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Course Progress Introductory level Electrostatic, Coulomb s Law Electric Field, Gauss Law Magnetic field, Maxwell s Equations Current, Resistance, Capacitance, and Dielectrics Inductance and Alternating Current Basic Circuits and Analysis Ohm s Law, Current Divider, Voltage Divider Semiconductor circuit elements (Diodes and Transistors) Essential Circuit analysis (Kirchhoff Laws, Wheatson Bridges) Electrical Biosignals and Detection Techniques Filters, Sensors ECG, EMG, GSR 2
Basic Electricity and Electronics Chapter 1 : Fundamentals 3
Units 4
Atomic Model and Material Classification All materials may be classified into one of three major groups conductors, insulators and semiconductors. In simple terms, the group into which a material falls depends on how many free electrons it has. The term free refers to those electrons that have acquired sufficient energy to leave their orbits around their parent atoms. In general we can say that conductors have many free electrons which will be drifting in a random manner within the material. Insulators have very few free electrons (ideally none), and semiconductors fall somewhere between these two extremes. 5
Atomic and Molecular orbitals, and Bands in solids Hydrogen Atom Isophthalic Acid Sodium Crystal 6
Conduction of charges in solids 7
Metals and Insulators 8
Semiconductors Band structure of a semiconductor at ordinary temperatures (T 300 K). The energy gap is much smaller than in an insulator, and some electrons from the valence band occupy states in the conduction band. 9
Electric current This is the rate at which free electrons can be made to drift through a material in a particular direction. 10
Electric Current Electric current is the rate of flow of charge through some region of space. The SI unit of current is the ampere (A). 1 A = 1 C / s The symbol for electric current is I = dq dt.! Current is considered to be the flow of charge carriers, but not the electron, however due to motion of electrons 11
Charge Carrier Motion in a Conductor The measurement of the motion of charge carriers is determined by the Current Density J I A = nqv d A is the cross-sectional area of the conductor. The amount of current density is directly proportional to the Electric Field J E This proportionality if determined by the conductivity σ of the material. J = σe Ohm s Law (Microscopic Definition) 12
Electromotive Force (emf) The random movement of electrons within a material does not constitute an electrical current. This is because it does not result in a drift in one particular direction. In order to cause the free electrons to drift in a given direction an electromotive force must be applied. Thus the emf is the driving force in an electrical circuit. The symbol for emf is E and the unit of measurement is the volt (V). Typical sources of emf are cells, batteries and generators. 13
Electromotive Force (emf) The amount of current that will fl ow through a circuit is directly proportional to the size of the emf applied to it. The circuit diagram symbols for a cell and a battery are shown in Fig. 14
Resistance (R) Although the amount of electrical current that will flow through a circuit is directly proportional to the applied emf, the other property of the circuit (or material) that determines the resulting current is the opposition offered to the flow. This opposition is known as the electrical resistance, which is measured in ohms ( ). 15
Resistance (R) Conductors, which have many free electrons available for current carrying, have a low value of resistance. Insulators have very few free charge carriers then insulators, therefore they have a very high resistance. Pure semiconductors tend to behave more like insulators in this respect. However, in practice, semiconductors tend to be used in an impure form, where the added impurities improve the conductivity of the material. 16
Resistor An electrical device that is designed to have a specified value of resistance is called a resistor. The circuit diagram symbol for a resistor is shown in Fig. 17
Resistors The circuit elements that control the amount of current is called resistor. Resistance of a wire is given by R = ρ l A R Resistance, ρ: Resistivity, l: length, A: cross sectional area 18
Resistor Color Code 19
Temperature dependency of resistance The resistance of a material also depends on its temperature and has a property known as its temperature coefficient of resistance. The resistance of all pure metals increases with increase of temperature. The resistance of carbon, insulators, semiconductors and electrolytes decreases with increase of temperature. For these reasons, conductors (metals) are said to have a positive temperature coefficient of resistance. Insulators etc. are said to have a negative temperature coefficient of resistance. 20
Temperature dependency of resistance Over a moderate range of temperature, the change of resistance for conductors is relatively small and is a very close approximation to a straight line. Semiconductors on the other hand tend to have very much larger changes of resistance over the same range of temperatures, and follow an exponential law. These differences are illustrated in Fig. 21
Temperature dependency of resistance The reference temperature usually quoted is 0 C, and the resistance at this temperature is referred to as R 0. Thus the resistance at some other temperature 1 C can be obtained from: 22
Potential Difference (p.d.) Whenever current flows through a resistor there will be a p.d. developed across it. The p.d. is measured in volts, and is quite literally the difference in voltage levels between two points in a circuit. Although both p.d. and emf are measured in volts they are not the same quantity. Essentially, emf (being the driving force) causes current to fl ow; whilst a p.d. is the result of current fl owing through a resistor. Thus emf is a cause and p.d. is an effect. It is a general rule that the symbol for a quantity is different to the symbol used for the unit in which it is measured. One of the few exceptions to this rule is that the quantity symbol for p.d. happens to be the same as its unit symbol, namely V. In order to explain the difference between emf and p.d. we shall consider another analogy. 23
Emf & Potential Difference (p.d.) 24
Conventional current and electron flow Whenever we are considering basic electrical circuits and devices we shall use conventional current flow i.e. current flowing around the circuit from the positive terminal of the source of emf to the negative terminal. 25
Ohm s Law This states that the p.d. developed between the two ends of a resistor is directly proportional to the value of current flowing through it, provided that all other factors (e.g. temperature) remain constant. Writing this in mathematical form we have: However, this expression is of limited use since we need an equation. This can only be achieved by introducing a constant of proportionality; in this case the resistance value of the resistor. 26
Internal Resistance (r) Internal Resistance (r) So far we have considered that the emf Evolts of a source is available at its terminals when supplying current to a circuit. If this were so then we would have an ideal source of emf. Unfortunately this is not the case in practice. This is due to the internal resistance of the source. As an example consider a typical 12 V car battery. This consists of a number of oppositely charged plates, appropriately interconnected to the terminals, immersed in an electrolyte. The plates themselves, the internal connections and the electrolyte all combine to produce a small but finite resistance, and it is this that forms the battery internal resistance. An electrolyte is the chemical cocktail in which the plates are immersed. In the case of a car battery, this is an acid/water mixture. In this context, finite simply means measurable. 27
Internal Resistance (r) Figure shows such a battery with its terminals on open circuit (no external circuit connected). Since the circuit is incomplete no current can flow. Thus there will be no p.d. developed across the battery s internal resistance r. Since the term p.d. quite literally means a difference in potential between the two ends of r, then the terminal A must be at a potential of 12 V, and terminal B must be at a potential of 0 V. Hence, under these conditions, the full emf 12 V is available at the battery terminals. 28
Internal Resistance (r) Figure shows an external circuit, in the form of a 2 resistor, connected across the terminals. Since we now have a complete circuit then current I will flow as shown. The value of this current will be 5.71 A (the method of calculating this current will be dealt with early in the next chapter). This current will cause a p.d. across r and also a p.d. across R. 29
Internal Resistance (r) These calculations and the consequences for the complete circuit now follow: 30
Internal Resistance (r) Results Assuming that the battery s charge is maintained, then its emf remains constant. But its terminal p.d. varies as the current drawn is varied, such that Rather than having to write the words p.d. across R it is more convenient to write this as V AB, which translated, means the potential difference between points A and B. In future, if no mention is made of the internal resistance of a source, then for calculation purposes you may assume that it is zero,i.e. an ideal source. 31
Energy (W) This is the property of a system that enables it to do work. Whenever work is done energy is transferred from that system to another one. The most common form into which energy is transformed is heat. Thus one of the effects of an electric current is to produce heat (e.g. an electric kettle). 32
Power (P) This is the rate at which work is done, or at which energy is dissipated. The unit in which power is measured is the watt (W). 33
Alternating and Direct Quantities The sources of emf and resulting current flow so far considered are called d.c. quantities. However, the other commonly used form of electrical supply is that obtained from the electrical mains. This is an alternating or a.c. supply in which the current flows alternately in opposite directions around a circuit. Again, the term strictly means alternating current, but the emfs and p.d.s associated with this system are referred to as a.c. voltages. Thus, an a.c. generator (or alternator) produces an alternating voltage. Most a.c. supplies provide a sinusoidal waveform (a sinewave shape). Both d.c. and a.c. waveforms are illustrated in Fig. 34
Alternating and Direct Quantities 35
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