This chapter is from the book
This chapter is from the book
This chapter is from the book
1.6. Forces Driving Current
Electrons do not flow (that is, current doesn’t exist) without a driving force. Some force must move the electrons along from one point to another. In general, from a practical standpoint, the two categories of force that move electrons are a voltage, or a charge difference, between two points, and the presence of a changing magnetic field that causes electrons to flow. How these forces come into being is the question.
1.6.1. Batteries
A common battery is an excellent example of a device that gives us a voltage (charge differential) at its terminals. The difference in charge comes about through a chemical reaction in the battery.7 Charles Augustin de Coulomb (1736–1806) is credited with Coulomb’s Law (1785), which states
- There are two kinds of charge, positive and negative. Like charges repel, unlike attract, with force proportional to the product of their charge and inversely proportional to the square of their distance.
Since there are unlike charges at each terminal of a battery (positive and negative), the charges are attracted from one terminal to the other. If we provide a path (circuit) through which the charge can flow, it will do so.
There are many different types of batteries, but the process is pretty similar for all of them. The electrodes (the positive and negative terminals) are dissimilar materials (usually some form of metal), and a chemical solution of some type (called an electrolyte) is between them. The electrolyte reacts (a process called an electrochemical reaction) with one or both of the electrodes to create at least one other compound and either some positive or negative ions. The ions are charged and provide the source of the charge for the battery. When the terminals of the battery are connected through a circuit, the electrons flow through the circuit to the other terminal/electrode, where they combine with the ions or compounds at that terminal.
Alessandro Volta (1745–1827) is credited with making the first battery in 1800. The measure of voltage is named after him (volt). He made his first battery with zinc and silver, with salt water as the electrolyte. Some common materials used in battery manufacture today include
- Zinc/carbon (standard carbon battery)
- Zinc/manganese-oxide (alkaline battery)
- Lithium-iodide/lead-iodide (lithium battery)
- Lead/lead-oxide (automotive battery)
- Nickel-hydroxide/cadmium (NiCad battery)
- Zinc/air
Each combination of materials and electrolytes produces a specific battery voltage (often ranging from a fraction of a volt to 1 or 2 volts). If more voltage is desired, more battery cells are placed in the series. For example, the automotive lead acid battery produces approximately 2 volts per cell. A 12-volt car battery has six internal cells. Typically, the current capacity of an individual battery is a function of the surface area between the metals and the electrolytes. If you want more current capacity from an individual battery, you need larger cells. That’s why standard battery sizes range from AAA to D. The chemical processes (and the voltage) are the same for each size, but the surface area is larger for larger physical sizes.
Batteries typically have a maximum charge flow (current flow) that they can provide. This is because as charge circulates through a circuit, additional charge (ions) must be made available at the terminals. Different electrochemical reactions differ at the rate at which they can provide charge. Batteries with plates with larger surface areas typically can provide more charge (current) at a specific point in time than can batteries with smaller plates.
In some batteries, the chemical process is reversible. Such batteries are rechargeable. If we circulate the charge from the battery through a circuit, we can then connect the battery to a charger (typically just a higher voltage) and restore the materials back to a charged condition. Other chemical processes are not reversible. These batteries will run down, or discharge, with use until the chemical process can no longer provide ions for the charge.
1.6.2. Generators
The other common source of force for generating current comes from electrical generation. Michael Faraday (1791–1867) formulated Faraday’s Law of Magnetic Induction (1831), which states that a changing magnetic field is accompanied by a changing electric field that is at right angles to the change of the magnetic field. We start with a magnetic field, which can come from a physical magnet or a current flowing through a conductor. In either of these cases, there is always a magnetic field. The most important word in Faraday’s Law is changing. Consider the presence of another wire or trace nearby. There will be an electric field, and therefore an electric force, generated in this conductor if the nearby magnetic field is changing.
The changing nature of the magnetic field can be caused by one or more of several different factors. The physical magnet may be moving relative to the conductor, or the conductor may be moving in relation to the magnet. The current that is creating the magnetic field may be changing. Any of these situations will create a changing relationship between the adjacent conductor and the magnetic field. Thus, any of these situations can create an electric field, and therefore a current flow, in the adjacent wire.
This is the basic principle behind all generators. Typically a wire coil is turned in the presence of a magnet field, generating a force in the coil, or a magnetic field is changed near a coil. In dams and oil- or coal-fired (even nuclear) generation plants, the electrical power comes from generators where (typically) coils are turned in the presence of strong magnets. Current or voltage is generated in the secondary coil of a transformer by the changing magnetic field caused by a changing current through the primary winding of the transformer. Almost all electrical force in our homes and offices originates from generators.
1.6.3. Static Electricity
We are all familiar with static electricity. That’s what causes the spark to jump between our hand, for example, and another object or person. Static electricity represents the force between two points that have a different charge. It is often caused by rubbing two different (insulating) materials together. Electrons transfer from one material to the other, creating the charge imbalance.
If we connect a conductor between two points charged with static electricity, current will flow until the charge difference between them is neutralized. In most practical cases, the amount of charge imbalance is not too great. There may be a significant voltage difference because of the charge imbalance, but the total amount of charge is usually small. A small spark may jump when the voltage difference is high but the amount of accumulated charge is small. An example is when we walk on a carpet, and the motion of our shoes against the carpet creates a charge difference between our bodies and nearby objects. Then when we touch a doorknob, the charge difference results in a spark. The spark is, in fact, charge (electrons, and therefore current) jumping between our bodies and the doorknob. The spark can be a “shock,” but it is over very quickly. In most practical cases, static electricity dissipates over time in the objects, or into the air, so the charge imbalance is not maintained for a long period of time.
For this reason, static electricity (static charge) is usually not a practical source of charge for current generation. Static discharge, however, can be destructive to electronic equipment. Although the total charge that jumps from one object to another may not be great, the initial charge spike caused by a high voltage, though brief, may be hot enough to burn, and therefore destroy, sensitive equipment. This can be a particular problem in semiconductors. Care must be taken to prevent static electricity sparks from traveling through a semiconductor junction. The localized heating, though small, may be enough to burn a hole through the junction and destroy it.
A common example of very large static electric charge differences is what occurs during an electrical storm. The static charge difference can be very high because the two objects being rubbed together (the earth and the atmosphere) are so large. When the charge difference gets high enough, a spark jumps between the objects, helping to neutralize the charge difference—in this case, lightning. Here, the voltage difference and the current magnitude are very large—large enough to be lethal.
The effects of a lightning strike also illustrate the limits of insulating materials. Wood, for example, is normally a pretty good insulator. But when lightning strikes a tree, the force (voltage) behind the strike is enough to change the state of the wood (it burns it) so that it becomes conductive.