3.18 Energy and force – what’s the difference?

We were discussing heat pumps – as you do these days. How times have changed – I remember hearing the phrase for the first time in a very remote corner of  my physics degree decades ago. Today it’s common parlance amongst plumbers and home improvers. A good thing, too. Members of a science discussion group were struggling with the seemingly nonsensical idea of getting heat out of the cool earth or air to warm up your home. That’s what heat pumps do; that’s why they are called ‘pumps’. Tyre pumps take air from a low pressure environment (the atmosphere) to a high pressure one (the tyre). Dutch windmills pump water from low-lying fields, against gravity, up to sea level. Heat pumps take heat energy from a low temperature environment and ‘pump’ it up to a higher temperature one.

At the heart of the conceptual difficulty lies an important issue in physics: distinguishing heat from temperature. It’s not helped by an inherent confusion in the English language: the word ‘hot’ – an adjective relating to temperature – sounds so close to the word ‘heat’ – a noun for a form of energy.

Heat pumps

You can imagine a tepid bath – it’s at a fairly low temperature. But think of all that hot water that went into: it contains quite a lot of heat energy, and you paid for it through your energy bill. All energy costs, one way or another. By contrast imagine a spark from a sparkler. It’s temperature is so high that it gives off a bright light – over 1000°C in fact. Yet if it lands on your skin it causes no harm because it contains very little heat energy due its small size. If there’s a lot of heat energy in a bath compared to a spark, imagine how much more there must be in the air or earth around your house. That’s where heat pumps get the energy to heat up your rooms; and it renews itself endlessly from the surrounding air and earth.

Admittedly this large supply of energy close to home comes at a pretty low temperature, especially outside the summer months. By analogy you could say the air that pumps up your bike or car tyre comes at a pretty low pressure – from the air all around. So you need a heat pump to raise the temperature of the heat energy you draw in from outside.

To do this it acts on the same principle and similar technology as your fridge. A fridge extracts heat energy from within its cabinet and pumps it out at a higher temperature into your kitchen through the black tubing at the back. So to by placing a kind of fridge carefully in relation to your external walls, you can extract the heat from outside – like having the interior cabinet of the fridge outside – and the part that gives off heat (the back of the fridge) remains indoors.


At this point discussion in the group might well have followed up the technological question of how a fridge or heat pump works: how it achieves this astonishing feat of raising or lowering the temperature of air. But, on this occasion it didn’t. What really got people talking was the deeper question of what we actually mean by ‘energy’. After all, it’s such a familiar word – we have to pay for it every month, at increasing cost; and we worry about where it’s all going to come from in the longer term, as oil and coal cease to be acceptable sources.

People in the group felt pretty hazy about what energy actually was despite it being such an everyday word – and exactly how it differed from the word ‘force’. The origin of the two words makes a useful starting point. Etymologically, the word ‘energy’ (from the ancient Greek) is associated with activity or work and ‘force’ with strength (from Latin). You might describe one person as energetic and another as forceful: the scientific meanings are quite close to these everyday connotations.


Force is used in science to refer to a push or pull on an object that can speed it up or slow it down, or make it change direction or change its shape. In other words it’s very close to our everyday understanding of the word. What’s a bit trickier to really grasp is that forces are not only exerted when things are in contact – like pulling on a rope, or hammering in a nail – but also when they are remote from one another. It seems magical when, as a child, you first become aware of a magnet’s ability to attract a pin some distance away, or a comb’s tendency to cling on to fine dry hair.

In Newton’s day, in the seventeenth century, it was thought ridiculous to claim that an invisible force called gravity, operating over vast distances, could hold the moon in its path around the Earth or the planets round the Sun. Today we take this for granted for practical purposes, but it remains very difficult to grasp intuitively.  

To cope with this imaginative difficulty scientists adopted the word ‘field’ as a metaphor to describe the zone in which one object experiences a force due to another. We say the Moon lies within the gravitational field of the Earth (and vice versa); a fridge door within the magnetic field of a door magnet; a lock of hair within the electrostatic field of a charged-up comb. So, forces are exerted on objects – by other objects or fields. Our bodies press down under gravity on a sandy beach, electric motors drive our washing machines and hefty cables pull up our lifts/elevators.


Our concept of energy is quite a different matter. Energy is an abstraction used to keep an account of changes we observe in systems. If that sounds vague, it’s meant to. Energy is notoriously difficult to define as a generalisation; it’s more readily described in particular situations – that’s how the concept is usually taught at school.

There are the kinds of energy: heat energy, nuclear energy, chemical energy, electrical energy, kinetic energy, potential energy…. but what is it that makes them all ‘energy’? The formal definition in physics is “the capacity to do work” which means the capacity to make a force move through some distance. This is easy to imagine in a mechanical context like the force driving a piston up and down in a car engine or the work you do in pushing a child’s swing through a certain distance. It’s less easy to conceptualise in the chemical, electrical or nuclear case.

In these situations we are aware of energy as it manifests itself in a transformation. A jet of gas bursts into flame on your hob, a vacuum cleaner jumps into action or a bomb explodes.

This image is of natural gas burning in a crater in Turkmenistan.

This idea of transformation underlies the deeper meaning of energy: it is an abstract quantity – a number – which remains the same throughout any transformation. This is known as the law of the conservation of energy; you may recall it being drummed into you at school: “energy is neither created nor destroyed”. So it’s no use trying to conjure up a physical image for energy: it’s a mathematical abstraction. Like the concept of money, it may take on physical appearances like cash or numbers on a bank statement, but in the end it’s an abstract human invention. Unlike money, however energy is always conserved!

Energy transformations

So, to get an intuitive feel for energy we need to look at particular manifestations of this abstraction. This is of enormous importance today as the existential threat to our planet comes largely from the damaging ways in which we transform energy. Let’s start with the problematic fossil fuels. As children learn at primary school, coal and oil are formed over very long periods of time from partially decayed plant material. The source of energy is the vegetation from which the fuel was formed.

So, for the vast and wide-ranging industries that depend on oil, gas and coal, energy from plants is the source. This is the origin of the energy not only for the cars, planes, cookers and heating systems with which we are in daily contact, but also for the less visible combustion processes in the manufacture of cement, steel and plastics and other essentials of modern life. Burning a fossil fuel in the presence of oxygen is what releases energy for so many of the things we do.

In this transformation chemical energy that’s been locked up in biological molecules for millions of years is released in the form of heat. When a full account is made of the energy remaining in the products of combustion (the fumes and ashes for example) plus the energy released as heat, it will be found to be the same as the energy content of the original molecules of the fossil fuel and oxygen.

Earlier energy transformations had of course occurred within the plants that made up the fossil fuel. It was light energy radiating from the surface of the Sun that had fallen on the leaves and propelled the process of photosynthesis. In this transformation, electromagnetic energy in the form of light gets locked up as chemical energy in the carbohydrate molecules that make up much of the plant. These carbohydrates are created from water drawn from the earth and carbon dioxide from the air which react together in the cells of the plant when the Sun shines.

Prior to this, the electromagnetic (light) energy radiating from the Sun had to arise from an even earlier transformation. In this case it is the nuclear reactions occurring in the extremely hot and pressurised core of the Sun that release the light energy that ultimately reaches its surface and radiates away.

Alternative sources of energy

The Industrial Age saw an explosion of interest in energy by innovators in manufacturing and transportation and also by an emerging community of scientists. The transformation of heat energy from fossil fuels into steam, and thence into mechanical energy to drive machines, was at the heart of much of industry. Pistons were forced by high pressure steam entering the cylinders of the engines that turned weaving machines, pumped out coalmines and spun the  wheels of railway locomotives. The subject of thermodynamics was invented to explain the energy transformation from burning furnace to moving piston.

Today however, the long-term cost of this era of industrialisation is plain to see. The carbon dioxide gas produced as a by-product of burning fossil fuels was gradually creating a blanket around our globe, warming our surroundings and leading to the potentially catastrophic consequences we are aware of today. Energy is going to have to come from other sources in the coming decades; different kinds of transformation are required.

Paradoxically, abundant quantities of energy, in many forms, surround us in the natural environment. Waves at sea rise and fall continuously, tossing boats up and down. Tides drive great masses of water up estuaries in a twice daily rhythm. Winds with energy sufficient to uproot trees come and go and sunshine warms our rooves and fields. Energy will need to be captured, transformed and stored for use from these alternative sources as fossil fuels are phased out. Devices to make this happen are continuously being developed and policies to promote them gradually emerging. The transformative technologies appear very different to one another – wind turbines, solar panels, tidal barrages, wave generators, heat pumps – but, in almost all cases, they end up transforming energy into its electrical form. What electrical energy is and how it’s produced is explored in the “Further Reading” section below.


In summary, the story is of energy, deriving ultimately from the very creation of the universe, being transformed though nuclear reactions inside the Sun, reaching the Earth by electromagnetic radiation. Here on our planet, it stirs up the winds, falls on solar panels and is transformed by photosynthesis into the carbohydrates that fuel our human existence. In its ultimate transformation energy departs from the surface of our bodies, the brake linings of our vehicles and the walls of our buildings in its final form, as heat. It radiates away from our warming planet across the boundless realms of outer space. Energy simply shifts from one form to another, without diminishing in quantity, but gradually degrading from its high temperature starting point to its low temperature end. These are the fundamental laws of thermodynamics – energy continually transforming, always conserved in quantity but diminishing in quality.

Not easy to define crisply, energy is manifest in numerous forms. It Is not a substance, has no physical presence, but our ability to understand its manifold transformations and how to influence them will determine the fate of our planet in the decades to come.

© Andrew Morris 27th August 2021

Further reading

Electrical energy

Electrical energy is created when metal wires (often copper) are made to rotate in the presence of a magnetic field. This simple process causes electrically charged particles (electrons) throughout the metal to acquire energy. Power stations of old, as well as the wind farms, tidal barrages and wave generators, all create electrical energy by turning metal wires through a magnetic field. Giant turbines are needed to do this turning and the energy they need for it comes from burning the coal, oil  or gas or more directly from the wind or sea

It’s these giant spinning electrical generators that take up the space inside the hall of a power station or tidal barrage or atop a wind turbine. The role of the wind, the wave or the shifting tide is to cause the mechanical movement of the metal inside the generator.

Photograph of La Rance tidal power station in Brittany by Fabioroques

In this way renewable mechanical forms of energy, rather than the one-off chemical form in fossil fuels, are transformed into the electrical form needed to power our modern lives.

Solar panels (or photovoltaics) create electrical energy in a different way, by using particular semiconductor materials that energise electrons directly by trapping energy from the sunlight falling on them rather than through rotation in a generator. The result is the same – energised electrons.

Using electrical energy

Chemical energy remains locked up in plants, foods and other chemicals until some kind of reaction happens to release it. It is in effect stored until needed. Electrical energy however is delivered at the very instant it is produced. As soon as electrons become energised in a power station, wind turbine or photovoltaic panel, they are energised at every point to which they are connected. As soon as your electric kettle or light bulb is switched on it becomes connected in a continuous line of metal to the sources generating the energy – the power stations and wind farms. The role of the UK National Grid is to adjust the amount of energy produced at any given moment to the demand for it at the that instant and to ensure it is distributed to where it is needed. Power stations are switched on and off during the course of the day in accordance with the pattern of demand.

For many purposes supplying electrical energy through the National Grid does the job. For freely roaming vehicles however – cars, lorries, scooters, bikes and, in the near future aeroplanes – electrical energy has to be stored in the moving vehicle. It’s hard to imagine overhead wires ever gracing the road system as they do the railways. The trick is to store up electrical energy in a battery.

Batteries transform electrical energy in a different way from generators. In place of magnets and spinning wires, they contain a combination of materials (such as lead oxide and sulphuric acid) which, when placed in a circuit, energise the electrons in it. At the flick of a switch they can be connected to a light bulb, mobile phone or vehicle engine which then transform the energy of the energised electrons into the light, sound or mechanical energy they need.

This image is a slice through a typical cylindrical battery (like an AA or AAA in the UK) consisting of alternating layers, rolled up like a Swiss roll.

As we all know from experience, however, the chemical process that drives a battery eventually runs down, rendering the battery flat. Technological advances have now solved this problem thanks to the rechargeable battery. When such a battery is put into charging mode, it consumes energy from power stations rather than delivering energy to a device. Its internal chemical process is reversed until the chemical conditions of the original fully charged battery are restored, as nearly as possible.

As battery technology advances, it is becoming possible for energy generated from renewable sources to be stored in rechargeable batteries powerful enough to supply a motor vehicle for hundreds of miles and even a light aircraft on a shortish journey. In effect energy transformed from mechanical form in wind, or electromagnetic form in sunlight is being transformed into a chemical form and stored inside batteries.

Countless ingenious ideas are being developed to reduce our dependence on fossil fuels. In Melbourne Australia, for example two PhD students have pioneered a way to repurpose depleted car batteries. When finished for use in a vehicle they may still have up to 80% of their original storage capacity so can be put to use. When power stations are idling – at night for example – these batteries can take in energy, and hold it till it’s needed when demand is high, at breakfast time. A tremendous saving in both energy supply and raw materials.