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WELCOME

WELCOME.

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WELCOME

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  1. WELCOME

  2. Conducting materialsby the end of this lesson you will be able to:1. State and explain the factors which affect the resistance of a resistor.2. define semi-conducting materials and explain how they conduct electricity.3. explain how the diode operates.4. explain the function of the diode in half- wave rectification.

  3. resistance RESISTANCE IS THE OPPOSITION TO CURRENT FLOW. ALL MATERIALS SHOW SOME RESISTANCE TO CURRENT FLOW. GOOD CONDUCTORS SHOW LITTLE RESISTANCE TO CURRENT FLOW. POOR CONDUCTORS(INSULATORS) SHOW HIGH RESISTANCE TO CURRENT FLOW.

  4. FACTORS WHICH AFFECT RESISTANCE: • Length of resistor. • Cross sectional area. • Type of material. • Temperature.

  5. FACTORS AFFECTING RESISTANCE

  6. Length of resistor. The longer the resistor Greater the resistance Lesser the current flow

  7. Cross sectional area. Thinner the resistor Higher the resistance Thicker the resistor Lower the resistance

  8. Type of material Silver allows electrons to flow through most rapidly. Copper is next best and is widely used since it is relatively cheap. When high resistance is needed, alloys are used such as constantan, manganin or nichrome. These have about 50-60 times the resistance of copper. Some materials allow current to flow based on its temperature.

  9. Temperature. The resistance of pure metals increases with temperature. The resistance of certain other conducting materials decrease with temperature. E.g. carbon. Certain other materials- germanium, silicon and selenium- show a decrease in resistance as the temperature increases. These materials are called semi-conductors. The resistance can be further decreased if these materials are doped with certain impurities.

  10. SEMI-CONDUCTORS • Semi-conducting materials can conduct current because of • Intrinsic conduction which is due to the natural material. • Extrinsic conduction which is artificial-due to the addition of impurities- ‘doping’.

  11. INTRINSIC CONDUCTION In pure silicon, all the outer electrons are occupied in bonding so no current will flow.

  12. When the temperature is high, few electrons become loose. As these electrons move, they leave ‘holes’, which are positive. INTRINSIC CONDUCTION

  13. INTRINSIC CONDUCTION

  14. INTRINSIC CONDUCTION The net effect is that of ‘holes’ moving in the direction of conventional current, while electrons move in the direction of real flow of current. Semi-conductors are better conductors at higher temperatures. Conduction in a semi-conductor by means of positive holes and thermal electrons is called intrinsic conduction.

  15. EXTRINSIC CONDUCTION Silicon has four electrons in its outermost shell. If it is doped with an atom which has five electrons in its outermost shell, then there will be extra electrons to carry current.

  16. EXTRINSIC CONDUCTION When the doping provides extra electrons, the semi-conducting material is called a n-type (negative) material.

  17. Silicon, when doped with atoms which contains less electrons in the outermost shell such as Aluminium- (3 electrons) will have less electrons to carry current EXTRINSIC CONDUCTION

  18. EXTRINSIC CONDUCTION When doping provides less electrons- i.e. more positive holes, for the conduction of current, the material is said to be p-type (positive). Conduction is by the movement of ‘holes’.

  19. THE JUNCTION DIODE p-n junction diodes can be used to control the direction of current flow in a circuit. The junction diode conducts electricity in one direction only.

  20. Whether or not the current will flow depends on how the diode is connected in the circuit. The connection can be connected in forward bias – current flow or reverse bias – no current flow. THE JUNCTION DIODE Forward bias - Current flows. Bulb lights. Reverse bias - No current flows. Bulb does not light.

  21. Electrons flow from the negative end of the battery and enters the n-type side of the diode. These electrons are attracted across the junction to the p-type end (positive). As electrons jump over, holes are left on the n-type side. The holes appear to move from the p-type to the n-type side. FORWARD BIAS

  22. Current will flow in the circuit and the diode is said to be forward bias. The negative end of the battery is connected to the n-type end of the diode. FORWARD BIAS

  23. FORWARD BIAS

  24. Electrons flow from the negative end of the battery to the p-type end of the diode. The electrons simply fill the ‘holes’ and do not jump across the junction. There is no attraction for the electrons to move across the junction. REVERSE BIAS

  25. REVERSE BIAS No current will flow in the circuit and the diode is said to be in reverse bias. The negative end of the battery is connected to p-type end of the diode.

  26. REVERSE BIAS

  27. HALF-WAVE RECTIFICATION Diodes are used to convert alternating current into direct current. The current is said to be rectified.

  28. The diode only allows current to flow in a certain direction as shown. HALF-WAVE RECTIFICATION

  29. If the diode is reversed, then it will only allow the current to flow in the opposite direction. HALF-WAVE RECTIFICATION

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