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Electricity
Electric Charge, Electric Field, and Electric Current
Image courtesy of Garry Platt at Amber Home.
Amber is the fossilized resin from trees similar to a fir or spruce that grew millions of years ago. Amber is a beautiful, translucent yellow-brown solid. It has been used as jewelry for thousands of years. As jewelry it was often vigorously polished with a soft cloth. Ancient Greeks discovered a strange property of amber. When it is rubbed with a cloth, it can attract nearby bits of straw or grain. From the Latin word for amber, electrum, we get the word electricity.
All materials show this behavior to varying degree when rubbed. The classic materials used to explore this strange attraction were glass rods rubbed with silk cloth and rubber rods rubbed with fur. Plastic combs or strips rubbed with wool or combed vigorously through dry hair will also attract bits of paper. In 1646 it was shown that repulsion was also possible. When two rods of glass were rubbed with silk, the rods repelled each other. When two rubber rods were rubbed with fur, the rods repelled each other. However, a glass rod rubbed with silk attracted a rubber rod rubbed with fur.
This mysterious attractive or repulsive property could also be passed to other materials. In the classic experiment two tiny balls of pith were hung next to each other without touching. A glass rod was rubbed with silk and touched to each pith ball. The pith balls would suddenly move away from each other.
These and many more experiments led scientists to a model used to summarize what they observed. The model consists of three rules using something called "electrical charge". The three rules are:
- There are only two kinds of electrical charge. One is called positive and the other is called negative.
- Two objects that have been charged alike repel each other.
- Two objects that have been oppositely charged attract each other.
The rules are simple but understanding charge is more difficult. There have been many theories about the nature of charge since the 1600s. Modern atomic theory defines charge in terms of the electron. The electron has a negative one charge. The more massive proton . has a positive one charge. In fact, the proton is 1000 times more massive than the electron. According to our modern model when an object has a negative charge, it has an excess of electrons. When an object is positively charged, it has lost electrons. The outer electrons of an atom move much more easily than a proton bound in a nucleus. The traditional unit of charge is the Coulomb, named after French physicist, Charles Coulomb .
Compared with an electron, the Coulomb is a very large charge. Each electron and proton have a charge of approximately 1.6 x 10-19 Coulomb. If two objects, each with a net charge of +1 Coulomb, were 1 meter apart, the repulsive force would be 9 billion Newtons or 1 million tons. The Coulomb is such a large charge that it is unwieldy in many applications. Often a micro-Coulomb (10-6) or pico-Coulomb (10-12) is used.
The influence of the charged object extends out into the space surrounding the charged objects. In the example above a meter of space separates the two objects! A force is exerted on each object, even though the objects are not touching each other. In the 1600s this was called "action at a distance", and it was very disturbing to many physicists, including Newton. Gravitational, magnetic and electrical forces all display this effect. William Gilbert studied this long-distance effect extensively using magnetism. He explained the action of a magnet by proposing that it had a "sphere of influence" surrounding it. This region of influence was called a field. One can see the presence of a magnetic field if one puts a magnet under glass and sprinkles iron filings on top of the glass.
The iron filings appear to be arranged along lines.
It is possible to see a similar effect of an electric field. A charged object placed in fine oil with tiny bits of thin thread will cause the thread to line up in the field. It might look like the picture below.
A drawing similar to the one below can represent this situation.
Image Courtesy of C.R. Nave Hyperphysics
Physicists conventionally give a direction to the fields with the field lines pointed away from a positive charge and pointed toward a negative. The field lines are more dense (closer together) near the charge indicating the electric field is stronger closer to the charge. Field lines drawn close together represent a strong field.
When two opposite charges are placed near each other in fine oil with fine bits of thread, the situation looks like the picture below.
If a positive test charge were placed in the field around a positive charge and released, the test charge would be repelled and would accelerate away from the positive charge. If this test charge were in the field of a negative charge, the test charge would accelerate toward the negative charge. But the test charge wouldn't 'know' whether it was pushed by a positive charge or pulled by a negative charge. The test charge would simply experience the local electric field in a particular direction. Scientists describe this as due to an electric field producing a force on a charged object. The size of the force depends upon the local strength of the field.
The picture below shows two parallel metal plates that are oppositely charged. Tiny bits of thread are suspended in oil between the plates. Notice that the threads line up parallel to each other between the plates. The threads arrange themselves along curved lines near the edges.
The drawing below represents what is happening between the plates. The top plate has an excess of positive charge and the bottom plate has an excess of negative charge. The electric field (E) is directed downward (from positive to negative). The drawing shows the direction of force (F) the field exerts on a positive charge and on a negative charge. If the charges were free to move, they would accelerate in the direction of the force.
Electric fields can, therefore, cause charges to move. The movement of charges is an important concept in electricity. Physicists call the net movement of charges an electric current. In the late 1700's scientists chose the direction of electric current to be the direction in which positive charges move in an electric field. This was before scientists knew that electrons and protons were the negative and positive charge particles, and that the electron moved more easily than the proton. We now know that in a copper wire the outer electrons of the copper atom move relative to the nucleus of the copper atom. Therefore, the charge carriers (electrons) move in the opposite direction to the current.
Electric charges, electric fields and electric current are critical to the study of the structure of the Sun, solar wind and the magnetosphere of the Earth. Moreover, electric current causes magnetic fields (see Electromagnetism) that are important to understanding dynamic characteristics of the Sun and how the Sun interacts with the Earth.
The next section, Electromagnetism, involves us in a unique and interesting exploration of the science of two men, Hans Christian Oersted and Michael Faraday, in the early 1800s, and provides us with the background necessary to understand the magnetism of the Earth.
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