Electricity: Part 1

Introduction: This is my first science post, but nevertheless, let’s leap in. The definition I like best is that electricity is the set of phenomena associated with the presence and flow of charged particles. The electricity that travels through wired circuits and has such an effect on our lives is created by the flow of negatively charged electrons, although, in general, electricity can be the flow of any type of charged particles. Here are some questions we’ll try and answer: What is charge? What is static electricity? How do we generate electricity, and what’s the difference between AC and DC? How does electricity do anything, for instance light a bulb or run a motor? How does natural electricity—like lightning—occur? What are volts and amps, and how are they connected? Why can electricity hurt us (but not birds on power lines)? Because there’s a lot here and we may take a diversion or two, I’ll split it into several parts. The rough plan is:

• Part 1: Electrostatics, Coulomb forces and applications
• Part 2: Atomic theory, explanation of electrostatics and what really is charge
• Part 3: Electric and gravitation fields, a comparison between the two, and the fundamental forces of nature
• Part 4: Volts, amps, the Earth’s charge and lightning
• Part 5: Magnetism and electromagnetism
• Part 6: AC and DC circuits, capacitors, inductors and the like
• Part 7: Power generation—batteries, generators, mains electricity and the national grid

Preamble: Since it may take a while to publish all the parts, here’s an advance summary:

The root of electricity is the Coulomb force which is either a force of attraction that exists between positively and negatively charged objects or a force of repulsion between two objects of the same charge. A charged object consists of the aggregation of the charges of a vast number of atomic particles, usually protons or electrons: an object is negative if it has more protons than electrons, and negative if it has more electrons. Charge is measured in coulombs. The reason why protons and electrons have charge, why their charges are equal and opposite, and why the Coulomb force between them is as it is—proportional to one over the square of their distance apart—is unknown, although quantum physics can (to an extent) explain the mechanism. Charged bodies can be created by rubbing certain materials together, which causes electrons to be transferred from one to the other. Static electricity—a short, sharp flow of electrons—is created when a charged body comes into contact with a conductor, and has many useful applications, as well as some unfavourable effects.

An electric current is a continuous flow of electrons. This can be created when a mechanism—such as chemical reactions in a battery—creates and maintains separate areas of charge. If a conducting path (or circuit) is placed between the two areas of charge, then electrons will flow from the negative side to the positive. It was discovered in 1820 that an electric current creates a magnetic field in the surrounding space, and, for example, a compass needle is deflected by a current. The reverse is also true: a magnetic field will create a current in a conducting wire, although only if there is relative motion between the two (either the wire or the magnet is moving). This is the principle behind both electricity generators and electric motors. Electricity generators in power stations either rotate turbines, consisting of large numbers of conducting wire coils, in the presence of a magnetic field; or rotate magnets around the conducting wire coils. This creates a current in the conducting wires, which is fed into the power grid. The mechanical energy to drive the rotation can come from various processes, for example coal-, gas- and nuclear-powered energy uses coal, gas, or uranium to create high temperatures which are used to heat water and generate high-pressure steam, which drives a set of turbines. Wind and hydroelectric power use wind and water to drive the turbines. The engineering details for efficient power generation are complex, but the creation of electricity by rotating magnets around conducting wires (or vice versa) is the basic principle. The electricity created is AC (alternating current) because the direction of the current reverses each time the turbine spins 180 degrees. AC is easier to transport across long distances than DC (direct current), and hence is used as the basis for national power grids. DC can be generated by using a mechanism to convert AC to DC, or directly by means of a battery. An electric motor does the reverse of a generator, and converts electric power to mechanical power. Magnets are positioned around wire coils, and when current flows, the wire will experience a magnetic force and will move, which can drive the turning of a shaft to do work, such as power a fan or turn a car engine. Electricity can do useful work in other ways; for example, current flow through a high-resistant light bulb filament heats the filament sufficiently to give off light.

The connection between electricity and magnetism is fundamental, and combined electric and magnetic effects are called electromagnetism. They’re described mathematically by Maxwell’s equations. Magnetism arises through the movement of charged particles (currents) and even permanent magnets have currents circulating within them. The deep explanation for why currents create magnetic effects needs us to look at relativity and quantum physics (and me to do some research).

Electric potential is measured in volts and is a measure of how much work it takes to move a charge from a reference point—typically ground or earth—to the point in question. The value depends on the charges (or the electric field) in the surrounding area. Electric potential difference, also measured in volts, is the difference in electric potential between two points—this is what is quoted in, for example, battery voltage. The higher the voltage between, e.g. the two terminals of a battery, then the stronger the forces will be that drive the electrons through a conducting circuit attached to the battery (because a higher voltage battery creates more charge at the terminals). Therefore, a higher voltage leads to a larger current, although the size of the current is also affected by the resistance of the circuit. Current is measured in amps, which show how fast the charge is flowing—one amp is one coulomb of charge per second. Electricity causes harm in humans due to two mechanisms: first, large currents generate a lot of heat and can cause burns and tissue damage; second, electricity disrupts natural electrical activity in the body, in particular the functioning of the heart. Birds on electricity pylons are safe because it’s easier for the current to flow through high-conducting wires than the bird’s body, which is more resistant to electricity. However, this would change if the bird also touched a route to a lower voltage, such as a second wire at a different voltage, or one of the ground poles. In this case a large potential difference would exist, and current would flow through the bird, with problematic effects. I’ll dig into this a little more in Part 4, because this can be explained better (how does the current know if the bird is connected to ground, what forces are acting?).

As we reach the end of this preamble, there are a few further points to quickly mention. First, electric and magnetic fields are a way of describing the electric and magnetic forces that would be felt by a charged particle in an area of space and consist of values and directions at each point (which can be represented by lines of force). Second, lightning is a complex example of static electricity, and is under active research—as a taster, though, the classic and familiar lightning strike is actually an up-strike from the ground and not a strike from the heavens down. Third, the Earth’s surface is negatively charged, which is more or less balanced by a positive charge in the atmosphere. Also, of course, the Earth has a magnetic field. And, finally, fourth, the flow of electrons in circuits is very slow and you would wait a long time to connect a telephone call across the world if you relied on this. What happens is that accelerating charges create electromagnetic waves, which move at the speed of light and transport electromagnetic energy. These waves travel outside the wires of a circuit, and the finer details of electricity transport are complicated (translation: I need to do some more research).

Electrostatics and charge: I’ve just rubbed a balloon against my head and stuck it to the wall. This effect—electrostatics or static electricity—has been known about since the ancient Greeks. Rubbing two materials together can change them such that they attract or repel other materials. The effect is more obvious on some objects. If you search YouTube for “electrostatic experiments” you’ll find some cool tricks to do at home, such as pushing coke cans or diverting flowing water with an ‘invisible’ force. Benjamin Franklin experimented with charge from 1746 and defined positive charge as that acquired by glass when electrified by rubbing with silk and negative charge as that acquired by rubber when rubbed with cat’s fur. Materials acting like electrified glass are said to be positively charged and those acting like electrified rubber negatively charged. Objects of a similar charge repel each other, while those of opposite charge attract each other. The choice of which was positive and which negative was arbitrary. (In fact, Franklin got it the “wrong” way round, because traditional circuit diagrams show charge flowing from positive to negative, whereas we now know that current flow in standard circuits is electrons flowing from negative to positive.) The French physicist Coulomb established, in 1784, the relationship for the force between two charged particles, showing it to be proportional to the magnitude of each charge and inversely proportional to the square of the distance they are apart. If the charges are the same sign, the force is attractive; if opposite, the force is repulsive.

The formula for Coulomb’s law is F = kq₁q₂/r², where the q₁ and q₂ are the charges, r is their distance apart in metres, F is the force between them in Newtons, and k is a constant. Charge is measured in coulombs, and the definition of one coulomb states that a charge is one coulomb if, when separated from a similar charge by one metre, a force of 8.98742 x 10⁹  Newtons is generated. That seems arbitrary, but it means that one coulomb (or 1 C) is the amount of charge transported by one amp in one second—we’ll get to amps later! Force is a vector quantity which means it has direction as well as magnitude. The vector formula for coulomb force uses vector notation, where vectors are denoted in bold: F = kq₁q₂r’/r², where r’ is a unit vector (it has a magnitude of one) in the direction between the two charges.  A key reason to mention vectors is that the vector equation shows that the force acts in the direction between the two charges. Another reason is that if a charged particle is subject to forces from more than one other charged particle, then the total force is the sum of the forces from all the other particles. This is called the principle of superposition. The sum of the forces is a vector sum, which means that the direction (as well as the magnitude) of the force is worked out by the sum. There are always subtleties, and the value of k is smaller if the space between the two charges is not a vacuum (and the force will be smaller). Here are some things to note about charged materials and the forces between them.

• Contact-induced static electricity: Examples of materials likely to gain a negative charge when rubbed are polyester, polystyrene and rubber; examples of materials becoming positive are human skin, human hair, nylon and glass. The triboelectric series orders materials from those most likely to become positive to those most likely to become negative; and rubbing a material higher on the list with one lower is likely to result in the former becoming positive and the latter negative. This is why you can get a shock from, for example, polyester clothes. The clothes rub against the skin and the skin becomes positively charged. This is contact-induced (or triboelectric) static electricity. If the charge builds up and you touch a conductor like a car door or even another person, there will be a rapid discharge of the static electricity which creates a spark and you’ll feel a shock through nerve stimulation as the charges flow through the body. To avoid this, the charge needs to gradually dissipate, either through the air or the ground. The charge will better dissipate in moist air because this is a better conductor of electricity than dry air, so you’re more likely to get a shock on a dry day. Similarly, if you’re wearing insulating shoes (like rubber), the charges are less likely to leak away through the ground because rubber is an insulator, which prevents the flow of charge—and again a shock is more likely. Humans can become positively or negatively charged, although positive is more likely because human skin is high up the positive side of the triboelectric series.
• Electrostatic induction: Walking in rubber shoes on a nylon carpet can cause a static shock for a slightly different reason. The shoes pick up charge from rubbing against the carpet, which creates contact-induced static charge on the shoes. But rubber is an insulator, so how does this charge get from the shoes to the human body? The answer is by a process called electrostatic induction, or induced charge. The shoes have a negative charge (and the carpet a positive charge) because of the rubbing. Positive charges in the body are attracted to the charge on the shoes and negative charges are repelled. The body remains neutrally charged as a whole, because charges can’t flow through the insulated shoes, but the charge distribution in the body is changed—some parts are positive (near the feet) and some are negative, for example the hands. Note that all the charged areas are on the surface—the human body is a conductor and any ‘clumps’ of charge inside the body quickly balance out. When the negatively charged hand touches a conductor, charges will flow and there will be a shock.
  Electrostatic induction also explains the charged balloon sticking to a neutral wall, although the mechanism is different. Unlike the human body, which is a conductor of electricity, the wall is an insulator, which means charges are not able to move around freely. What happens is that the atoms in the wall orient themselves so that negative charges are pushed into the wall (because they’re repelled by the negative balloon) and therefore positive charges more prevalent at the wall’s surface, which then attract the balloon. Atoms lining up in a particular direction like this is called polarisation. The same effect is at play when dust is attracted to a TV screens or computer monitor—the back of the screen is negatively charged (because electrons are fired at them) and the front of the screens are positively charged and therefore attract polarised dust particles. This applies to the older style CRT (cathode ray tube) screens; the modern LCD (flat screen) monitors use a different mechanism, but I think they still create some static and dust attraction, and you can get antistatic cleaners and sprays for either. To be honest, dust even sticks to mirrors, so I’ll leave a certain mystery around dust and why it sticks to everything. If you can invent a machine that zaps the dust away without destroying everything else, you’ll be on to a winner.
• Expert advice: All in all, the above is why it’s not a good idea to play squash on a nylon carpet while wearing rubber shoes and polyester clothes.
• Electrostatic discharge and sparks: An electrostatic discharge happens when a charged material (e.g. your finger) touches a conductor and the charge rapidly flows from one to the other. When you see a spark, it occurs across an air gap between the two surfaces, just before the materials touch. It’s caused by electrons leaping the gap and heating up the air molecules, so they glow. The discharge is over in a fraction of a second, and the size of the spark (and whether a spark is seen) depends on the amount of charge and the distance between the materials. We haven’t spoken about volts and amps yet, but, for reference, voltage is a measure of the difference in charge between two surfaces, and current measures how much charge flows per second. Humans can be charged to several thousand volts when they get a static shock—but this doesn’t cause any danger to health because it is the current flow that can cause harm, and this is low and short-lived in normal static electricity scenarios. Lightning is an extreme and complicated example of a static discharge, and we’ll look at that later. Lightning is dangerous!
• Problems of static electricity: Although a direct static shock is unlikely to endanger humans, static sparks around flammable materials can cause fires and explosions and there have been many related industrial explosions and deaths. The Hindenburg tragedy in 1930 is believed to have been caused by static electricity—see Hindenburg mystery solved. Fuelling operations, for example fuelling aircraft through pipes, are susceptible since fluids such as kerosene or diesel can accumulate charge during their flow through pipes and a spark can ignite fuel vapour. The small and intricate nature of electronic components makes them susceptible to static electricity, for example by the fusing of small wires due to heat or by voltage breakdown (which occurs when a static voltage is high enough for an insulating component to start conducting electricity, which is likely to permanently damage the component). Damage to electronics can either cause an immediate malfunction or be latent, with the effects appearing later. Aircraft accumulate contact-induced static electricity due to friction (rubbing) against the air molecules in flight. The amount of static and how it discharges depends on atmospheric conditions, and rapid discharge could disrupt radio communications or cause danger in fuel areas. Spacecraft also accumulate static electricity, and rocket launches are sometimes delayed due to cloud conditions. In addition, space exploration itself is subject to static electricity—moving over dry planetary terrain creates a static build up; and with no atmosphere (on the moon) or a very dry one (on Mars), this is less likely to be dissipated until an astronaut touches the rocket or other conducting equipment. It’s interesting that the Apollo astronauts didn’t report electrostatic shocks. However, the moon is believed to go through an eighteen-year cycle of varying charge (due to its orbit through the Earth’s magnetic field) and was at a minimum during the Apollo missions; so it may be that a permanent presence on the moon would need to pay more attention to static electricity. Hospitals need to take static-avoidance measures for several reasons: the effects on sensitive equipment; the use of flammable oxygen; and that if the high resistance of the skin can be bypassed (in a patient with a wound or being operated on), then a patient is much more conductive to electricity.
• Antistatic measures: Industries where static electricity is a risk will have anti-static policies, processes and devices. Antistatic devices are also available for every-day use. The basic principle is to gradually dissipate static electricity away by providing a conduction path. This can be as simple as opening a window or using a humidifier to make the air more conductive. I’ll describe three main techniques here; the sophistication and expense will depend on the potential impact.
  Grounding: This is used to provide a conducting path to ensure the static leaks to the ground. Examples are antistatic wrist straps, antistatic mats and flooring. These provide conductive material which is built into the device and connected to the ground. To connect to the ground, you’ll have a connection (probably via a cord or a socket) that leads to an electrical grounding system—which is a system that conducts electricity into the ground, for example an eight-foot-long copper grounding rod driven into the ground. I believe that the earthing wire in a house electrical socket often connects to a grounding rod near the house, and you could therefore connect an antistatic mat or strap to a standard electrical socket—but don’t quote me on that (read the manual). Although these grounding devices allow the flow of static electricity, they’re also designed to prevent the flow of more serious electricity from shocks. They do this by having enough resistance to prevent low-voltage flow (which is associated with mains electricity shocks), but not to prevent the higher-voltage static electricity dissipation.
  Antistatic agents: These are chemicals that are either coated on the surface or mixed into a material, and work by making the material slightly conductive so that static can be dissipated away. Examples are antistatic bags for storing electronic equipment (a substance is added to the surface), aircraft fuel (chemicals are added to the fuel), or washing powder (chemicals are added to the powder to reduce static on clothes).
  Ionizers: the principle here is to use an ionizer (also called an ionizing or static bar) to create positive and negative ions in the air, which will be attracted to a statically charged material. If the material is negatively charged it will attract the positively charged ions and become neutral, and vice versa. This is often used on manufacturing production lines. Ions are molecules with extra electrons or missing electrons and are created by applying an electric current to a sharp metallic point which creates a corona discharge which creates the ions. Basically, electrons are occasionally knocked out of atoms to create ions anyway, but under a corona discharge, the electric field is strong enough that these ‘free’ electrons knock other electrons out of atoms and a chain reaction occurs with lots of ions being created. You may also need an airflow, such as a fan, to blow the ions in the right direction to eliminate the static electricity. Ionizers are also used as air purifiers, for example in homes. In this case the ions are intended to attract airborne particles such as dust, allergens, viruses or bacteria. This is simple induced static electricity—the particles are attracted to the charged air molecules and fall to the floor. This is different from air purifiers which use air flow and filters to capture small particles.
• Piezoelectricity and quartz watches: There are other ways in which static electricity can form. Piezoelectric materials, which were discovered in 1880, generate a charge on their surface when subjected to a mechanical force or pressure. This is related to their structure, and particularly the polarisation of their molecules. Examples are crystals such as quartz, some ceramics, and biological materials such as bone or DNA. The reverse also holds—if an electric field is applied to piezoelectric materials, then a mechanical deformation is produced in the material (it bends or changes shape slightly). The amount of charge produced (or the amount of deformation in the reverse case) is very precise, which leads to practical applications. One example is the quartz watch, which works something like this: First, note that all materials have a natural frequency they vibrate at, whether they’re tuning forks, guitar strings or tall buildings. The frequency depends on their structure. Resonance happens when a force is applied to a material at its natural frequency (or some multiple of its natural frequency – these are called resonant frequencies), and this creates larger vibrations. This can be a problem in, for example, machinery or buildings or bridges, and avoiding resonance is an important part of building design.
  Anyway, back to the watch. A slice of quartz, usually shaped like a tuning fork, has bursts of electric current applied to it via the battery. The reverse piezoelectric effect means that the quartz is deformed and then restored to normal size by the electricity—that is, the crystal vibrates. Because the crystal vibrates, the (forward) piezoelectric effect creates static electricity on its surface. A conducting circuit allows the static electricity to discharge and create an electric current (or electric pulses) at the same frequency that the crystal vibrates at. The natural frequency of quartz depends on the angle of cut of the crystal, and its size and shape. Quartz crystals for watches are engineered so that the frequency is 32,768 vibrations per second (32,768 Hz). Going back to the electric pulses that have been output, electronic ‘dividers’ filter the pulses so that only one pulse each second is registered (32,768 is a power of 2, which helps the electronic logic). For an analogue watch, the one second pulses power a motor which powers mechanical gears which move the hands; for a digital watch, the pulses connect to electronic counters which keep track of the time and feed the numbers into an LED display. LED stands for light-emitting diode, which is a semiconductor device that emits light when a current passes through it. A lot of electronic engineering is required to design the circuitry of a digital watch, including (in better models) subtleties like correcting for temperature because the natural frequency varies slightly with temperature. The key point is that the piezoelectric properties of quartz allow it to vibrate at a precise frequency which can be used to accurately measure time with the help of electronic circuits. Quartz is chosen ahead of other piezoelectric materials because its natural frequency is stable with respect to environmental changes, and is ‘sharp’ (it’s easy to identify amongst other frequencies which are also created—technically-speaking, the crystal has a high Q factor). One final point to note, is that the electrical current generated by the vibrating quartz is amplified and fed back into the crystal—this creates resonance and helps keep the crystal vibrating at its resonant frequency.
  There are many other applications of piezoelectric materials, some relating to sound. Since the materials can be made to vibrate by application of a current, this means sound is created (even if we can’t always hear it, because the frequency is outside our hearing range). This can be detected and measured, which is the principle used in, for example, ultrasound therapy or ultrasound imaging.
• Pyroelectricity and infrared sensors: pyroelectric materials show a similar effect to piezoelectric materials except that a charge is created on the material when it is heated or cooled instead of when pressure is applied. Again, the reason is down to the internal structure of the material. One example of its application is in infrared sensors, since the heat of a nearby human or animal can produce enough charge separation to trigger a detectable current.

Applications of electrostatics: Other applications of electrostatics include:

• The Van de Graaf Generator: This is a staple of physics demonstrations of electrostatic charge, and also has applications in nuclear medicine and as a research tool for accelerating atomic particles. It was invented by the American physicist Robert Van de Graaf in 1929. Electric charge from a battery is sprayed by a moving belt on to the top of a hollow metal globe above an insulated column. See the Van de Graaf Wikipedia page for details.
• Various types of printing, including xerography (used by photocopiers) and inkjet: The concept in xerography is roughly (mostly using Wikipedia): 1. The surface of a cylindrical drum is electrostatically charged to become negative; 2. The drum is coated with a photoconductive material (which means that it becomes conducting when exposed to light); 3. A bright light shines on the document to be copied, and the white areas reflect onto the drum (or, in more modern, digital versions, a laser or LED is used to scan the image to the drum) ; 4. Where light shines on the drum, the surface becomes conducting and the negative charge is discharged. The parts of the drum which aren’t exposed to light (corresponding to the image or text on the document) remain negative; 5. The toner powder is positively charged and applied to the drum. The toner sticks to the negative portions of the drum surface by electrostatic attraction to create an image; 6. The toner image is transferred to a piece of paper, again by electrostatic forces (the paper is negatively charged and the toner particles transfer to it), and then the toner is bonded to the paper by heat and pressure rollers. Color toner particles have precise electrostatic properties according to their colour (cyan, magenta, yellow, or black), and a colour image is created by superimposing separate images from each toner colour. Inkjet printers also use electrostatics, and I’ll leave you to look up the details.