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RAE practice questions

Chapter 4: Resistance and Ohm’s Law

Free practice questions on Resistance and Ohm’s Law, chapter 4 of the SARL's Class A manual, for South Africa's Radio Amateur Examination (RAE). Every question has the right answer and an explanation. Sections covered:

  • 4.1 Resistance
  • 4.2 Symbols in mathematical equations
  • 4.3 Rearranging Ohm's Law

79 questions

  1. 1. In the water-pipe picture of electricity, a narrow pipe holds back the flow. Which electrical quantity does the narrowness stand for?
    Show answer

    Resistance

    A thin pipe resists water flow just as resistance opposes the flow of current; the flow itself is like the current.

  2. 2. Resistance is best described as:
    Show answer

    The opposition to current flow

    Resistance is how strongly a conductor opposes a current, so more resistance means less current for the same voltage; the flow of charge itself is the current, not the resistance.

  3. 3. What is the effect on current if resistance increases and voltage stays the same?
    Show answer

    Current decreases

    With the voltage fixed, I = V ÷ R, so a larger resistance gives a smaller current; 'current increases' has the effect the wrong way round.

    Try it: Ohm's Law & Power
  4. 4. A resistor draws 2,4 A from a 30 V supply. It is replaced by one with three times the resistance. What current flows now?
    Show answer

    0,8 A

    With the voltage fixed, current is inversely proportional to resistance (I = V ÷ R), so three times the resistance gives a third of the current: 2,4 A ÷ 3 = 0,8 A; 7,2 A wrongly multiplies.

    Try it: Ohm's Law & Power
  5. 5. A 15 Ω resistor and a 60 Ω resistor have the same voltage across them. How does the current in the 15 Ω resistor compare with the current in the 60 Ω one?
    Show answer

    Four times as large

    At the same voltage, current is inversely proportional to resistance (I = V ÷ R), so a quarter of the resistance passes four times the current; 'a quarter as large' has it the wrong way round.

    Try it: Ohm's Law & Power
  6. 6. Two points have a voltage between them. Apart from that voltage, what decides how much current flows when a conductor joins them?
    Show answer

    The resistance of the conductor

    The current depends on the voltage and on the conductor's resistance: the more resistance, the less current; insulation only keeps the current where it belongs, so its thickness does not set the current.

    Try it: Ohm's Law & Power
  7. 7. In the water-flow picture of a circuit, which of these stands for a low resistance?
    Show answer

    A short, wide pipe

    A wide pipe lets the water through freely, like a low resistance; a fine filter or a narrow pipe holds the flow back, like a high resistance, and a pump stands for the voltage that drives the flow.

  8. 8. A problem gives a resistor's value and the current through it and asks for the voltage across it. Which law do you use?
    Show answer

    Ohm's Law

    Ohm's Law, V = I × R, links the voltage across a conductor to the current through it and its resistance; Kirchhoff's Current Law, in chapter 6, is about currents meeting at a junction.

    Try it: Ohm's Law & Power
  9. 9. What is the unit of electrical resistance?
    Show answer

    Ohm

    Resistance is measured in ohms; volts measure voltage, amperes current and watts power.

  10. 10. The symbol Ω represents:
    Show answer

    Ohm

    Ω, the Greek capital letter omega, stands for the ohm, the unit of resistance; the ampere is written A, the watt W and the volt V.

  11. 11. If a conductor has 1 ohm of resistance and 1 volt is applied, how much current flows?
    Show answer

    1 A

    That is how the ohm is defined: one volt across one ohm drives one ampere (I = V ÷ R = 1 ÷ 1); 0,1 A would mean a resistance of 10 Ω.

    Try it: Ohm's Law & Power
  12. 12. A resistance of 68 000 Ω can also be written as:
    Show answer

    68 kΩ

    Kilo means a thousand, so 68 000 Ω ÷ 1000 = 68 kΩ; 6,8 kΩ would be only 6800 Ω, a slip of one place.

    Try it: Unit Converter
  13. 13. 1,5 kΩ is equal to:
    Show answer

    1500 Ω

    Kilo means a thousand: 1,5 × 1000 = 1500 Ω; 150 Ω multiplies by only 100, and 0,0015 Ω divides by 1000 instead of multiplying.

    Try it: Unit Converter
  14. 14. A resistor is marked 2,2 MΩ. What is that in kΩ?
    Show answer

    2200 kΩ

    Mega is a million and kilo a thousand, so 1 MΩ = 1000 kΩ and 2,2 MΩ = 2200 kΩ; 220 kΩ is ten times too small, and 2 200 000 is the value in ohms.

    Try it: Unit Converter
  15. 15. Which of these is the same resistance as 0,47 MΩ?
    Show answer

    470 kΩ

    A megohm is 1000 kΩ, so 0,47 MΩ × 1000 = 470 kΩ; 47 kΩ moves the decimal comma one place too few, and 470 Ω treats mega as kilo.

    Try it: Unit Converter
  16. 16. What happens to the voltage if current doubles, assuming resistance is constant?
    Show answer

    It is twice as large

    With R fixed, V = I × R, so the voltage follows the current: twice the current gives twice the voltage; it would halve only if the current halved.

    Try it: Ohm's Law & Power
  17. 17. For a given conductor, the voltage V is directly proportional to which quantity?
    Show answer

    Current

    For a given conductor the voltage rises in step with the current; its resistance is the fixed constant that links the two, so it is not the quantity that varies.

    Try it: Ohm's Law & Power
  18. 18. A resistor has 7 V across it when 2 mA flows through it. What voltage is across it when 6 mA flows?
    Show answer

    21 V

    For a fixed resistance the voltage is proportional to the current: 6 mA is three times 2 mA, so the voltage is 3 × 7 V = 21 V; 2,3 V wrongly treats them as inversely proportional.

    Try it: Ohm's Law & Power
  19. 19. A resistor passes 2,4 mA with 9 V across it. Which is nearest to the current when the voltage is raised to 14 V?
    Show answer

    3,7 mA

    For a fixed resistor the current is proportional to the voltage: 2,4 mA × 14 ÷ 9 = 3,73 mA, so 3,7 mA is nearest; 4 mA is 0,27 mA away, and 1,5 mA turns the ratio upside down.

    Try it: Ohm's Law & Power
  20. 20. The voltage across a resistor is doubled, and the current through it doubles too. What has happened to its resistance?
    Show answer

    It has stayed the same

    Resistance is the voltage divided by the current, and doubling both leaves that ratio the same; it would double only if the voltage doubled while the current stayed the same.

    Try it: Ohm's Law & Power
  21. 21. A resistor's value and the current through it are known. Which expression gives the voltage across it?
    Show answer

    V = I × R

    The voltage is the current times the resistance, V = I × R, so 3 A through 7 Ω gives 21 V; V = I ÷ R would give only 0,43 V.

    Try it: Ohm's Law & Power
  22. 22. Which symbol is used for electric current?
    Show answer

    I

    Current is written I, a symbol that does not match its English name; C would be the natural guess but is the coulomb's symbol, while V stands for voltage and R for resistance.

  23. 23. Apart from V, which letter is often used for a voltage (an electric potential)?
    Show answer

    E

    Besides V, the manual uses E for electric potential, so the law may also be written E = I R; P would match 'potential', but P is the symbol for power.

  24. 24. In V = I R, what does writing I and R side by side mean?
    Show answer

    I multiplied by R

    Symbols placed side by side are multiplied, so I R means I × R; the × sign is dropped to save writing, while a division always needs ÷ or /.

    Try it: Ohm's Law & Power
  25. 25. Why does the manual write quantities as single letters in its formulas?
    Show answer

    It saves writing out their full names each time

    Symbols are a shorthand that saves writing each name in full; they are not units, although the letter V happens to stand both for voltage and for its unit, the volt.

  26. 26. In the line 'V = 12 V', what does each V stand for?
    Show answer

    First the quantity, then the unit

    Before the equals sign V is the symbol for voltage, the quantity; after the 12 it abbreviates the unit, the volt, and the context shows which is meant.

  27. 27. A circuit has a current of 0,5 A and resistance of 10 Ω. What is the voltage?
    Show answer

    5 V

    V = I × R = 0,5 A × 10 Ω = 5 V; 20 V comes from dividing 10 by 0,5, 0,05 V from dividing the other way, and 5 Ω has the wrong unit.

    Try it: Ohm's Law & Power
  28. 28. A current of 2 A flows through a 3 Ω resistor. What is the voltage across it?
    Show answer

    6 V

    V = I × R = 2 A × 3 Ω = 6 V; dividing, 3 ÷ 2, gives the wrong 1,5 V, and adding the two numbers gives 5 V, which means nothing.

    Try it: Ohm's Law & Power
  29. 29. What is the potential difference across a 220 Ω resistor carrying 0,1 A of current?
    Show answer

    22 V

    V = I × R = 0,1 A × 220 Ω = 22 V; 2,2 V puts the decimal comma one place out, and 2200 V divides 220 by 0,1 instead of multiplying.

    Try it: Ohm's Law & Power
  30. 30. A current of 0,25 A flows through a 200 Ω resistor. What is the voltage?
    Show answer

    50 V

    V = I × R = 0,25 A × 200 Ω = 50 V; 5 V and 500 V put the decimal comma one place out, and 800 V comes from dividing 200 by 0,25.

    Try it: Ohm's Law & Power
  31. 31. If the current is 0,86 A and the resistance is 73 Ω, what is the voltage?
    Show answer

    62,78 V

    V = I × R = 0,86 A × 73 Ω = 62,78 V; 6,278 V and 627,8 V put the decimal comma one place out, and 84,88 V divides 73 by 0,86 instead of multiplying.

    Try it: Ohm's Law & Power
  32. 32. If the current is 0,42 A and the resistance is 13 Ω, what is the voltage?
    Show answer

    5,46 V

    V = I × R = 0,42 A × 13 Ω = 5,46 V; 54,6 V and 0,546 V put the decimal comma one place out, and 30,95 V divides 13 by 0,42 instead of multiplying.

    Try it: Ohm's Law & Power
  33. 33. If the current is 0,33 A and the resistance is 85 Ω, what is the voltage?
    Show answer

    28,05 V

    V = I × R = 0,33 A × 85 Ω = 28,05 V; 2,805 V and 280,5 V put the decimal comma one place out, and 257,6 V divides 85 by 0,33 instead of multiplying.

    Try it: Ohm's Law & Power
  34. 34. If the current is 1,69 A and the resistance is 79 Ω, which is nearest to the voltage?
    Show answer

    130 V

    V = I × R = 1,69 A × 79 Ω = 133,5 V, so 130 V, 3,5 V away, is nearest; 150 V is 16,5 V away, and 47 V divides instead of multiplying.

    Try it: Ohm's Law & Power
  35. 35. If the current is 1,33 A and the resistance is 77 Ω, which is nearest to the voltage?
    Show answer

    100 V

    V = I × R = 1,33 A × 77 Ω = 102,4 V, so 100 V, 2,4 V away, is nearest; 110 V is 7,6 V away, and 58 V divides 77 by 1,33 instead.

    Try it: Ohm's Law & Power
  36. 36. If the current is 1,85 A and the resistance is 94 Ω, which is nearest to the voltage?
    Show answer

    170 V

    V = I × R = 1,85 A × 94 Ω = 173,9 V, so 170 V, 3,9 V away, is nearest; 190 V is 16,1 V away, and 51 V divides 94 by 1,85 instead.

    Try it: Ohm's Law & Power
  37. 37. A transceiver draws 20 A through a supply lead whose two wires have 0,05 Ω of resistance in all. How much voltage is lost in the lead?
    Show answer

    1 V

    V = I × R = 20 A × 0,05 Ω = 1 V, so a 13,8 V supply delivers only 12,8 V at the radio; 0,1 V and 10 V misplace the decimal comma, and 400 V divides 20 by 0,05.

    Try it: Ohm's Law & Power
  38. 38. A 330 Ω resistor carries a current of 25 mA. What is the voltage across it?
    Show answer

    8,25 V

    V = I × R = 0,025 A × 330 Ω = 8,25 V, as 25 mA is 0,025 A; 82,5 V takes it as 0,25 A and 0,825 V as 0,0025 A, and 13,2 V divides 330 by 25.

    Try it: Ohm's Law & Power
  39. 39. A 180 Ω resistor carries 12 mA. What is the voltage across it?
    Show answer

    2,16 V

    V = I × R = 0,012 A × 180 Ω = 2,16 V, since 12 mA is 0,012 A; 0,216 V and 21,6 V misplace the decimal comma, and 15 V divides 180 by 12.

    Try it: Ohm's Law & Power
  40. 40. A 270 Ω resistor carries 20 mA. What is the voltage across it?
    Show answer

    5,4 V

    V = I × R = 0,02 A × 270 Ω = 5,4 V; 54 V takes 20 mA as 0,2 A and 0,54 V as 0,002 A, and 13,5 V divides 270 by 20.

    Try it: Ohm's Law & Power
  41. 41. A current of 15 mA flows through a 2,2 kΩ resistor. What is the voltage across it?
    Show answer

    33 V

    V = I × R = 0,015 A × 2200 Ω = 33 V; milliamperes times kilohms give volts directly (15 × 2,2); 3,3 V and 330 V misplace the decimal comma, and 0,033 V forgets the kilo.

    Try it: Ohm's Law & Power
  42. 42. A current of 2,7 mA flows through a 6,8 kΩ resistor. Which is nearest to the voltage across it?
    Show answer

    18 V

    V = I × R = 0,0027 A × 6800 Ω = 18,36 V, so 18 V (0,36 V away) is nearest; 20 V is 1,64 V away, and 1,8 V and 180 V misplace the decimal comma.

    Try it: Ohm's Law & Power
  43. 43. A 4,7 kΩ resistor carries a current of 4 mA. What voltage is across it?
    Show answer

    18,8 V

    V = I × R = 0,004 A × 4700 Ω = 18,8 V, or simply 4 mA × 4,7 kΩ; 1,88 V and 188 V misplace the decimal comma, and 0,0188 V forgets the kilo.

    Try it: Ohm's Law & Power
  44. 44. A current of 50 µA flows through a 33 kΩ resistor. What is the voltage across it?
    Show answer

    1,65 V

    V = I × R = 0,000 05 A × 33 000 Ω = 1,65 V; 16,5 V slips the decimal comma, and 1650 V takes microamperes for milliamperes.

    Try it: Ohm's Law & Power
  45. 45. A 20 µA current flows through a 220 kΩ resistor. What voltage is across it?
    Show answer

    4,4 V

    V = I × R = 0,000 02 A × 220 000 Ω = 4,4 V; 0,44 V slips the decimal comma, and 4400 V treats the microamperes as milliamperes.

    Try it: Ohm's Law & Power
  46. 46. A leakage current of 4 µA flows through a 1,2 MΩ resistor. What voltage does it produce across the resistor?
    Show answer

    4,8 V

    V = I × R = 0,000 004 A × 1 200 000 Ω = 4,8 V, as micro and mega cancel; 0,48 V slips the decimal comma, and 4,8 mV treats mega as kilo.

    Try it: Ohm's Law & Power
  47. 47. A 2,7 kΩ resistor carries a current of 0,12 A. What is the voltage across it?
    Show answer

    324 V

    V = I × R = 0,12 A × 2700 Ω = 324 V; 32,4 V slips the decimal comma, 0,324 V forgets that 2,7 kΩ is 2700 Ω, and 22,5 kV divides instead.

    Try it: Ohm's Law & Power
  48. 48. You know the voltage across a resistor and its resistance. How do you work out the current?
    Show answer

    Divide the voltage by the resistance

    Dividing both sides of V = I × R by R gives I = V ÷ R, so divide the voltage by the resistance; multiplying instead would make the current grow with the resistance, the wrong way round.

    Try it: Ohm's Law & Power
  49. 49. Which of these is the correct rearranged form for finding a resistance?
    Show answer

    R = V ÷ I

    Dividing both sides of V = I × R by I leaves R = V ÷ I; R = I ÷ V is upside down, and V × I is the power, not the resistance.

    Try it: Ohm's Law & Power
  50. 50. To get I on its own from V = I × R, what must be done to both sides?
    Show answer

    Divide both sides by R

    Dividing both sides by R cancels the R beside the I, leaving I = V ÷ R; dividing by I instead gives R = V ÷ I, the form for finding a resistance.

    Try it: Ohm's Law & Power
  51. 51. Which branch of mathematics is used to rearrange a formula so that a different quantity stands on its own?
    Show answer

    Algebra

    Rearranging a formula is simple algebra: do the same to both sides until the wanted quantity stands alone; the triangle trick may look like geometry, but it is only a memory aid.

  52. 52. What is the triangle trick in the manual meant to help with?
    Show answer

    Rearranging the formula at a glance

    The triangle puts V at the top and I and R below it, so covering the quantity you want shows how to get it from the other two; colour codes are a chapter 5 topic.

  53. 53. If the current in a circuit doubles while the voltage stays the same, the resistance must have:
    Show answer

    Halved

    R = V ÷ I, so twice the current at the same voltage means half the resistance; a doubled resistance would have halved the current instead.

    Try it: Ohm's Law & Power
  54. 54. If the voltage is 20 V and resistance is 4 Ω, what is the current?
    Show answer

    5 A

    I = V ÷ R = 20 V ÷ 4 Ω = 5 A; 0,2 A comes from dividing the wrong way, 4 ÷ 20, and 80 A from multiplying.

    Try it: Ohm's Law & Power
  55. 55. What is the current if the voltage is 10 V and resistance is 2,5 Ω?
    Show answer

    4 A

    I = V ÷ R = 10 V ÷ 2,5 Ω = 4 A; 0,25 A divides the wrong way, 2,5 ÷ 10, and 25 A multiplies instead of dividing.

    Try it: Ohm's Law & Power
  56. 56. A heating element has a resistance of 20 Ω when hot. What current does it draw from a 230 V supply?
    Show answer

    11,5 A

    I = V ÷ R = 230 V ÷ 20 Ω = 11,5 A; 0,087 A divides the wrong way, 20 ÷ 230, and 4600 A multiplies instead of dividing.

    Try it: Ohm's Law & Power
  57. 57. A 560 Ω resistor has 15 V across it. Which is nearest to the current through it?
    Show answer

    27 mA

    I = V ÷ R = 15 V ÷ 560 Ω = 0,026 79 A = 26,79 mA, so 27 mA is nearest; 2,7 mA and 270 mA misplace the decimal comma, and 37 A divides 560 by 15.

    Try it: Ohm's Law & Power
  58. 58. A 48 V relay coil has a resistance of 1200 Ω. What current does it draw?
    Show answer

    40 mA

    I = V ÷ R = 48 V ÷ 1200 Ω = 0,04 A = 40 mA; 4 mA is ten times too small, and 25 A divides 1200 by 48 instead.

    Try it: Ohm's Law & Power
  59. 59. A 150 Ω resistor has 1,5 V across it. What current flows through it?
    Show answer

    10 mA

    I = V ÷ R = 1,5 V ÷ 150 Ω = 0,01 A = 10 mA; 1 mA is ten times too small, and 100 A divides 150 by 1,5 instead.

    Try it: Ohm's Law & Power
  60. 60. A 48 V supply is connected across a 50 kΩ resistor. What current flows?
    Show answer

    0,96 mA

    I = V ÷ R = 48 V ÷ 50 000 Ω = 0,000 96 A = 0,96 mA, as volts divided by kilohms give milliamperes; 0,096 mA and 9,6 mA misplace the decimal comma, and 0,96 A forgets the kilo.

    Try it: Ohm's Law & Power
  61. 61. A 13,8 V supply is connected across a 15 kΩ resistor. Which is nearest to the current that flows?
    Show answer

    0,9 mA

    I = V ÷ R = 13,8 V ÷ 15 000 Ω = 0,92 mA, so 0,9 mA is nearest; 1 mA is 0,08 mA away, four times as far, and 0,9 A forgets that the resistance is in kilohms.

    Try it: Ohm's Law & Power
  62. 62. A 10 kΩ resistor has 4 V across it. What current flows through it?
    Show answer

    0,4 mA

    I = V ÷ R = 4 V ÷ 10 000 Ω = 0,0004 A = 0,4 mA; 4 mA and 0,04 mA misplace the decimal comma, and 0,4 A forgets that the resistance is in kilohms.

    Try it: Ohm's Law & Power
  63. 63. A digital voltmeter has a resistance of 10 MΩ between its probes. What current does it draw while reading 2 V?
    Show answer

    0,2 µA

    I = V ÷ R = 2 V ÷ 10 000 000 Ω = 0,000 000 2 A = 0,2 µA; 2 µA and 0,02 µA misplace the decimal comma, and 0,2 mA treats mega as kilo.

    Try it: Ohm's Law & Power
  64. 64. A 0,01 Ω resistor in a power lead has 75 mV across it. What current is flowing?
    Show answer

    7,5 A

    I = V ÷ R = 0,075 V ÷ 0,01 Ω = 7,5 A, the idea behind an ammeter shunt; 0,75 A slips the decimal comma, and 7500 A forgets that 75 mV is 0,075 V.

    Try it: Ohm's Law & Power
  65. 65. A 150 kΩ resistor is connected across a 2 kV supply. Which is nearest to the current through it?
    Show answer

    13 mA

    I = V ÷ R = 2000 V ÷ 150 000 Ω = 13,3 mA, so 13 mA is nearest; 15 mA is 1,7 mA away, and 1,3 mA slips the decimal comma.

    Try it: Ohm's Law & Power
  66. 66. What is the resistance if the voltage is 12 V and the current is 8 A?
    Show answer

    1,5 Ω

    R = V ÷ I = 12 V ÷ 8 A = 1,5 Ω; 0,67 Ω divides the wrong way, 8 ÷ 12, and 96 Ω multiplies, which gives the power in watts, not a resistance.

    Try it: Ohm's Law & Power
  67. 67. A 3 V supply drives 6 A through a load. What is its resistance?
    Show answer

    0,5 Ω

    R = V ÷ I = 3 V ÷ 6 A = 0,5 Ω; 2 Ω divides the wrong way, 6 ÷ 3, and 18 Ω multiplies the two instead of dividing.

    Try it: Ohm's Law & Power
  68. 68. What is the resistance of a device drawing 3 A at 13,8 V?
    Show answer

    4,6 Ω

    R = V ÷ I = 13,8 V ÷ 3 A = 4,6 Ω; 0,46 Ω and 46 Ω misplace the decimal comma, and 41,4 Ω multiplies 13,8 by 3, which gives the power in watts.

    Try it: Ohm's Law & Power
  69. 69. A 13,8 V supply drives 0,6 A through a load. Which is nearest to the load's resistance?
    Show answer

    22 Ω

    R = V ÷ I = 13,8 V ÷ 0,6 A = 23 Ω, so 22 Ω, 1 Ω away, is nearest; 27 Ω is 4 Ω away, and 8,3 Ω multiplies the two instead of dividing.

    Try it: Ohm's Law & Power
  70. 70. A supply lead loses 0,45 V when it carries 15 A. What is the resistance of the lead?
    Show answer

    0,03 Ω

    R = V ÷ I = 0,45 V ÷ 15 A = 0,03 Ω; 0,3 Ω slips the decimal comma, 33 Ω divides the wrong way, and 6,75 Ω multiplies.

    Try it: Ohm's Law & Power
  71. 71. A 3 V battery delivers 60 mA to a small lamp. What is the lamp's resistance?
    Show answer

    50 Ω

    R = V ÷ I = 3 V ÷ 0,06 A = 50 Ω, as 60 mA is 0,06 A; 5 Ω takes 60 mA as 0,6 A, and 0,05 Ω forgets to convert the milliamperes.

    Try it: Ohm's Law & Power
  72. 72. A 3 V supply drives 75 mA through a resistor. What is its resistance?
    Show answer

    40 Ω

    R = V ÷ I = 3 V ÷ 0,075 A = 40 Ω; 4 Ω and 400 Ω misplace the decimal comma, and 0,04 Ω forgets to convert the milliamperes.

    Try it: Ohm's Law & Power
  73. 73. A resistor passes 1,5 mA when 24 V is across it. What is its resistance?
    Show answer

    16 kΩ

    R = V ÷ I = 24 V ÷ 0,0015 A = 16 000 Ω = 16 kΩ, as volts divided by milliamperes give kilohms; 1,6 kΩ and 160 kΩ misplace the decimal comma, and 16 Ω forgets the milli.

    Try it: Ohm's Law & Power
  74. 74. A resistor passes 3,3 mA with 12 V across it. Which is nearest to its resistance?
    Show answer

    3,6 kΩ

    R = V ÷ I = 12 V ÷ 0,0033 A = 3636 Ω, so 3,6 kΩ is nearest; 3,9 kΩ is about 260 Ω away, and 3,6 Ω forgets to convert the milliamperes.

    Try it: Ohm's Law & Power
  75. 75. A resistor passes 12 µA when 7,5 V is across it. What is its resistance?
    Show answer

    625 kΩ

    R = V ÷ I = 7,5 V ÷ 0,000 012 A = 625 000 Ω = 625 kΩ; 62,5 kΩ and 6250 kΩ misplace the decimal comma, and 625 Ω takes microamperes for milliamperes.

    Try it: Ohm's Law & Power
  76. 76. A 9 V battery drives 2 µA through a high-value resistor. What is its resistance?
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    4,5 MΩ

    R = V ÷ I = 9 V ÷ 0,000 002 A = 4 500 000 Ω = 4,5 MΩ; 0,45 MΩ and 45 MΩ misplace the decimal comma, and 4,5 kΩ takes microamperes for milliamperes.

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  77. 77. A test current of 25 mA through a coil gives 120 mV across it. What is the coil's resistance?
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    4,8 Ω

    R = V ÷ I = 0,12 V ÷ 0,025 A = 4,8 Ω, as millivolts divided by milliamperes give ohms; 0,48 Ω slips the decimal comma, and 4,8 kΩ leaves the 120 mV unconverted.

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  78. 78. A switch contact carrying 300 mA has 60 mV across it. What is its resistance?
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    0,2 Ω

    R = V ÷ I = 0,06 V ÷ 0,3 A = 0,2 Ω; 2 Ω slips the decimal comma, 200 Ω leaves the 60 mV unconverted, and 5 Ω divides the wrong way.

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  79. 79. What resistance limits the current from a 1 kV supply to 0,5 mA?
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    2 MΩ

    R = V ÷ I = 1000 V ÷ 0,0005 A = 2 000 000 Ω = 2 MΩ; 0,2 MΩ and 20 MΩ misplace the decimal comma, and 2 kΩ forgets to convert the milliamperes.

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