Basic Electrical Properties

Basic Electrical Properties
Basic Electrical Properties
Basic Electrical Properties
Basic Electrical Properties
Basic Electrical Properties
Basic Electrical Properties
Basic Electrical Properties
Basic Electrical Properties
Basic Electrical Properties
Basic Electrical Properties
Basic Electrical Properties
Basic Electrical Properties
Basic Electrical Properties
Basic Electrical Properties

DC voltage sources:

If the polarity of an electrical energy source does not change over time, it is called a direct current source. If the magnitude of the voltage is invariable, the source is termed as being a fixed voltage source. The following graphic depicts the circuit diagram symbol for such a source. 

Galvanic elements:

The principle of a galvanic elements involves using electro-chemical processes to generate a voltage. Such an element contains two materials of differing conductivity (e.g. zinc and carbon) that are used as electrodes immersed in a so-called electrolyte.  Elements of this kind are called primary elements. The magnitude of the voltage produced by a galvanic element depends on the materials used for the electrodes.

Among the examples of such galvanic elements are common household batteries. These are available nowadays in a variety of shapes and forms (e.g. as cylindrical batteries, button cells or in block form - see right). A typical voltage output is 1.5 V or some multiple of this number (e.g. 9V).

Accumulators:

Unlike primary elements, so-called accumulator batteries can be recharged on many occasions. They are defined as secondary galvanic elements. The best known is the lead accumulator usually used as the starter battery in motor vehicles (see right). Such batteries usually provide a voltage of 12 V. Most types of household battery are also available in a rechargeable accumulator form nowadays.  

Mains power supplies:

A mains power supply or transformer provides power supplied from the AC mains network (which has a specified voltage, currently defined in Europe as 230 V). This usually involves using a transformer to step the voltage down to the required voltage for the appliance and to ensure galvanic isolation (for safety reasons it is usually forbidden to have a direct connection to the mains), In a stabilised power supply, a closed-loop controller (voltage stabiliser) will ensure that the output voltage remains generally constant in spite of changing load or input. Such power supplies are also available in many forms, e.g. the adjustable laboratory power supply shown here (above right) or the fixed voltage supply that is often used for toys or music and games equipment (bottom right). Other forms include plug in power supplies or the power supply in a computer that provides various different but constant voltage outputs.  

Direction of conventional current:

 

Direction of electron flow:

Electrons flow in a conductor (in the circuit outside of the voltage source) from the negative of the source where electrons are in surplus to the positive pole where there is a relative deficit. Inside the source the electrons are forced away from the positive pole towards the negative. (see graphic below). The source ensures that there is always a potential difference between the two poles. 

Basic Electrical Properties

To avoid one frequently made error, it needs to be pointed out that the electrons are not actually created by the source; the source merely sets existing free electrons in the conductors of the circuit in motion. Similarly the electrical devices being powered, that are generally called "loads" (e.g. the lamp in the above circuit),  do not actually "use up" electrons. What they do use is some of the energy that the moving electrons carry.

Direction of conventional current:

Before the theory of electrons came to be known, even though the terminology, 'positive' and 'negative', had been chosen arbitrarily, it was assumed that any carrier of current was actually in excess at the positive pole and in deficit at the negative so that current would flow from positive to negative. Despite the more recent knowledge about electricity, the assumptions about the direction of flow had been become so established that it was thought practical to stick to the convention that current flows towards the negative pole. Thus the direction of an electric current is, by convention, in the opposite direction to the electron flow. It is common to speak of conventional current in this respect. The following graphic illustrates what this means. 

Basic Electrical Properties

To sum up:

Electric current flows by convention from positive pole to negative pole in any circuit external to a voltage source.

Electric charge:

 

If an atom contains its usual number of electrons, the atom or groups of similar atoms display no electrical properties. Such an atom or atoms are termed electrically neutral. If electrons are lost or captured by the atoms, they become electrically charged. Positively or negatively charged atoms or combinations of atoms are called ions (from the Greek ion: to wander).

Long before electrons were discovered, it was known in the field of static electricity that there were two types of electric charge known as positive and negative. Rubbing a hard rubber rod with wool, for example, causes electrons to be transferred from the wool to the rubber, giving the latter a negative charge while the lost electrons from the wool cause it to be positively charged.

The smallest possible charge that there can be (elementary charge e) is the charge on one electron. This charge is regarded as negative as a result of some arbitrary decisions made during the history of man's study of electricity. 

Electrons carry a negative elementary charge of
e = -1.602·10-19 C.

Where the "C" is the unit of charge the coulomb, named after the French physicist. In formulae the charge is very often represented by the letter "Q". One coulomb of charge comes about as the result of 1/e = 6.25·1018 electrons.

Charged objects are attracted or repelled by forces that depend on the nature of the charges.

Objects with the same charge repel one another while objects with differing charges attract each other.

The magnitude of the force F depends on the magnitude of the two charges and on the distance between the objects. For two point charges Q1 and Q2, separated by a distance r the following relationship, Coulomb's law, applies.

Basic Electrical Properties

The force thus decreases with the square of the separation of the two charges. The value e0 = 8.85·10-12 As/Vm is a constant called the absolute permittivity of free space (vacuum) and er is called the relative permittivity or dielectric constant, the value of which depends on the material in the space separating the charges. If either of the charges has a charge of 0, no force is apparent. The direction of the force (attracting or repelling) depends on the polarity of the charges.

The following animation illustrates the relationship. Give the two balls various charges by dragging a charge onto either ball with the mouse and see what happens. 

Basic Electrical Properties

Electric current:

It has already been noted that conductors are distinguished by the fact that possess large numbers of free electrons that are able to move between atoms in the atomic lattice. If there is no voltage across the conductor, the movement of such electrons is purely arbitrary, i.e. there is no overall direction of motion or specific destination, as shown in the following graphic.

 Basic Electrical Properties

If a DC voltage is applied to the conductor, however, the electrons now move in a specific direction through the conductor and an electric current flows from one pole of the voltage source to the other. The following graphic illustrates this. 

Basic Electrical Properties

Electrons can move more quickly the higher the voltage applied and the less resistance they encounter from the atomic lattice. The current I is  defined as the charge Q that flows through unit a cross section of the conductor in unit time, i.e. 

Basic Electrical Properties

The unit of current is named after the French physicist Ampere (and is abbreviated to "A"). A current of 1 ampere (or amp for short) means that in a unit time of 1 second, a charge of 1 coulomb flows.

 

Electric field:

The forces exerted on one another by electric charges are related to an electric field that surrounds any charged body. The magnitude of this field is given by the electric field strength E. If a charge Q is present within an electric field (resulting from another different charge), it is subjected to a force F. The relationship between the force and the field strength is given by the following expression:

 

The force itself thus obeys the following equation:

The force on a charge in an electric field is therefore stronger when the field is stronger and when the charge is itself is greater.

Electric field is not solely defined by the magnitude of the force on the charge, however, but by its direction. Electric fields are thus portrayed in the form of field line diagrams, that indicate the direction of the force. The form of an electric field is given by the geometric shapes of the charges that give rise to the field and by their position with respect to one another. The field lines indicate at any point in the field in which direction the electrical force will act on another charge. The following graphic shows an electric field around a positive point charge (left). The field lines radiate in straight lines from the charge. The direction of the lines is indicated by arrows. The arrows indicate the direction that a positive point charge (the smaller points in the graphic) would move if free to do so.  Field lines always radiate from a positive charge (or infinity). The density of the field lines (how close they are together) shows the strength of the electric field, so that in this case the field strength decreases with increasing distance from the point charge. 

If positive and negative charges are evenly distributed on two metal plates positioned parallel to one another (as is the case with a plate capacitor as we shall see later), the field lines between the plates are parallel as shown in the following graphic. The field lines emerge from the positively charged plate and end at the negatively charged plate. Since the density of lines inside a plate capacitor is always the same, the electric field strength E between the plates is also the same.  Such an electric field is described as being uniform

Note: field lines also run outside the capacitor but these are curved and are not shown here since we shall not be studying them in detail. 

Voltage:

If, for example, you rub a hard rubber rod with a woollen cloth, charged particles are exchanged between one substance and the other. Some electrons from the previously neutral wool are rubbed off onto the surface of the rubber. The rubber then has an excess number of electrons and is thus negatively charged. At the same time the wool has a deficit of electrons and is thus charged positively. A similar effect can be observed if you pull a tight nylon sweater over your head. That can really make your hair stand on end.

In current sources such as batteries or generators, positive and negative charges that exist in all materials are separated from one another by the effects of some energy. One terminal of the source thus has an excess of electrons (the negative pole) while the other displays a deficit (the positive pole). An electric field thus exists between these two charges and the system will try to even this imbalance out so that charges flow from one terminal to the other and generate a so-called electric current

If both poles are connected via conductors the charges seek to even out by passing along these conductors and giving rise to a current. This involves the current source (e.g. a battery) exerting an amount of work W on the charge Q that has been transported. The voltage U of the source is now defined as the quotient of the work and the charge: 

Basic Electrical Properties

(Note: usage of U as the symbol for voltage is conventional in some European countries; elsewhere, the letter V may be used instead.)

The unit of electrical voltage is called the Volt, or "V" for short. A voltage can only exist between two points (e.g. the poles of an electricity source).

The meaning of the concept of voltage can be illustrated by an analogy from the world of physics. The electricity source is equated with a water pump. A pump is able to exert an amount of lifting work on each litre of water it raises up to a certain maximum height. Thus for each litre of water it provides a certain amount of energy. Considering the two cases sketched in the following graphic, Pump P2 (right) has to exert twice the amount of work on each litre as Pump P1 (left) since the height is twice as high. The quotient of the work and the volume is thus twice as large. The analogy with electricity would then suggest that the right-hand pump had double the "voltage".

Basic Electrical Properties

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