Wednesday, March 4, 2015

An Introduction to Electric Circuits

Electrical/Electronic System Block Diagrams

An electrical/electronic system is a group of components connected together to perform a desired function. Figure 40.1 shows a simple public address system, where a microphone is used to collect acoustic energy in the form of sound pressure waves and converts this to electrical energy in the form of small voltages and currents; the signal from the microphone is then amplified by means of an electronic circuit containing transistors/integrated circuits before it is applied to the loudspeaker.

A sub-system is a part of a system that performs an identified function within the whole system; the amplifier in Figure 40.1 is an example of a sub-system.

A component or element is usually the simplest part of a system that has a specific and well-defined function — for example, the microphone in Figure 40.1.

The illustration in Figure 40.1 is called a block diagram and electrical/ electronic systems, which can often be quite complicated, can be better under-stood when broken down in this way. It is not always necessary to know precisely what is inside each sub-system in order to know how the whole system functions.

As another example of an engineering system, Figure 40.2 illustrates a temperature control system containing a heat source (such as a gas boiler), a fuel controller (such as an electrical solenoid valve), a thermostat and a source of electrical energy. The system of Figure 40.2 can be shown in block diagram form as in Figure 40.3; the thermostat compares the actual room temperature with the desired temperature and switches the heating on or off.

There are many types of engineering systems. A communications system is an example, where a local area network could comprise a file server, coaxial cable, network adapters, several computers and a laser printer;an electromechanical system is another example, where a car electrical sys- tem could comprise a battery, a starter motor, an ignition coil, a contact breaker and a distributor. Block diagrams may represent all such systems as these.

image

image

Standard Symbols for Electrical Components

Symbols are used for components in electrical circuit diagrams and some of the more common ones are shown in Figure 40.4.

Electric Current and Quantity of Electricity

All atoms consist of protons, neutrons and electrons. The protons, which have positive electrical charges, and the neutrons, which have no electrical charge, are contained within the nucleus. Removed from the nucleus are minute negatively charged particles called electrons. Atoms of different materials differ from one another by having different numbers of protons, neutrons and electrons. An equal number of protons and electrons exist within an atom

clip_image007

Figure 40.4

and it is said to be electrically balanced, as the positive and negative charges cancel each other out. When there are more than two electrons in an atom the electrons are arranged into shells at various distances from the nucleus.

All atoms are bound together by powerful forces of attraction existing between the nucleus and its electrons. Electrons in the outer shell of an atom, however, are attracted to their nucleus less powerfully than are electrons whose shells are nearer the nucleus.

It is possible for an atom to lose an electron; the atom, which is now called an ion, is not now electrically balanced, but is positively charged and is thus able to attract an electron to itself from another atom. Electrons that move from one atom to another are called free electrons and such random motion can continue indefinitely. However, if an electric pressure or voltage is applied across any material there is a tendency for electrons to move in a particular direction. This movement of free electrons, known as drift, constitutes an electric current flow. Thus current is the rate of movement of charge.

Conductors are materials that contain electrons that are loosely connected to the nucleus and can easily move through the material from one atom to another. Insulators are materials whose electrons are held firmly to their nucleus.

image

Electrical Potential and e.m.f.

The unit of electric potential is the volt (V), where one volt is one joule per coulomb. One volt is defined as the difference in potential between two points in a conductor which, when carrying a current of one ampere, dissipates a power of one watt, i.e.

image

A change in electric potential between two points in an electric circuit is called a potential difference (p.d.). The electromotive force (e.m.f.) provided by a source of energy such as a battery or a generator is measured in volts.

For a continuous current to flow between two points in a circuit a potential difference or voltage, V, is required between them; a complete conducting path is necessary to and from the source of electrical energy.

Figure 40.5 shows a cell connected across a filament lamp. Current flow, by convention, is considered as flowing from the positive terminal of the cell, around the circuit to the negative terminal.

image

Resistance and Conductance

The flow of electric current is subject to friction. This friction, or opposition, is called resistance R and is the property of a conductor that limits current. The unit of electric resistance is the ohm (Z); 1 ohm is defined as the resistance that will have a current of 1 ampere flowing through it when 1 volt is connected across it.

image

where V is the potential difference across the two points, in volts, and I is the current flowing between the two points, in amperes.

The reciprocal of resistance is called conductance and is measured in siemens (S). Thus,

image

Basic Electrical Measuring Instruments

An ammeter is an instrument used to measure current and must be connected in series with the circuit. Figure 40.5 shows an ammeter connected in series with the lamp to measure the current flowing through it. Since all the current in the circuit passes through the ammeter it must have a very low resistance.

A voltmeter is an instrument used to measure p.d. and must be connected in parallel with the part of the circuit whose p.d. is required. In Figure 40.5, a voltmeter is connected in parallel with the lamp to measure the p.d. across it. To avoid a significant current flowing through it a voltmeter must have a very high resistance.

An ohmmeter is an instrument for measuring resistance.

A multimeter, or universal instrument, may be used to measure voltage, current and resistance. An ‘Avometer’ is a typical example.

The cathode ray oscilloscope (CRO) may be used to observe waveforms and to measure voltages and currents. The display of a CRO involves a spot of light moving across a screen. The amount by which the spot is deflected from its initial position depends on the p.d. applied to the terminals of the CRO and the range selected. The displacement is calibrated in ‘volts per cm’. For example, if the spot is deflected 3 cm and the volts/cm switch is on 10 V/cm then the magnitude of the p.d. is 3 cm x10 V/cm, i.e. 30 V.

(See Chapter 50 for more detail about electrical measuring instruments and measurements)

Linear and Non-linear Devices

Figure 40.6 shows a circuit in which current I can be varied by the variable resistor R2. For various settings of R2, the current flowing in resistor R1 ,

image

displayed on the ammeter, and the p.d. across R1 , displayed on the voltmeter, are noted and a graph is plotted of p.d. against current. The result is shown in Figure 40.7(a) where the straight-line graph passing through the origin indicates that current is directly proportional to the p.d. Since the gradient, i.e.

p.d./current is constant, resistance R1 is constant. A resistor is thus an example of a linear device.

If the resistor R1 in Figure 40.6 is replaced by a component such as a lamp then the graph shown in Figure 40.7(b) results when values of p.d. are noted for various current readings. Since the gradient is changing, the lamp is an example of a non-linear device.

Ohm’s Law

Ohm’s law states that the current I flowing in a circuit is directly proportional to the applied voltage V and inversely proportional to the resistance R, provided the temperature remains constant. Thus,

image

image

Multiples and Sub-multiples

Currents, voltages and resistances can often be very large or very small. Thus multiples and sub-multiples of units are often used, as stated in chapter 1. The most common ones, with an example of each, are listed in Table 40.1.

For example, a 100 V battery is connected across a resistor and causes a current of 5 mA to flow. The resistance of the resistor is given by:

image

Conductors and Insulators

A conductor is a material having a low resistance that allows electric current to flow in it. All metals are conductors and some examples include copper, aluminium, brass, platinum, silver, gold and carbon.

An insulator is a material having a high resistance that does not allow electric current to flow in it. Some examples of insulators include plastic, rubber, glass, porcelain, air, paper, cork, mica, ceramics and certain oils.

Electrical Power

The product of potential difference V and current I gives power P in an electrical circuit. The unit of power is the watt, W.

image

Electrical Energy

Electrical energy = power × time

If the power is measured in watts and the time in seconds then the unit of energy is watt-seconds or joules. If the power is measured in kilowatts and the time in hours then the unit of energy is kilowatt-hours, often called the ‘unit of electricity’. The ‘electricity meter’ in the home records the number of kilowatt-hours used and is thus an energy meter.

image

For example, electrical equipment in an office takes a current of 13 A from a 240 V supply. If the equipment is used for 30 hours each week and 1 kWh of energy costs 7p, the cost per week of electricity is calculated as follows:

image

image

Main Effects of Electric Current

The three main effects of an electric current are: (a) magnetic effect

(b) chemical effect (c) heating effect

Some practical applications of the effects of an electric current include:

Magnetic effect: bells, relays, motors, generators, transformers, telephones, car-ignition and lifting magnets (see chapter 47)

Chemical effect: primary and secondary cells and electroplating (see chapter 42)

Heating effect: cookers, water heaters, electric fires, irons, furnaces, kettles and soldering irons

Fuses

A fuse is used to prevent overloading of electrical circuits. The fuse, which is made of material having a low melting point, utilises the heating effect of an electric current. A fuse is placed in an electrical circuit and if the current becomes too large the fuse wire melts and so breaks the circuit. A circuit diagram symbol for a fuse is shown in Figure 40.1, on page 207.

For example, an electric toaster has a power rating of 1 kW, and is connected to a 240 V supply.Power taken, P = VI, from which,

image

Hence a 5 A fuse is most appropriate for the toaster.