Author: S.Ashok
Electrons in the outer orbits of an atom are attracted to the nucleus by less force than electrons whose orbits are near the nucleus. These outer electrons may be easily forced from their orbits, while electrons in the inner orbits are called “bound” electrons since they cannot be forced out of their orbits.
Atoms and molecules in a material are in continuous random motion, the amount of this motion determined by the material, temperature and pressure. This random motion causes electrons in the outer rings to be forced from their orbits, becoming “free” electrons. “Free” electrons are attracted to other atoms which have lost electrons, resulting in a continuous passage of electrons from atom to atom within the material. All electrical effects make use of the “free” electrons forced out of the outer orbits. The atom itself is not affected by the loss of electrons, except that it be- comes positively charged and will capture “free” electrons to replace those it has lost.
The random movement of the “free” electrons from atom to atom is normally equal in all directions so that electrons are not lost or gained by any particular part of the material. When most of the electron movement takes place in the same direction, so that one part of the material loses electrons while another part gains electrons, the net electron movement or flow is called current flow.
ELECTROMAGNETIC FIELD
Current flowing through a wire generates a magnetic field whose direction is determined by the direction of the cur- rent flow. The direction of the generated magnetic field is found by using the left-hand rule for a current carrying conductor.
MAGNETIC FIELD OF A LOOP OR COIL
A loop generates a magnetic field exactly the same as a bar mag- net. If many loops are added in series forming a coil, a stronger magnetic field is generated. The left-hand rule for a coil is used to determine the coil’s magnetic polarity.
FIELD STRENGTH
Increasing the number of turns of a coil increases the field strength and increasing the coil current also increases the field strength. An iron core may be inserted to greatly concentrate the field (increase flux density) at the ends of the coil. The ampere -turn is the unit used in comparing the strength of magnetic fields.
PERMANENT MAGNET and ELECTROMAGNET FIELDS
Electromagnet fields are much stranger than the permanent magnet type, and are stool used in most practical electrical machinery. When electromagnets are used, the field strength can be varied by varying the amount of current flow through the field coils.
Units of current flow
Current flow is a measure of how many electrons are passing through a material in a given length of time. The coulomb is a measure of the number of electrons so that, by counting the coulombs which pass in a given amount of time, the current flow is measured. The unit of current flow is the ampere. One ampere of current is flowing when one coulomb of electrons passes through the material in one second, two amperes when two coulombs pass per second, etc.
Since amperes mean coulombs per second, the ampere is a measure of rate at which electrons are moving through a material. The coulomb, which represents the number of electrons in a charge, is a measure of quantity.
What causes current flow
What is EMF
Current flow takes place whenever most of the electron movement in a material is in one direction. You have found out that this movement is from a (-) charge to a (+) charge and occurs only as long as a difference in charge exists.
To create a charge, electrons must be moved, either to cause an excess or a lack of electrons at the point where the charge is to exist. A charge may be created by any of the six sources of electricity which you have studied about previously. These sources furnish the energy required to do the work of moving electrons to form a charge. Regardless of the kind of energy used to create a charge, it is changed to electrical energy once the charge is created; and the amount of electrical energy existing in the charge is exactly equal to the amount of the source energy required to create this charge.
When the current flows, the electrical energy of the charges is utilized to move electrons from less positive to more positive charges. This electrical energy is called electromotive force (emf) and is the moving force which causes current flow. Electrons may be moved to cause a charge by using energy from any of the six sources of electricity; but, when electrons move from one charge to another as current flow, the moving force is emf.
An electric charge, whether positive or negative, represents a reserve of energy. This reserve energy is potential energy as long as it is not being used. The potential energy of a charge is equal to the amount of work done to create the charge, and the unit used to measure this work is the volt. The electromotive force of a charge is equal to the potential of the charge and is expressed in volts.
When two unequal charges exist, the electromotive force between the charges is equal to the difference in potential of the two charges. Since the potential of each charge is expressed in volts the difference in potential is also expressed in volts. The difference in potential between two charges is the electromotive force acting between the charges-commonly called voltage.
Voltage or a difference in potential exists between any two charges which are not exactly equal. Even an uncharged body has a potential difference with respect to a charged body; it is positive with respect to a negative charge and negative with respect to a positive charge. Voltage exists, for example, between two unequal positive charges or between two unequal negative charges. Thus voltage is purely relative and is not used to ex- press the actual amount of charge, but rather to compare one charge to another and indicate the electromotive force between the two charges being compared.
How EMF is maintained
Of the six sources of electricity, you will usually use only magnetism and chemical action. Electric charges obtained from friction, pressure, heat and light are only used in special applications and are never used as a source of electric power.
In order to cause continuous current flow, electric charges must be maintained so that the difference of potential remains the same at all times. At the terminals of a battery, opposite charges exist caused by the chemical action within the battery, and as current flows from the (-) terminal to the (+) terminal the chemical action maintains the charges at their original value. A generator acts in the same manner, with the action of a wire moving through a magnetic field maintaining a constant charge on each of the generator terminals. The voltage between the generator or battery terminals remains constant and the charges on the terminals never become equal to each other as long as the chemical action continues in the battery and as long as the generator wire continues to move through the magnetic field.
If the charges were not maintained at the terminals, as in the case of two charged bars shown below, current flow from the (-) terminal to the (+) terminal would cause the two charges to become equal as the excess electrons of the (-) charge moved to the (+) charge. The voltage between the terminals then would fall to zero volts and current flow would no longer take place.
Voltage and Current Flow
Whenever two points of unequal charge are connected, a current flows from the more negative to the more positive charge. The greater the emf or voltage between the charges, the greater the amount of current flow. Electrical equipment is designed to operate with a certain amount of current flow, and when this amount is exceeded the equipment may be damaged. You have seen all kinds of equipment such as electric lamps, motors, radios, etc. with the voltage rating indicated. The voltage will differ on certain types of equipment, but it is usually 230 volts. This rating on a lamp, for example, means that 230 volts will cause the correct current flow. Using a higher voltage will result in a greater current flow and “burn out” the lamp, while a lower voltage will not cause enough current flow.
If a motor is designed to operate on 230 volts and you connect it to a 300 – volt electric power line, the motor will be “burned out” due to excessive current flow; but the same motor placed across a 100 -volt line will not operate properly because not enough current will flow. While current flow makes equipment work, it takes emf or voltage to cause the current to flow, and the value of the voltage determines how much current will flow.
Electromotive force-voltage-is used like any other type of force. To drive a nail you might use any number of different size hammers but only one size would furnish exactly the right amount of force for a particular nail. You would not use a sledge hammer to drive a tack nor a tack hammer to drive a large spike. Choosing the correct size hammer to drive a nail is just as important as finding the correct size nail to use for a given job.
Similarly, electrical devices and equipment operate best when the correct current flows, but for a given device or equipment you must choose the correct amount of voltage to cause just the right amount of current flow. Too large a voltage will cause too much current flow, while too small a voltage will not cause enough current flow.