Current electricity

Conductors and insulators

From an electrical point of view, all materials can be classified according to their behavior in an electric circuit. Materials which conduct electricity quite easily are called conductors. Materials which offer a high opposition to the flow of electricity are called non­conductors or insulators. There is a third group of materials called semiconductors, which conduct under some conditions and not under others.

Whether a material is a good or bad conductor cannot be decided in some arbitrary fashion because there is no sharp dividing line. All materials have some opposition to the movement of electrons and the degree of opposition governs the use of the material. For example, an electric light receives electricity by means of a good conductor (copper) and, to ensure the electrons flow only where desired, the copper wires are encased in a plastic sheathing (a poor conductor, or a good insu­lator). The circumstances governing a material’s use also have an effect on the selection.

Some salts are insulators when solid but are good conductors when molten. Neon is an insulator when not put under an electrical stress but becomes a conductor when (lie applied voltage is high enough to ionise the gas.

It is important to realise that there are degrees of conductivity. Under normal everyday use there is no perfect conductor, nor is there a perfect insulator. Semi­conductors in the highly refined state are very poor con­ductors (i.e. good insulators). As their content of impurities is increased, their ability to conduct electrons also increases. Semiconductors are first purified and then “doped” to the required degree of impurity to control the rate of flow of electrons through them.

Insulators that are porous and capable of absorbing moisture must be treated with caution because as they become damp their insulating qualities decrease mark­edly. These materials (e.g. wood) are often impregnated with varnish to prevent the absorption of moisture.

Conductors

The obvious characteristic of most good conductors of electricity is that they are metals. The common feature for any good conductor involves the outermost electrons in the valency ring or shell (see section 2.3.3). In the case of copper, the single electron in the valency ring is not held tightly to the nucleus because it is furthest away and shielded by the other orbits. It is easily removed by another atom and can be passed on from atom to atom. Because of this it is called a `free’ electron. Any good conductor has large numbers of free electrons. Not all metals conduct equally well. The best is silver, followed by copper, gold and then aluminum. The most practical conductor for general use is copper-arising from its good performance as a conductor and its relatively low cost.

Insulators (non-conductors)

The most outstanding characteristic of an insulator is that it contains very few or no free electrons under normal conditions. Without free electrons there can be no current flow and the electrons in the atom remain strongly bound to the nucleus. As a general guide only, the greater the number of electrons in the valency ring the better the material is as an insulator.

Semiconductors

The term semiconductors must not be misunderstood. They are not half-conductors. Pure semiconductors are insulators at low temperatures and reasonably good con­ductors at higher temperatures. The elements germanium and silicon are the most important of the semiconductors in the electrical and electronics industries, although there are some compounds used in special cases. Semicon­ductors belong to group 4 of the Periodic Table, that is, they have 4 electrons in their valency ring. Pure semi­conductors are seldom used; they are usually doped with elements from groups 3 and 5 of the Periodic Table.

Approximate resistivity in ohm-metres
Category Material   (see section 2.10.3)
Insulator high opposition to glass 10 000 000 000 (1010)
electron flow Bakelite 10 000 000 000 (1012 )
    rubber 1 0 000 000 000 000 (1013)
sulfur 10 000 000 000 000 (1013)
Semiconductor silicon 0.8
germanium 0.89
Conductor low opposition to silver 1.63 X 10
electron flow copper 1.72 X 10
aluminium 2.83 X 10
tungsten 5.50 X 10

 

Electric circuits

There are three basis types of circuit. All are shown in Figure 2.13.

Open circuit

If the current path is not continuous between two points of potential difference, the circuit is referred to as an open circuit. Refer to Figure 2.13(a). The circuit has a break in it. No current can flow and the lamp is not alight. This condition occurs in a normal circuit when it is switched off.

Closed circuit

If the circuit is complete as in Figure 2.13(b) then current can flow and the lamp glows. This condition occurs in a normal circuit when it is switched on. A closed circuit is an essential condition for a current flow. To maintain a continuous current flow, a continuous source of electrical energy must be provided to maintain the potential difference at the beginning of the circuit. In Figure 2.13 a car battery is shown as the source of energy but other devices such as generators may be pro­vided when greater quantities of energy are needed. Static electric charges generated by friction are not usually able to carry out this function.

In normal electric curcuits of this type there are further types of circuits called series and parallel circuits. These; are discussed in chapter 4.

Short circuit

This type of circuit is to be avoided whenever possible. It is shown in Figure 2.13(c). The lamp is bypassed by a conductor connected direct from one terminal of the battery to the other. The expression used is that the lamp is `shorted’ out of the circuit, or the circuit has a `dead short’ on it. No current flows through the lamp but an excessive current flows direct from one terminal of the energy supply source to the other. The current flow is not limited by the load as in Figure 2.13(b). In this particular case, unless the battery is protected against excessive current flow in some way the battery and con­ductors can easily be damaged.

Direction of current flow

When electrons are in motion between two points in an electric circuit, the electron flow is referred to as current electricity. The path taken by the current is called an electric circuit. The terminals of the energy source are called its poles. One terminal. of a battery is its positive pole and the other its negative pole.

The positive terminal is the pole which is charged positively. That is, it has a deficiency of negative charges or electrons. The negative terminal is the pole which has a ready supply of electrons freely able to move through an electric circuit.

Figure 2.14 shows two methods, of indicating the direction of electric current flow. In an electric circuit the current flow is always from a point of higher electron potential to one of a lower potential.

In the early days of electrical experimentation it was wrongly assumed that current flow was always from a positive polarity to a negative one. This view lasted for so many years that it is still used today and is called `conventional’ current flow.

Only more modern experimentation showed that a current flow consisted of negative charges flowing towards the positive terminal. That is, an electric current flow was actually an `electron flow’. The conventional current flow version had been in use for so long that it was considered unwise to adopt electron flow as the standard. However, in the USA, electron flow is often used, and to avoid the confusion caused by this practice, a wise student will always check which method is used when using circuits from the USA. This also applies to textbooks as well.

When no convention is stated, it is always assumed that the conventional current flow is being referred to. If electron flow is used, it must be clearly stated that it is being used, to avoid confusion. For the remainder of this book, unless it is specifically stated otherwise, con­ventional current flow is to be assumed.

Current electricity units

Potential difference

The amount of current that flows in any particular circuit depends primarily on the amount of potential difference between the terminals of the circuit.

Potential means the charging of a body or the charg­ing of one end of a conductor. This results in a differ­ence of potential between two bodies or between two ends of a conductor. The term ‘potential difference’ is often shortened to the one word ‘potential’, having the same meaning.

Figure 2.15(a) shows that the rate of flow of water depends on the head or pressure. Increasing the pressure increases the flow of water.

In an electrical circuit the rate of flow of current also depends on the pressure (p.d.), if no other part of the circuit is altered (Fig. 2.15(b)).

If the resistance to flow remains the same, doubling the pressure on the water in a tank forces twice as much water through the pipe. Similarly, doubling the potential difference doubles the current. If a difference in potential of 2 units causes I A to flow, then a potential difference of 4 units causes a 2 A flow. That is, making the voltage twice as large will make the current flow twice as large, provided the opposition or resistance to flow is kept constant.

An electrical pressure is measured in units called volts. By definition (Chapter l) a volt is the pressure causing 1 watt to be dissipated in a circuit when 1 ampere is flowing. By Ohm’s law (see section 2.8) it is also the pressure causing 1 ampere to flow through a resistance of I ohm.

The following abbreviations and symbols are used to indicate electrical pressure:

pressure (p.d.)       V (general symbol)

volt                      V

millivolt                 mV

microvolt               μV

kilovolt                  kV

 

Electrical resistance

If the movement of an electron in a conductor were traced, it could be shown that its movement, while irreg­ular in direction, resulted in a general drift from negative to positive polarity along the conductor. The free elec­trons moving towards positive polarity collide with pos­itive ions in the conductor and tend to recombine to form neutral atoms.

At the time of the collision the electron is slowed down. It can accelerate again due to the applied e.m.f. but suffers further collisions with other ions. The slowing down is a measure of the opposition to current flow by the conductor. The outside source of energy must supply extra energy to accelerate the elec­tron again, and the exchange of energy with further collisions results in the generation of heat and a rise in temperature of the conductor. For the same current flow, the lower the temperature rise, the lower is the conduc­tor’s electrical opposition to current flow.

This opposing force is called the electrical resistance of the circuit. Resistance is that property of a material which opposes the flow of electrons. The resistance of a conductor depends on its length, cross-sectional area, type of material and temperature. The following abbre­viations and sumbols are used to indicate resistance:

Resistance         R (general symbol)

Ohm                 Ω

MicroOhm          μΩ

KiloOhm            kΩ

MegaOhm          MΩ

 

Electric current

Current has been shown to be a flow of negative charges called electrons. For convenience these small charges are grouped into larger units called coulombs. The rate of flow is given as coulombs per second. In section 1.2 the ampere was given as file base electrical unit of flow and defined according to the magnetic force between parallel conductors. By definition, a current flow of I ampere. transfers a charge of I coulomb (6.24 X 1018) electrons each second. The following units and symbols are used to indicate electric current.

Current                            I (general symbol)

Ampere                            A

Milliampere                        Ma

Microampere                     μA

 

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