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##### Electric and Electronic Principles

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**Diode**Earth Resistors Transformer LED Op Amp Transistor Thermistor Circuit symbols**EMF**• Electromotive "force" is not considered a force, as force is measured in newtons, but a potential, or energy per unit of charge, measured in volts • PD • Potential difference measured between two points (eg across a component) if a measure of the energy of electric charge between the two points Definitions**Current**• The flow of electric charge • Resistance • The resistance to current • Capacitors • Store charge in circuit Definitions**The ammeter is in series with components in the circuit**The voltmeter is connected in parallel with the components in the circuit Simple circuit**Current stays the same all the way round a series circuit**Current in a series circuit**The voltage (pd) across the battery terminals is shared**between all the components in the circuit voltage in a series circuit**The total current is shared by the components in a parallel**circuit Current in a parallel circuit**The electrical resistance of an electrical conductor is the**opposition to the passage of an electric current through that conductor Resistance**αΔT = ΔR/R₀**• ΔR = αR₀ΔT Temperature coefficient of resistance**Question**• A copper wire has a resistance of 400 Ω at 0o C • 1, Calculate the resistance at 30oC if the temperature coefficient of copper is 0.0043/oC Question**If mercury is cooled below 4.1 K, it loses all electric**resistance • The critical temperature for superconductors is the temperature at which the electrical resistivity of a metal drops to zero. The transition is so sudden and complete that it appears to be a transition to a different phase of matter;. Several materials exhibit superconducting phase transitions at low temperatures. superconductors**The thermistors we normally**refer to are NTC where the resistance increases when the temperature decreases PTC thermistor resistors Increase resistance with Increasing temperature**In the above test the open circuit The open circuit voltage**was measured. The decade box was then set to a maximum and connected as the load. The resistance of the box was reduced so that the voltage across it decreased by 10% each time. From this information the load current and the power in the load was calculated for each voltage. Graphs of load voltage VL against load current IL and power in the load PL against load resistance RL were plotted.**Graph of**VL against IL VO/C VL IL Calculating the gradient of the graph gives us the internal resistance of the source**Graph of**PL against RL PL The peak (maximum power) is where the load resistance is equal to the internal resistance of the source RL RL = RS**Using Kirchoff’s second Law The sum of all the PD’s**around the circuit is equal to the e.m.f. of the source. If the load resistance is equal to the internal resistance then the PD across each must be the same. Thus VL must be half the e.m.f. of the cell r VI R VL This means that maximum power is obtained when the load resistance is equal to the internal resistance. As was show in the experiment**The need for Maximum power transfer is when there is a high**source impedance and power is scarce. This is contrasted to when power is abundant (i.e. low source impedance)and a constant voltage is available Power is inversely proportional to load resistance. That is the higher the load resistance the lower the power**V out = V in x R2/ R1 +R2**Basic voltage divider circuit**Internal or source resistance is always less than**the lowest of R1 or R2 When measured in a half voltage test**Capacitance is typified by a parallel plate arrangement and**is defined in terms of charge storage: Capacitors**A dielectric is an electrical insulator that can be**polarized by an applied electricfield. When a dielectric is placed in an electric field Capacitors**A dielectric is an electrical insulator that can be**polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material as they do in a conductorbut only slightly shift from their average equilibrium positions causing dielectric polarization. Capacitors**In an insulating material, the maximum electric field**strength that it can withstand intrinsically without breaking down, i.e., without experiencing failure of its insulating properties. Field strength E = V/d • V = potential across the plates • D = distance between the plates Capacitors**In a test on a 1mm thickness of polymer, it is ruptured by**an applied voltage of 20kV. • a) Calculate the dielectric strength of the material • b) Describe what happens in the material when the rupture occurs c) Explain why a solid insulator with a hairline crack through it breaks down at a lower voltage than the rated voltage Capacitors**The permittivity of a substance is a characteristic which**describes how it affects any electric field set up in it. A high permittivity tends to reduce any electric field present. We can increase the capacitance of a capacitor by increasing the permittivity of the dielectric material. Permittivity**The permittivity of free space (or a vacuum), e0, has a**value of 8.9 × 10-12 F m-1. • The absolute permittivity ε of all other insulating materials is greater than ε0. • The ratio ε / ε0 is called relative permittivity of the material and is denoted by K (or εr). • K = ε / ε0 = Absolute permittivity of medium / Absolute permittivity of air Permittivity**Capacitance is increased by the use of a dielectgric**Permittivity**The energy stored in a capacitor can be expressed as**W = 1/2 C V2 (1) where W = energy stored (Joules) C = capacitance (Farad) V = potential difference (Voltage) Energy stored in a capacitor**A 2.0kV power supply unit has an internal 2.6μF capacitor**connected across the output. • a) Calculate the charge stored • b) Calculate the energy stored • c) State how stored charge creates a hazard • d) Describe how the hazard may be reduced Example question**A variable capacitor is a capacitor whose capacitance may be**intentionally and repeatedly changed mechanically or electronically Variable capacitor**Types of variable capacitors**• Mechanically controlled In mechanically controlled variable capacitors, the distance between the plates, or the amount of plate surface area which overlaps, can be changed Variable capacitor**Electronically controlled**• The thickness of the depletion layer of a reverse-biased semiconductor diode varies with the DC voltage applied across the diode. Any diode exhibits this effect (including p/n junctions in transistors) • Their use is limited to low signal amplitudes Variable capacitor**Transducers**• In a capacitor microphone (commonly known as a condenser microphone), the diaphragm acts as one plate of a capacitor, and vibrations produce changes in the distance between the diaphragm and a fixed plate, changing the voltage maintained across the capacitor plates. Variable capacitor**An air-spaced variable capacitor has semi-circular plates.**Minimum capacitance is 20pF (at 0°) and maximum capacitance is 400pF when the shaft is rotated 180°. a) Sketch a graph of capacitance against angle of rotation of the shaft b) Calculate the capacitance when the shaft is rotated 90° c) Calculate the maximum capacitance if a polymer film of relative permittivity 2.3 is placed in the airspace between the plates**CT = C1 + C2etc**Capacitors in parallel**1/CT = 1/C1 + 1/C2 + 1/C3 etc**Capacitors in series**C = Q/V Q = CV**Q = CVmax(1 – e-t/RC) V max Voltage I = (V/R) – e-t/RC current Time Capacitor Charging