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Electrostatics, Electricity, and Magnetism

Electrostatics is the interactions between electric charges. It is often called static electricity. You are familiar with the terms positive and negative and should recall that atoms are composed of positively charged protons, negatively charged electrons, as well as neutral neutrons. During your study of atoms you learned that protons were 2000 times more massive than electrons, however, the magnitude of electric charge is the same for each. So, one electron and one proton result in no net electric charge: neutral. The basic rule of electrostatics is the similar charges repel each other and opposite charges attract each other. The forces that result from electric charges are on the order of 1037 times larger than the force of gravity. You have probably noticed the effect gravity more than the effect of electric force for the basic reason that gravity only attracts, while electric forces can attract and repel, thus canceling each other out. The amount of electric force experienced by electric charges increases as the amount of charge increases and decreases as the distance between the charges increases. The strength of the electric force in the space around a charge is known as an electric field. The unit of electric charge is called a coulomb (C) and is equal to the amount of charge resulting from 6.25 x 1018 electrons. 

Everyday objects often become charged in three basic ways: friction, conduction, and induction. Charging by friction is simply the scraping off of electrons from one object onto another by rubbing them together. The object losing the electrons becomes positively charged and the object scraping the electrons becomes negatively charged. Conduction simply involves contact between a charged and a charged or neutral object. When the two object touch electrons are transferred away from a negatively charged object or toward a positively charged object. If one of the objects is neutral it either gains or loses electrons based on the charge of the other object. Charging by induction involves a charged object coming near an object. If a negatively charged rod were brought near a neutral conductor the electrons will be repelled to the far side of the conductor leaving the closer side positively charged. If you were to provide a pathway to the ground for the electrons on the far side (grounding) the electrons would be further repelled through that pathway. The result would be a positively charged conductor due to the loss of electrons. You can ground a conductor by simply toughing it with your finger, your body is then the pathway to the ground. As it turns out the earth itself is a virtually infinite reservoir of electrons. Let’s reverse the induction example provided above. Bring a positively charged rod were brought near a neutral conductor the electrons will be attracted to the near side of the conductor leaving the far side positively charged. If you were to provide a pathway to the ground, more electrons flow onto the conductor due to the attraction of the positively charged rod. Remember the electric force is extremely large. The result would be a negatively charged conductor due to the gain of electrons. Another example of charge induction is found in the production of lightning. 

Clouds in a thunderstorm are often polarized; the top and bottom regions of the cloud have opposite charges. If the bottom of a certain cloud were negative it would induce a positive charge on the earth’s surface below. This induced positive charge attracts more negative charge to the bottom of the cloud. If the electrostatic buildup of electrons becomes to great there is a discharge of the electrons from the cloud to the ground, which we know as lightning. Ground to cloud, cloud-to-cloud, and inter-cloud lightning are all results of the discharge of huge amounts of electrons. You may experience tiny lightning if you buildup charge and then touch another object, often a conductor, and get shocked.

In either of these cases a change has taken place, which as we know requires energy. There is an electric potential energy (EPE) much as there is a gravitational potential energy (GPE). You must do work on a positively charged particle in order to move it closer to another positively charged particle and is the gained EPE of the particle. A change in the amount of charge also changes the EPE, much like changing the mass of an object changes its GPE. EPE is not usually a useful quantity, but is incorporated in the idea of electric potential, which is the EPE divided by the amount of charge present. Electric potential and the potential difference are more commonly called voltage. The unit of voltage is the volt (V) or 1 joule per coulomb. A lightning strike is on the order of 108 volts. Potential difference specifically refers to the difference in electric potential between two points. In order for electric charges to move, as in lightning, there must be a potential difference. This is the electric version of water flowing downhill. Water has to be uphill before it can flow downhill, a potential difference due to location. 

We have used lightning as a sudden and short-lived movement of electric charges, electrons usually. One of the major scientific and technological advances of the nineteenth century was the harnessing of the flow of electrons along conductors, or an electric circuit. In a circuit, electric charge flows due to the movement of free electrons in the conductors, usually metal wires. The flow of electric charge is caused by a potential difference in the circuit and is called electric current. The reason a circuit is different from an electric discharge is that there is an “electric charge pump” in the circuit, a voltage source, such as a battery (which is actually a series of electric cells). Think some more about the idea of water flowing. Specifically, let’s look at a garden or park fountain. There is a reservoir of water that is pumped up and out through the fountain. The water falls back to the reservoir due to gravity and is again pumped up through the fountain… A battery works much like the pump in the fountain in that it “pumps” electric charge back up to a higher electric potential. The electric charge than “falls” through the circuit to the bottom of the charge reservoir (all the materials that compose the circuit provide the electrons) and is “pumped” back to a higher electric potential by the battery… The rate of electric current is measured in amperes (A). One ampere is defined as one coulomb of charge passing a point in one second. The current in a circuit does not normally cause the circuit to be electrically charged. The electric charge that flows is part of the circuit, which is almost always neutral in charge to start with. 

The simplest electric circuit is composed of a voltage source, conducting material, and a resistor. A voltage source is often a battery, which is a series of electric cells. There are two types of electric cells, wet and dry. Both involve chemical reactions between metal and a compound. A wet cell involves an aqueous electrolyte, usually an acid, and metal plates. The chemical reaction between the acid and the metal produces energy, which results in a potential difference between the metal plates, terminals, of the cell. A dry cell works basically the same but with a solid “paste” as opposed to a liquid. Everyday batteries are actually dry cells. In order to have a battery, by definition, you must have multiple cells. A “AA” battery is actually a “AA” cell. A resistor is just that, something that resists the electric current. Much like water in a pipe the thicker and shorter a wire is the less resistance is provides to the electric current. Resistance also varies with the specific material and the temperature of the material; copper is less resistant than steel and high temperature gives high resistance. Resistance is measured in ohms (W) and is related to current and voltage according to Ohm’s Law which states that the current in a circuit is directly proportional to the voltage established across the circuit, and is inversely proportional to the resistance of the circuit, as written in the equation: I = current, V = voltage, and R = resistance. Electrical devices such as light bulbs are resistors. One last point about current itself, an electric field is established along the path of the wires in a circuit. The electric field travels at nearly the speed of light, however, the individual electrons have a random motion of about 3 million km/h and only move along the wire at a rate of about one meter every hour. This occurs in a circuit with a direct current (DC), here the electric field lines are maintained in one direction in the circuit. In a typical household circuit the current is an alternating current (AC), here the direction of the electric field is changed 180 degrees at a regular rate, much like a wave. In an AC circuit the electrons in the wires do not travel along the wires at all. 

There are two basic types of circuits: series and parallel. In a series circuit there is only one path for the current to flow. The amount of current is the same in all parts of the circuit and the voltage is divided proportionally to the resistance of each part of the circuit. A 3 volt battery will provide a voltage around 3 volts, but the voltage of each resistor in the circuit will add up to that 3 volts. A break anywhere in the circuit will cause the current to stop flowing in the entire circuit. 

A parallel circuit provides multiple pathways for the current to flow between the same two points. As a result the voltage across each path is the same and is equal to the voltage source. Each path behaves as a separate series circuit and the current in each path is inversely proportional to the resistance of each path. As the number of paths increase the total resistance decreases and the total circuit current increases. A parallel circuit with three bulbs on three paths will drain a battery about three times faster than the same three bulbs a series circuit. If one of the paths is broken (a bulb blows out) the current will remain in the other paths of the circuit. 

Another important aspect of electrical components is their power, the rate at which work is done. Electrical power is simply the product of electric current and voltage within a circuit. 

The other piece of the electromagnetic puzzle is magnetism, which is also caused by the motion of charged particles, usually electrons in atoms. Magnetic forces originate at magnetic poles, north-seeking and south-seeking. All magnets have a north and a south pole. Unlike electric charges, which you can have a positive without a negative; you cannot have a north pole without a south pole. If you were to break a magnet in half you would have two magnets with north and south poles. This idea gives rise to the current explanation of magnetism that each atom is actually a tiny little magnet due to the motion of unpaired electrons in atoms. When large clusters of atoms line up with their north poles all aligned a magnetic domain is formed. If the magnetic domains in a substance are all fairly well aligned with each other the result is a magnet. The magnetic domains in iron, cobalt, and nickel are relatively easily aligned, as a result a magnet is usually composed a material containing one of these elements. An iron nail attracted to a magnet because its domains will align with the magnetic field of the magnet in such a way as to point the south poles of the domains at the north pole of the magnet. If you leave a nail in a magnetic field for a long time the domains become snuggly aligned and the nail can become magnetized. Over time the nail will lose its magnetism as the domains gradually return to their natural alignment. Striking a magnet or heating it will both speed up the un-aligning process by jostling some of the domains. 

An extension of the cause of magnetism, moving charges, is that a wire with an electric current in it creates a magnetic field around according to the right hand rule (refer to class discussion). A compass, which lines up with a magnetic field will exhibit that same behavior when near a current carrying wire. Due to this phenomenon a current carrying wire can be deflected by a magnetic field. This same behavior assisted in defining the charge of electrons as they were observed in the beam of a cathode ray tube. By wrapping wire around an iron post and putting current through the wire you can produce an electromagnet, which you can turn on and off, as well as reverse the poles quite easily by changing the direction of the current. Electric meters, which allow the measurement of current and voltage, are based on this same deflection due to a magnetic field. In the simplest variation a coil of wire is placed around a steel needle on a pivot. When the current from a circuit is allowed to pass through the meter the needle is deflected based on the amount of current. This deflection can be calibrated to measure current or voltage. Most voltmeters, ammeters, and galvanometers (the example described) are of different design but operate on the same basic concept. 

A simple electric motor works due to the magnetic field produced by an electric current. Basically a magnet is used to produce a region with a magnetic field, a loop or coil of wire is placed in this magnetic field. When a current flows the through the wire it causes the coil to rotate, the north part of the field attracts the south part of the coils field. If that were all, the coil would stay stationary. However, in the motors design are two stationary contacts about which the two ends of the coil of wire rotate while staying in contact to allow the current flow. As the coil rotates the direction of the current reverses so that when the south of the coil meets the north of the magnet, the coil becomes a north on the side and repels from the north of the magnet, thereby keeping the coil in motion. If you then attach a shaft to the center of the wire coil…Voila! an electric motor. Electrical energy has been converted into mechanical energy. Next, we reverse the process in what is called a generator. Put a handle on the shaft, have your friend rotate the shaft, the coil spins in the magnetic field and voltage is produced in the wire, and we know voltage is a potential difference which causes an electric current. Now just connect a light bulb to the two ends of the wire coil and you have converted mechanical energy to electrical energy to light energy! 

The last point of importance is that of a transformer. This device is composed of two separate coils of wire around a single iron core. When the powered or primary coil is swithced on a magnetic field is momentarily established and an electric current is momentarily induced in the secondary coil. At this point it is about as useful as an electrostatic discharge. If, however, you turn the primary coil on-off-on-off… you induce a continuous current in the secondary coil. This may not seem terribly useful until you recognize that by differing the number of turns between the primary and secondary coils you can change the voltage. If the secondary has more turns than the voltage of the secondary is greater than in the primary. The change in voltage is directly proportional to the ratio of the number of turns in each coil. The advantage of this is that electricity can be supplied from the power company at maybe 20,000 volts can be “stepped down” to the usual 120 volts at you house and then stepped up or down to meet the needs of your specific electrical devices in your house. Each stepping of the voltage is done by a transformer. The ease with which alternating current can be stepped is one of the main reasons it is more prevalent than direct current.