Units 1,2,3,9,12 Delmars Standard Textbook of Electricity

Transcription

1 Units 1,2,3,9,12 Delmars Standard Textbook of Electricity 1. What are the two basic types of electric sources? Alternating and Direct Current 2. What is the effect of unlike charges on each other? Attract 3. What is the effect of like charges on each other? Repel 4. What is matter and what are the three states? Any substance that has mass and occupies space 5. What is an element? Substance that cannot be divided chemically into two or more simpler substances 6. What are the three parts of an atom? Neutron, proton, and electron 7. What value determines the atomic number of an element? Number of protons 8. What type of charge do electrons have? Negative 9. What is centripetal (centrifugal) force? Tendency of an object to remain at rest or continue moving in a straight line unless acted upon by some external force 10. How many electrons can be held in the third orbit of an atom? 18 (2x(N) 2 ) 11. What is the maximum number of electrons that can be contained in any orbit? What is the valence shell of an atom? Outer shell 13. How many valence electrons do conductors typically have? How many valence electrons do insulators have? What statement can be made about an atom with seven or eight valence electrons? Extremely stable and does not give up electrons easily 16. How many valence electrons do semiconductors have? What is the unique property of semiconductors? Resistance decreases as temperature increases 18. What are the six methods for producing electricity? Magnetism, Chemical Action, Pressure, Heat, Friction, and Light 19. What is the side effect of an electron entering a valence orbit? Heat is often produced

2 Unit 2 1. What is a coulomb? 6.25 x electrons (charge transferred by a current of one ampere in one second) 2. What is an ampere? One coulomb per second 3. What is electron flow theory? Current flows from the most negative part of a circuit to the most positive part 4. How long would it take for electricity to flow around the earth 10 times? 1.3 seconds 5. What is an open circuit? No complete path so no current flows 6. What is a short circuit? Very little or no resistance so large currents flow 7. What is a grounded circuit? When the hot conductor comes into contact with the safety grounding conductor (connected to the case or non-current carrying metal parts) 8. What is the term used to describe electrical pressure? Voltage 9. What is the term used to describe the opposition to current flow? Resistance 10. What is the result of current flowing through a resistance? Heat 11. What is impedance? Total opposition to current flow (Resistance + Impedance + Capacitance) 12. What is a watt? Measurement for the amount of power used in a circuit. (Volts x amps) 13. What is the wattage of a toaster that uses 10 amps of current when connected to a 120 Volt outlet? 1200 W 14. How many watts are in 1 HP? 746 W 15. What is ohm s law? One volt will push one amp through one ohm 16. State ohms law three different ways using E=voltage, I=current, and R=ohms. E = I x R, I = E/R, and R=E/I 17. How do you calculate power in watts when you know the current and resistance of a circuit? Find the voltage E = I x R then find the watts W = E x I

3 18. What is the voltage of a device that draws 11 amps and has a resistance of 21.8 ohms? E = 11 x 21.8 (I x R) = What does the metric prefix mega mean? 1,000, What does the metric prefix milli mean? What does the metric prefix micro mean? What does the metric prefix kilo mean? How is mass measured in the metric system? Kg 24. What is the amperage drawn by a 3000W electric water heater connected to a 240 volt source? 12.5 A 25. What is the resistance of a 2000W electric heater on a 240 volt supply? (Find amperage first) I = 2000/240 (W/E) = 8.33 amps R = E/I =240/8.3 = ohms 26. Fill in the missing chart values (on back of this sheet) for the ohm s law practice problems on page 78. Unit 3 1. What are some of the common uses of static electricity? Copy machines, air cleaners, and paint spray operations 2. What is the most common and annoying demonstration of static electricity? Shock when we walk across a carpet and touch a door knob 3. Under what circumstances can an electrostatic charge build up on a conductor? If it is electrically insulated from surrounding objects 4. What is the result of rubbing a glass rod with a wool cloth? Electrons are removed leaving a positive charged rod 5. What is the result of rubbing a rubber rod with a wool cloth? Wool deposits electrons on the rod giving it a negative charge 6. What is the average charge in a lightening bolt? 15,000,000 Volts 7. What type of charge does a thunder cloud have? (Positive or negative) Either or both

4 8. How does a lightening arrester function? One end is grounded and it is placed near the object to be protected. If the object is struck the lightening jumps across to the arrester and bleeds to ground 9. How are static charges eliminated on vehicles carrying hazardous materials? By dragging a metal chain to bleed off any buildup of static charge 10. How can static charges from touching a door knob in the home be minimized? Humidifying the air or carrying a metal object to touch to the door knob 11. What is the unique property of selenium that makes it useful in copy machines? Changes conductivity with a change in light intensity Unit 9 1. What are the two general types of meters? Analog and digital 2. What are two features of analog meters? Pointer and scale 3. What is the principle of operation for analog meters? Like magnetic poles repel each other 4. What change must be made in AC voltage before it can be read on a moving-coil analog voltmeter? Rectified to DC 5. What type of analog meter is typically used to measure AC voltages? Moving vane 6. How is a voltmeter connected to a circuit? Parallel 7. What property of a voltmeter allows it to be connected across the source? Very high resistance in series with the meter movement 8. What setting should be used on a multirange voltmeter to test a known voltage? Smallest scale that is higher than the known voltage 9. What happens to the value of resistance connected in the circuit as the voltage scale increases? Resistance increases 10. What factor will determine which scale is used on a multimeter? Value to be measured 11. How does the resistance (impedance) of an ammeter compare to the resistance of a voltmeter? Resistance in an ammeter is very small (.1 ohm typically)

5 12. What is the result of connecting an ammeter across a supply voltage? Damage to the ammeter due to the low impedance and high current flow 13. What is the term used to describe a low-resistance device that conducts most of the current away from a meter movement? Shunt 14. What is the resistance of an ammeter shunt designed to have a voltage drop of 50 millivolts when 25 amperes of current flow through it?.002 ohms (2 milliamps) 15. What is a make-before-break switch? Next set of contacts close before the previous ones open 16. Don t worry about the Ayrton Shunt section!! 17. What device is used to change scale values on most AC ammeters? Selector switch 18. What is turns ratio? Ratio of turns in the primary to turns in the secondary (1:3 or 10:1, etc) 19. What is the formula for the current and turns relationship in an AC ammeter? N p /N S = I S /I P 20. How many turns are in the secondary winding of a current transformer if the primary has 20 turns, the primary current is 15 amps, and the secondary current is 5 amps? N S = How many turns are in the secondary winding of a current transformer if the primary has 40 turns, the primary current is 20 amps, and the secondary current is 10 amps? N S = What is the secondary current in a current transformer when there are 30 turns in the primary, 15 turns in the secondary, and the primary current is 7.5 amps? I S = 15 A 23. What is the secondary current in a current transformer when there are 50 turns in the primary, 10 turns in the secondary, and the primary current is 2 amps? I S = 10 A 24. What is the primary current in a current transformer when there are 120 turns in the primary, 30 turns in the secondary, and the secondary current is 4 amps? I P = 1 A 25. What is the primary current in a current transformer when there are 90 turns in the primary, 15 turns in the secondary, and the secondary current is 6 amps? I P = 1 A

6 26. How many turns are in the primary of a current transformer when there are 12 turns in the secondary, the primary current is 20 amps, and the secondary current is 5 amps? N P = How many turns are in the primary of a current transformer when there are 60 turns in the secondary, the primary current is 30 amps, and the secondary current is 10 amps? N P = What is the result of a short circuit across the terminals of a current transformer? No problem 29. What is the standard secondary voltage of current transformers? 5 V 30. What is the result of an open in the secondary of a current transformer? Very dangerous and high voltages 31. What is the result of connecting a clamp-on ammeter around two wires in a circuit? Cancelled fields and no reading 32. What is the result of looping a wire twice through a clamp-on ammeter? Double the reading (and turns ratio) 33. Will a current transformer type ammeter work with DC voltage? Why? No. Current transformers work on induction which results from the current changing direction. DC current is unidirectional 34. What type of DC ammeter can measure both AC and DC amps? Hall effect meters 35. Which scale on a multimeter is not linear? Ohmmeter 36. What are the two types of analog ohmmeters? Series and shunt 37. What is the first task when resistance is to be measured with an ohmmeter? Zero the needle 38. How accurate are digital ohmmeters? About 1% 39. What is the main precaution that must be taken when connecting an ohmmeter into a circuit? Make sure the power is off 40. Why does a multimeter have more than one (the largest) scale for each function? More accurately measure smaller values 41. What meter displays a two-dimensional image of voltage? Oscilloscope

7 42. Which axis of an oscilloscope displays time? Horizontal or X 43. What other feature of a waveform can be seen on an oscilloscope. (Other than voltage and time) Length, frequency, and duration of a pulse 44. What is a chopped waveform? Alternating several times during one sweep 45. How many terminals are on a wattmeter? Four 46. Skip recording meters and bridge circuits!! Unit What is the most common method of producing electricity? Alternators 2. What is the definition of a battery? Several cells connected together 3. What is a voltaic cell? Constructed from two different metals and an acid, alkaline, or salt solution 4. What property determines the voltage of an individual cell? Materials from which it is made 5. What is the difference between a primary and a secondary cell? Primary cannot be recharged and must be disposed of. Secondary can be recharged many times 6. What are the main parts of a carbon-zinc cell? Carbon rod as an electrode, zinc container, and a mixture of ammonium chloride, manganese dioxide, and granulated copper as an electrolyte 7. What is the main advantage of an alkaline cell compared to carbon-zinc cell? Longer life (3-5 times) 8. What is the main disadvantage of an alkaline cell compared to a carbon-zinc cell? Cost 9. What are the two main types of button cells? Mercury-zinc and silver-zinc 10. Which part of a lithium cell actually contains lithium? Anode 11. What is the shelf life of a lithium battery? Decades 12. What is the result of using an improper charging current for a lithium battery? May cause the cell to explode

8 13. What property of a cell determines the current available? Surface area of the plates 14. How are primary cells rated for available current? Milliampere-hour 15. What effect does aging and usage have on the electrodes and electrolyte in a cell? Deterioration 16. What effect does aging and usage have on the internal resistance of a cell? Increases 17. What is a secondary cell? One that can be recharged many times 18. What is the most common secondary cell? Lead-acid 19. What is the term used to describe the device for measuring the specific gravity of a cell? Hydrometer 20. What is the indication of a lead-acid battery becoming more discharged? Lower specific gravity 21. How is a lead-acid cell charged? Reverse the chemical action that occurred during the discharge by connecting a DC power source to the terminals 22. As a secondary cell charges, what is produced at the positive and negative plates? Negative-hydrogen and positive-oxygen 23. What happens in a lead-acid cell when too much charging current is used? Lead sulfate does not have a chance to dissolve back into the electrolyte and become acid so it flakes off and goes to the bottom 24. What is the most explosive element known? Hydrogen 25. What is the general rule for the amount of charging current for a cell? Less than 1/10 th of the ampere-hour rating of the battery 26. What is one of the main advantages of a gel cell? Prevents spillage and allows for mounting in any position 27. How are lead-acid cells rated? Ampere-hour 28. What is the best test for a lead-acid cell? Load test (Load of 3 times the amperehour rating of the battery connected for 3 minutes and voltage should not drop below 80% of rated

9 29. What is the main advantage of a nickel-iron cell? Recover from deep discharge and being left discharged for long periods 30. What are the two main disadvantages of nickel-iron cells? Cost and high internal resistance 31. What is the main advantage of the nicad cell? Extremely long life span (2000 cycles) 32. What are the three disadvantages of nicad cells? Less voltage developed in cells, initial cost high, and memory (remember charge/discharge cycles) 33. What are the main advantages of NiMH cells? 40% higher energy density, less memory accumulation, and more environmentally friendly 34. What material is used for the negative electrode in a Ni-MH cell? Metal Hydride 35. Where are lithium-ion cells used? Portable equipment such as notebook computers, video cameras, cell phones, etc. 36. What are the main advantages of lithium-ion cells? High energy density, cell voltage, and weight-energy density, no memory problems 37. What is the total voltage of 3-12 volt, 40 ampere-hour batteries connected in series? 36 Volts 38. What is the ampere-hours rating for 3-12 volt, 40 ampere-hour batteries connected in series? 40 ampere-hours 39. What is the total voltage of 3-12 volt, 40 ampere-hour batteries connected in parallel? 12 Volts 40. What is the total ampere-hour rating for 3-12 volt, 40 ampere-hour batteries connected in parallel? 120 Volts 41. What precaution should be taken when connecting batteries in parallel? Make sure the voltages are the same 42. What is a photon? Small package of pure energy (light) that contains no mass 43. What property determines the amount of current a solar cell can deliver? Material from which it is made 44. What is a thermocouple? Junction of two different metals that produces a small voltage when heated

10 45. What two factors determine the voltage of a thermocouple? Type of materials and the temperature difference between the junction and the free ends 46. What is a thermopile? Series connection of thermocouples 47. What is one application of a piezoelectric device? Needle on a record player or pickup for a microphone