How Electrical Energy Really Flows: A Journey through Poynting’s Vector
Today I have a special piece for you. I will not wear my literary mantle, but I will instead take advantage of my status as an engineer and try to reveal a scientific truth to you with simplicity and accuracy. This particular truth concerns the greatest misunderstanding in the scientific community and that is electricity.
DISCLAIMER: This article is an inspiration preceded by this excellent video.
Imagine you have a giant circuit consisting of a battery, a switch, a light bulb, and two wires, each 300,000 kilometers long. That’s the distance light travels in just one second. These wires would reach halfway to the moon before coming back to connect to the light bulb, which is only one meter away. Now, I have a quiz for you. If you close the switch, how long would it take for the bulb to light up? Is it half a second, one second, two, and 1/c seconds, or none of the above?
Please commit to an answer and put it down in the comments so you can’t say, oh yeah, I knew that was the answer. Play by the rules, Medium Community! In this article, we’ll explore the fascinating world of electrical energy transmission, debunk some common misconceptions, and uncover the truth about how electrical energy really flows.
The Shape of the Circuit
For this circuit to function correctly, some simplifying assumptions must be made. For instance, the wires must have no resistance and the light bulb must immediately turn on when current passes through it.
The Lies We Were Taught
To understand how electrical energy flows, we need to address the misconceptions that many of us were taught about electricity. One common belief is that electrons themselves carry potential energy and are pushed or pulled through a continuous conducting loop. Additionally, it’s often assumed that electrons dissipate their energy in a device. However, these ideas are fundamentally flawed. Consider the inaccuracy of this theory from the point of view that if electron motions really did result in energy transfer, then why did the energy only transfer in one direction? From the source to the coil and not simultaneously from the device back to the source?
The Breakthrough: Maxwell’s Equations
In the 1860s and 70s, Scottish physicist James Clerk Maxwell made a groundbreaking discovery. He realized that light consists of oscillating electric and magnetic fields that are perpendicular to each other and in phase. Maxwell formulated equations, known as Maxwell’s equations, to describe the behavior of these fields and their associated waves.
The Poynting Vector
In 1883, one of Maxwell’s former students, John Henry Poynting, delved deeper into the conservation of energy. Poynting’s work led to the development of an equation describing energy flux, known as the Poynting vector (S). This vector helps us understand how electromagnetic energy flows from one place to another. In simpler terms, we are trying to decipher the movement of these 2 fields. We want to find out how much electromagnetic energy passes through an area per unit of time. That’s what Poynting was trying to explain to us.
How Energy Really Flows
Now, let’s consider a simple circuit with a battery and a light bulb. When the battery is connected to the circuit, its electric field extends through the wires at the speed of light. This electric field pushes electrons, causing them to drift slowly in one direction, creating an electric current. However, the motion of electrons is minimal, about a tenth of a millimeter per second. This current, known as conventional current, flows in the opposite direction to electron motion but is responsible for powering devices.
The Role of Electric and Magnetic Fields
As electrons move through the wires, they create both electric and magnetic fields around the circuit. According to Poynting’s theory, energy flows through these fields, not through the movement of electrons. This energy is transmitted as electromagnetic waves, and it travels at the speed of light.
Historical Lessons: Undersea Telegraph Cables
To further emphasize the importance of electromagnetic fields in energy transmission, we can look at historical examples. In the mid-19th century, undersea telegraph cables suffered from signal distortion over long distances. Scientists like Lord Kelvin initially believed that electrical signals traveled through the cables much like water through a tube. However, it was eventually proven that it was the electromagnetic fields around the wires that carried the energy and information.
The Reality of Electrical Energy Transmission
In conclusion, the answer to our initial question about the giant circuit and the light bulb is that the light bulb will turn on almost instantaneously, in roughly 1/c seconds. This may seem counterintuitive to some who imagine that the electric field needs time to travel through the long wire. However, what truly matters is the propagation of electric and magnetic fields, which can transmit energy at the speed of light.
The Takeaway
Understanding how electrical energy truly flows challenges common misconceptions about electricity. Instead of focusing on the movement of electrons within wires, we should recognize the role of electromagnetic fields in energy transmission. This knowledge helps us appreciate the complex yet fascinating journey of electrical energy from power plants to our homes. So, the next time you flick on a light switch, remember that it’s not just the wires but also the invisible fields around them that bring light to your life.
