the technology at that time, and on recent geological exploration. Here are the UK figures for three resources over time.

The changing rates of energy resource usage are worth looking at. The UK government publishes a digest of all the energy figures every July, and these are made available on the gov.uk website.

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Supply and demand

Energy and Thermal Physics

Supply and demand

Physics Narrative for 11-14

Energy supplied to your house – fluctuating demands

Running out of fuel in a car is always very inconvenient, but it's not the end of the world! A short trip to the petrol station to collect a few litres of fuel and you are off again. The fuel is low in mass and releases energy from its store quickly. You can accelerate away to make up for lost time.

Apart from occasional catastrophic failures, the energy supply to your house fortunately does not run out. The national grid controller matches the input to the demands on the system by varying the rate of depletion of energy stores (associated with fuels such as gas, coal and oil). The variation in demand can be enormous.

The biggest fluctuations are associated with breaks in TV programmes. Here are some (relatively recent pickups (short term increases):

04 July 1990 2800 megajoule / second : World Cup Semi Final – England v West Germany (end of extra time)

21 June 2002 2600 megajoule / second : World Cup 2002 Quarter Final – England v Brazil (half time)

12 June 2002 2400 megajoule / second : World Cup 2002 – England v Nigeria (half time)

05 April 2001 2300 megajoule / second : EastEnders

22 November 2003 2100 megajoule / second: Rugby World Cup Final – England v Australia (half time)

In other words, on Saturday 22 November 2003, with a large fraction of the nation watching the Rugby World Cup Final on the TV, there was a huge increase in demand for electricity (an increase of 2100 million joule of energy each second at half time), as everybody made cups of tea to settle their nerves.

Given these fluctuations in demand, power stations that can quickly change their rate of supply are essential, and some of their generating capacity is held in reserve. Hydro-electric and gas powered stations are two such types of station. Nuclear stations are very slow to change their output. Solar, tidal, wind and wave powered electricity generation cannot be varied on demand other than switching in or out what is available at that time.

Nor can energy be easily stored up against demand. One of the few effective ways is to pump water uphill, to be run down through the power station again when the demand is there. Another may be to generate hydrogen, but scientists, engineers and politicians are still working on that one.

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Pathways to and from stores

Energy and Thermal Physics

Pathways to and from stores

Physics Narrative for 11-14

Electric circuits

Electrical loops may be used to shift energy from one store to another. When teaching electric circuits in schools, this aspect of the behaviour of electric current is the first one to be emphasised.

For example, a battery may be used to run an electric motor, in which case energy is shifted from the chemical store of the battery to the kinetic store of the motor (as the motor spins around) and to the thermal store of the surroundings (as they undergo heating).

Power engineers work on a bigger scale: for them it is important to know that a total of 1,078,800,000,000,000,000 joule (1.0728 × 1018 joule) of energy was shifted in the 2001/2002 financial year by the UK national grid. Engineers do track the energy in a system in this way. They consider the ways in which the energy in the stores is depleted or augmented and measure this flow.

As any process occurs (such as lighting a bulb with a battery, or lifting a heavy bag up onto a table), energy is shifting from one store to another. In doing so, we can imagine that the energy is shifting along a pathway. The pathway tells us about the rate of accumulation of energy in stores. It gives hints about about the process or mechanism by which energy is shifted between stores and also how much energy is shifted. For example, in the case of the electric circuit, the bulb lights when the current meets resistance in the filament of the bulb. Here energy is shifted through electrical working along an electrical pathway.

In the case of lifting a bag onto the table, this is achieved by applying a force to the bag as it is lifted through the distance up to the table. Here the energy is shifted through mechanical working, along a mechanical pathway.

Energy and pathways

It turns out that there are a limited number of ways (or pathways) by which energy can be shifted from one store to another. We shall focus on just four:

  1. Electrical working – the electrical pathway.
  2. Mechanical working – the mechanical pathway.
  3. Heating by particle movement.
  4. Heating by radiation – including radiation we can happen to be able to see.

For each pathway you can calculate how much energy has been shifted and this is what enables us to settle on just four pathways (just like the restricted number of energy stores). We will say more about each of these pathways in the following sections.

Identifying pathways

It is worth remembering that the picture of shifting energy along pathways between stores belongs to the energy perspective. You'd never be able to find, and point at, an energy store or pathway (see episode 02). Having said that, each kind of pathway is based on a very real mechanism (such as a current in a resistor, or a force exerted to lift a bag) by which energy is shifted from one store to another.

Selecting pathways

So now we can add the idea of pathways to our descriptions of energy being shifted between stores. This will not always be necessary, but in some situations this description leads to a calculation of the changes in energy. The pathway can give us information about how much energy is being shifted to or from a store.

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Light and sound as pathways

Energy
Energy and Thermal Physics

Light and sound as pathways

Physics Narrative for 11-14

Thinking about light and energy

Light is often seen as a place where energy can be found. Here we show a better way to think about light and energy.

In episode 02 we suggested that it makes more sense to think of lighting as a pathway rather than as an energy store. What does this mean when it comes to describing a simple system from an energy perspective?

Let's take the example of a bulb being lit from a battery. Suppose the bulb is switched on, left on for a few minutes and then switched off. At the point of switching off, the bulb and surroundings are warmer and we can easily describe this process in terms of energy stores.

Energy is shifted from the chemical store to the thermal stores of the bulb and surroundings. Notice that light is not referred to in this energy store description.

Circuits in terms of pathways

We can also describe the process in terms of pathways. When the bulb is connected to the battery, it warms up as energy is being shifted by electrical working (as the current passes through the resistance of the filament).

As the bulb warms up it gives out light and warms up the surroundings, mainly though conduction and convection. Try putting your hand close to a domestic light bulb. You can't feel any heating until very close, which suggests that there is relatively little heating by radiation.

The process in terms of pathways

We can describe this physical process in terms of pathways.

In words, the bulb:

  • Is continuously supplied by the electrical working pathway (as current passes through the resistance of the filament).
  • Continuously supplies the heating by radiation pathway (as the bulb gives out light and other frequencies of electromagnetic radiation) along with the heating by particles pathway (as conduction and convection processes take place around the bulb).

Lighting appears in this description simply because it provides a pathway for shifting energy around. A good light bulb needs to do a lot of lighting and not too much heating. The design of the bulb needs to ensure that the best part of the electromagnetic spectrum accounts for most of the heating by radiation pathway. On the other hand, when designing an electric fire, there needs to be lots of heating and not too much lighting. This time it is the heating by particles pathway that needs to be as big as possible.

Sound as a pathway

Loudspeakers function much like bulbs, enabling you to hear by turning the to-and-fro movements of electrical currents, which encode the sound, into the to-and-fro movements of air. Your ear is affected by these to-and-fro movements, once they arrive after traveling through the medium.

The loudspeaker is working remotely on your ear and only while that is happening can you hear the sound. So hearing a sound is a process. So it cannot be sensibly thought of as an energy in a store, which is as a result something that has happened: sound is happening, or it is not sound.

In everyday life this makes perfect sense: You buy a sound system rated by watts (must be measured over a duration), not joules(atemporal).

Making connections between sound and energy descriptions is best done through power. The sounding is a pathway that empties or fills stores of energy. Sound is not a store of energy.

The power of a sound we hear is tiny, so not very significant in energy descriptions, but it best thought of as filling or emptying stores through the mechanical working pathway.

Teacher Tip: Hearing sounds is about power in pathways—best thought of as the mechanical working pathway, because of the mechanisms of creation and destruction, and the closer link between pathways and mechanism as opposed to the abstract description posed by energy.

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First thoughts about temperature and energy

Temperature
Energy and Thermal Physics

First thoughts about temperature and energy

Physics Narrative for 11-14

Temperature and energy

Here is a simple situation. Put a hot object, say a pasty, in an insulated picnic box, cut off from the outside so that it is near enough isolated. There are no great surprises when you look later, the pasty is cooler, and the air inside the box is warmer. The pasty can no longer be used to warm your hands, but neither can the air inside the box. In some way the energy is less useful. Although appearing in the same store (thermal store of energy), it is shared out more evenly after the pasty has cooled. The same quantity of energy is present, but it is now spread more widely in the energy stores associated with the pasty and surrounding air.

There is a measure of how useful the energy is and that is the temperature of the object. The higher the temperature of an object, the greater is its potential usefulness. A large mass of stuff at a low temperature is of little use in warming things, whereas a smaller mass at a higher temperature is useful (even though both may represent the same total amount of energy in a thermal store). Most useful, of course, are objects of large mass at high temperatures. One spectacular example of this is the exploitation of the hot interior of the Earth in geothermal power stations.

As the pasty cools down, energy is shifted to a wider range of stores, and we say that the energy is dissipated (see episode 02). Dissipation always involves the move to less concentrated, less useful energy stores.

Energy in thermal stores

The same quantity of energy can appear in thermal energy stores in two ways. A large block of matter can be at a low temperature, or a small block of matter can be at a high temperature. The same amount of energy is shared out amongst a few particles or amongst many particles. Temperature is a measure that does not depend on how much of the stuff is present whereas energy is a measure that does. We'll come back to this in more detail in episode 05.

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Renewable and non-renewable energy resources

Energy and Thermal Physics

Renewable and non-renewable energy resources

Physics Narrative for 11-14

Renewable and non-renewable sources

Think of a gas fire at home. As the gas burns, energy is continuously shifted from the concentrated chemical store of the gas to the thermal store of the surroundings as they warm up. Once the gas combines with oxygen, energy from the chemical store is dissipated and there's no getting it back. This image of the continuous dissipation of energy gives us good reason to be careful in our use of energy resources. The full range of energy resources that we use can be classified as renewable or non-renewable. Renewable resources are those that can be replaced within human time scales. Non-renewable resources are those that cannot.

It takes millennia for decaying trees to become oil (a non-renewable resource), yet only decades for trees to grow ready to be harvested as firewood (a renewable resource). Both of these resources ultimately depend on photosynthesis to shift energy arriving from the nuclear store in the Sun to the chemical store in the hydrocarbons and oxygen. Here the pathway that links the Sun with the plants involves the passage of light: the nuclear store is emptied by radiation.

Renewable or not?

Renewable resources are not infinite: they depend on the Sun. For the next 5 billion years or so, the Sun will shift energy by radiation at a fairly steady rate, so depleting its nuclear store. As long as our demands stay within this fixed rate of supply, sensible use can be made of renewable resources. Bio-fuel plantations, wind, wave, solar and hydro-power stations can all be used to divert some energy from the Sun's store for our use.

Tidal flows are somewhat different, drawing on the energy in the kinetic store associated with the orbiting of the Moon around the Earth and both the Earth and Moon around the Sun, so (very gradually) reducing the speed of their orbits.

As outlined earlier, national and international energy demands fluctuate with time. One key consideration in meeting these demands lies in the use of nuclear power. Nuclear power stations are based on the process of nuclear fission. This process involves mining the raw materials from which fuel rods are made: this generates radioactive waste products that will be around for millennia. Fission also happens quite naturally at low rates throughout the Earth's interior wherever the appropriate nuclei are found, providing some of the energy that keeps the Earth warm. This is tapped into by geothermal power stations.

Given that our society is reluctant to live within the rate of replacement of energy resources provided by the sun, an understanding of our consumption of energy resources will continue to be an essential part of the education of every citizen.

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Resources, stores, pathways

Energy and Thermal Physics

Resources, stores, pathways

Physics Narrative for 11-14

Resources are not conserved; energy is conserved

Energy resources are depleted, and are not conserved. Some resources are reckoned as renewable: this depends on the timescale over which the stores is refilled.

Power stations deplete resources at rates depending on their design, and so switch power to the national grid. Energy is shifting from stores at the designed rate, and power in the electrical pathway is switched again by the final 'consumer' to perform some useful task. The national grid is a power distribution system.

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