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Chapter 2

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Chapter 2

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  1. ENT 114: CIRCUIT THEORY Chapter 2

  2. Voltage, Current and Resistance

  3. Model of atom The Bohr atom is a tool for visualizing atomic structure. • The nucleus is positively charged and has the protons and neutrons. • Electrons are negatively charged and in discrete shells. • The atomic number is the number of protons and determines the particular element. • In the neutral atom, the number of electrons is equal to the number of protons.

  4. Atomic structure The outer shell is called the valence shell. Electrons in this shell are involved in chemical reactions and in metals they account for electrical and thermal conductivity. A neutral Si atom is shown. There are 4 electrons in the valence shell. Question: Is Si a conductor, insulator, or semiconductor? Semiconductor

  5. Force There is a force (F) between charges. Like charges repel; unlike charges attract. • The force is directly proportional to charge. • The force is inversely proportional to square of distance.

  6. The force of attraction or repulsion between two charged bodies Q1 and Q2 can be determined by Coulomb’s Law: • Where F is in Newtons (N), k a constant = • Q1 and Q2 are the charges in coulombs

  7. Voltage Voltage (V) is the energy (W) per charge (Q); it is responsible for establishing current. Work is done as a charge is moved in the electric field from one potential to another. Voltage is the work per charge done against the electric field.

  8. Voltage Definition of voltage One volt is the potential difference (voltage) between two points when one joule of energy is used to move one coulomb of charge from one point to the other.

  9. Voltage Voltage Voltage is responsible for establishing current. Sources of voltage include batteries, solar cells, and generators. A Cu-Zn battery, such as you might construct in a chemistry class, is shown.

  10. Current Current (I) is the amount of charge (Q) that flows past a point in a unit of time (t). One ampere is a number of electrons having a total charge of 1 C move through a given cross section in 1 s. Question: What is the current if 2 C passes a point in 5 s? 0.4 A

  11. Motion of negatively charged electrons in a copper wire when placed across battery terminals with a difference in potential of volts (V).

  12. Flow of electric charges -ve charge (electron) flow to +ve terminal. + ve charge flow to –ve terminal This motion creates electric current It is conventional to take the current flow as the movement of positive charge, that is opposite to the flow of electron. Current (I) is the amount of charge (Q) that flows past a point in a unit of time (t).

  13. Electric current is the time rate of change of charge, measured in amperes (A) 1A=1C/s Direct current (dc): current that remains constant with time. Alternating current (ac): current that varies sinusodally with time. i = current in ampere q= charge in coulomb t = time in second Current

  14. Current direct current Alternating current Damped current exponential current

  15. The flow of charge through any material encounters an apposing force similar in many respect to mechanical friction. This opposition, due to the collision between electrons and between electron and other atoms in material, which converts electrical energy into heat, is called the resistance of material. The unit of measurement of resistance is the ohm, for which the symbol is Ω. Resistance

  16. The circuit symbol for resistance : The resistance of any material with a uniform cross-sectional area is determined by the following factors: Material Length Cross-sectional Area Temperature Resistance

  17. Resistance • At a fixed temperature of 20°C, the resistance is related to the other 3 factor by, • Where; • ρ (Greek letter rho) is the characteristic of the material called resistivity, • lis the length of the sample • A is the cross-sectional area of the sample

  18. Resistance • The higher the resistivity, the greater the resistance of conductor • The longer the conductor, the greater the resistance • The greater the area of a conductor, the less the resistance

  19. Resistance: Temperature Effect • Temperature have a significant effect on the resistance of conductors, semiconductors and insulators. • Conductors • For good conductors, an increase in temperature will result in an increase in the resistance level. Consequently conductors have a positive temperature coefficient. • Semiconductor • For semiconductor materials, an increase in temperature will result in a decrease in the resistance level. Consequently, semiconductors have negative temperature coefficient. • Insulators = Semiconductors

  20. Resistance: Temperature Effect • For a moderate range of temperature, such as 100°C, the change of resistance is usually proportional to the change of temperature; • The ratio of the change of resistance per degree change of temperature to the resistance of some definite temperature are called coefficient of resistance, α.

  21. Conductance is the reciprocal of resistance. Resistance Resistance is the opposition to current. One ohm (1 W) is the resistance if one ampere (1 A) is in a material when one volt (1 V) is applied. Components designed to have a specific amount of resistance are called resistors.

  22. Resistance

  23. Resistance Question What is the resistance and tolerance of each of the four-band resistors? 5.1 kW ± 5% 820 kW ± 10% 47 W ± 10% 1.0 W ± 5%

  24. Resistance Alphanumeric Labeling • Two or three digits, and one of the letters R, K, or M are used to identify a resistance value. • The letter is used to indicate the multiplier, and its position is used to indicate decimal point position.

  25. Resistance Variable resistors include the potentiometer and rheostat. A potentiometer can be connected as a rheostat. The center terminal is connected to the wiper

  26. Basic Circuit A basic circuit consists of 1) a voltage source, 2) a transmission system and 3) a load 4) a control apparatus. An example of a basic circuit is the flashlight, which has each of these.

  27. One element on simple circuit is a mathematical model for electric apparatus that have two terminals. Basic Circuit Elements • Active Elements • Could supplied power to circuits • Example : Voltage and Current source • Passive Elements • Only could absorb power • Example : resistor, inductance, capasitance, diod and etc. Basic Circuit Elements

  28. An ideal Independent source is an active element that provides a specified voltage or current that is completely independent of other circuit element An Ideal Dependent source is an active element which the source quantity is controlled by another voltage or current. Voltage and Current Source

  29. Independent source Voltage Current

  30. Dependent source dependent voltage- Controlled voltage source dependent current- controlled voltage source dependent voltage- controlled current source dependent current- controlled current source

  31. DMM An important multipurpose instrument is the DMM, which can measure voltage, current, and resistance. Many include other measurement options.

  32. Selected Key Terms Ampere AWG Charge Circuit The unit of electrical current (American Wire Gauge) A standardization based on wire diameter An electrical property of matter that exists because of an excess or a deficiency of electrons. Charge can be either + or -. An interconnection of electronic components designed to produce a desired result. A basic circuit consists of a source, a load, and an interconnecting path.

  33. Selected Key Terms Conductance Coulomb Current Electron Ground Ohm (W) The ability of a circuit to allow current. The unit is the siemans (S). The unit of electrical charge. The rate of flow of electrical charge. A basic particle of electrical charge in matter. The electron possesses a negative charge. The common or reference point in a circuit. The unit of resistance.

  34. Selected Key Terms Potentiometer Resistance Rheostat Siemens Volt Voltage A three-terminal variable resistor. The opposition to current. the unit is the ohm (W). A two-terminal variable resistor. The unit of conductance The unit of voltage or electromotive force. The amount of energy per charge available to move electrons from one point to another in an electric circuit.

  35. Quiz 1. The atomic number is the number of a. protons in the nucleus b. neutrons in the nucleus c. protons plus neutrons in the nucleus d. electrons in the outer shell

  36. Quiz 2. Valence electrons are a. in the outer shell b. involved in chemical reactions c. relatively loosely bound d. all of the above

  37. Quiz 3. The atomic particle responsible for electrical current in solid metallic conductors is the a. proton b. electron c. neutron d. all of the above

  38. Quiz 4. The symbol for charge is a. C b. W c. Q d. W

  39. Quiz 5. The definition for voltage is a. b. c. d.

  40. Quiz 6. A battery stores a. electrons b. protons c. ions d. chemical energy

  41. Quiz 7. The unit of conductance is the a. ohm b. coulomb c. siemen d. ampere

  42. Quiz 8. A four-color resistor with the color bands gray-red-black-gold is a. 73 W b. 82 W c. 680 W d. 820 W

  43. Quiz 9. A 330 kW± 5% resistor has the color bands a. red-red-brown-gold b. orange-orange-yellow-gold c. yellow-yellow-red-gold d. yellow-yellow-green-gold