Energy, Work, and Power

Podcast Transcript

Everything we know in the world is ultimately dependent on energy. Energy fuels our bodies as well as our civilization. Energy is literally everywhere and all around us.

Yet for the longest time, we had no idea what energy really was. It wasn’t until relatively recently that scientists had a grasp on energy as a concept, and once they did, they unlocked the related concepts of work and power.

Learn more about energy, work, and power, what they are, and how they are different from each other on this episode of Everything Everywhere Daily.

One of the first things that you learn in physics is the nature of Energy. While the concept of energy seems obvious, for most of human history, we lacked a clear understanding of its nature.

The concept of energy originated as a philosophical notion about what drives things to happen and later evolved into a precise, conserved quantity that connects every aspect of physics.

In classical antiquity, Aristotle used terms like energeia to discuss actuality or activity, rather than a measurable substance, so his notion of energy had more to do with metaphysics than mechanics.

In classical Chinese thought, qi is the vital breath or material force that animates and organizes the world, present in heaven, earth, and living beings. It condenses to form things and disperses to dissolve them. This was more of an anatomical concept than a physical one, but it was an early idea of some force responsible for physical animation.

The idea that motion, heat, and chemical energy could all be manifestations of the same thing wasn’t something that anyone had thought of.

Early modern mechanics replaced the ancient outlook with mathematical accounts of motion. Galileo demonstrated that a falling body trades height for speed in a systematic manner, suggesting an underlying principle behind changes in motion.

Seventeenth-century thinkers then argued over what caused it. Rene Descartes proposed a conserved “quantity of motion,” proportional to mass times speed.

At the same time, ChristiaanHuygens and Johann and Daniel Bernoulli discovered that the square of speed governed many problems in collisions and impacts, a quantity Gottfried Leibniz called vis viva, which is equivalent to the product of mass times the velocity squared.

The debate over which magnitude was fundamental set the stage for the modern split between momentum and energy.

The real change in our understanding of energy, as so many things, began with Isaac Newton.

Newton basically established our modern understanding of motion and mechanics. That is why it is called Newtonian physics.

For the purpose of this episode, the important thing he did was to spell out the equation that defines force and define his three laws of motion.

The definition of a force is any action that tends to maintain or alter the motion of a body or to distort it. It could be a push or a pull.

Newton’s famous formula was that Force = Mass x Acceleration. The unit of force that we use today is called the Newton, for obvious reasons.

As we’ll see in a bit, the concept of Force is vital to understanding energy.

One of the things that should be understood is what acceleration means in physics. Acceleration is a change in velocity. Velocity, in physics, doesn’t just mean speed.

Velocity is a vector, which means it consists of a speed and a direction. This is important because you can change velocity by changing speed or by changing direction. If you have ever been in a car that has turned sharply, you have experienced a force, even if your speed didn’t change.

Throughout the 18th and 19th centuries, research was done on a wide variety of subjects, including heat, magnetism, and electricity.

Eventually, it was realized that each of these could be converted into the other. Mechanical motion could be converted to electricity with a generator. Electricity could be used to create motion with a motor, or heat with a resistance coil.

The development of the steam engine also had an important role to play. Engineers had to figure out the efficiencies of steam engines in terms of getting the maximum amount of motion from a given unit of heat.

It was in the 19th century that the term energy began to be used. In particular, the ability of energy to perform work.

Thomas Young popularized the term “energy” in 1807. Jean-Victor Poncelet and Gaspard-Gustave de Coriolis defined and quantified “work” as force through distance for machines, and William Rankine introduced “potential energy” to describe energy of position in 1853, complementing kinetic energy of motion.

As early as the 18th century, the concept of energy conservation was proposed, although again, that name did not refer to it at the time. The French scientist Émilie du Châtelet was one of the first to propose that energy was conserved. This was similar to what Johann and Daniel Bernoulli proposed, the conservation of what was called vis viva.

This idea was eventually codified into the first law of thermodynamics, which states that energy cannot be created or destroyed, only transformed from one form to another.

There is a lot I’ve crammed in here so far, and I’ll probably do a deeper dive into many of these concepts in future episodes.

Now, I want to shift gears a bit and discuss what energy is, as it is understood in modern physics, and how it relates to similar concepts.

The contemporary definition of Energy is that Energy is the capacity to do work.

This then raises the question, what exactly is work?

Work is the result of applying a force to an object, causing it to move. If you push a box across the floor, you’ve done work. In physics, work is defined as force multiplied by the distance moved in the direction of the force.

Something I’ve never really touched on before is the concept of units. Units are extremely important in science, and they separate one measurement from another.

Back to the example of moving a box, work is force x length.

Force is mass x acceleration.

Acceleration is the change in velocity, and velocity is length over time, such as in miles or kilometers per hour.

If velocity is length divided by time, then acceleration would be length over time squared. You’ve probably heard this in the acceleration of gravity being 9.81 m/s² or 32.2 ft/s².

So force is mass x length divided by time squared, and then work would be mass x length squared divided by time squared.

The metric unit of work is called the Joule, named after the English physicist James Prescott Joule.

Because energy is just the potential to do work, energy is measured in the exact same units as work.

Energy is stored until it is used to do work, and there are various ways that energy can be stored.

Kinetic energy is the energy of motion. For a mass m moving at speed v, KE = ½mv². Again, it is expressed in a slightly different way, but mv² is still mass x length² / time².

Gravitational potential energy is based on the acceleration of gravity and the distance an object can fall. It is expressed as mass x the acceleration of gravity x height. Again, the expression is a bit different, but the units are all the same.

There are also various types of energy, including magnetic, nuclear, sound, chemical, thermal, elastic, mechanical, electric, and others.

There are other units of energy that you might be familiar with.

A calorie is also a unit of energy. It is simply a particular amount of energy that is measured in the form of heat.

The calorie is actually a confusing term because there are two different types of calorie units. One is known as the small calories, and the other is the large calories, also known as the kilocalorie.

The small calorie is equivalent to 4.184 Joules, and the kilocalorie is 4184 Joules.

The kilocalorie, or large calorie, is what is usually used on food packaging to measure energy.

There are other units of energy used for various purposes.

The Electronvolt is a unit of microscopic energy used for atomic and particle physics. The elementary charge defines it, so 1 eV = 1.602×10?¹?. It’s really, really tiny.

Another popular non-metric unit of energy is the British thermal unit or BTU.

A BUT is defined as the heat required to raise 1 pound of water about 1 °F.
1 BTU ? 1,055.06 J.

Another rather antiquated unit of energy is the foot-pound. More on that in a bit.

Going back to our example of applying a force to a box to do work. There is something that is missing. Time.

Moving a box one meter over the course of a minute is very different than moving that same box over the course of a second.

That difference in the speed of doing work is called power.

Power is the rate at which work is done, or energy is transferred.

The unit of measure for power is the Watt, which was named after James Watt. The Watt is defined as Joules per second.

Or to put it in units, it is mass x L² / T³.

If you are familiar with the Watt, it is probably because you have encountered the kilowatt, which is, of course, just 1000 watts.

Power companies don’t actually charge you for power as it is defined in physics; they charge you for energy.

This is usually in the form of kilowatt-hours. By multiplying power by time, the time cubed factor becomes times squared, which brings you right back to energy.

A traditional unit of power is the horsepower, which is still often used in engines.

Horsepower was coined by James Watt in the late 1700s to market his steam engines against those powered by real horses used to drive mine pumps and mill wheels. From observing ponies turning a mill, then scaling up, he defined one horsepower as the rate of doing 33,000 foot-pounds of work per minute, which is 550 foot-pounds per second, about 745.7 watts.

Historians note that Watt likely chose a generous figure so an engine rated at one horsepower would outperform a typical draft horse in practice. Continental engineers later adopted a slightly different “metric horsepower” equal to 75 kilogram-force meters per second or about 735.5 watts.

While the names of the units might be different depending on the application, force, work, energy, and power are all concepts that you are probably familiar with.

To summarize:

Force is an action that pulls or pushes something.

Work is the distance that a force is applied.

Energy is the potential to do work, which can be stored in any number of ways.

Power is the amount of work performed in a given time.

There are different units for different amounts of energy and power, but they are all fundamentally measuring the exact same thing.

The way we know they are all measuring the same thing is because all of the base units of mass, length, and time are in the same proportions.

Our understanding of energy, work, and power is one of the fundamental concepts in physics, which has contributed to the development of our modern world.