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Subelement B
Electrical Math
Section 9
Ohm’s Law-1
What value of series resistor would be needed to obtain a full scale deflection on a 50 microamp DC meter with an applied voltage of 200 volts DC?
  • Correct Answer
    4 megohms.
  • 2 megohms.
  • 400 kilohms.
  • 200 kilohms.

Resistance = Volts / Amps (R = E / I)
So, R = (200 volts) / (50 microamps)
or 200V / .00005A = 4,000,000 Ohms
(or 4 megaohms)

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Which of the following Ohms Law formulas is incorrect?
  • I = E / R
  • Correct Answer
    I = R / E
  • E = I x R
  • R = E / I
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If a current of 2 amperes flows through a 50-ohm resistor, what is the voltage across the resistor?
  • 25 volts.
  • 52 volts.
  • 200 volts.
  • Correct Answer
    100 volts.

Ohm's Law:

E = I x R (or Volts = Amps x Ohms)

E = (2 amps) x (50 ohms) = 100 volts

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If a 100-ohm resistor is connected across 200 volts, what is the current through the resistor?
  • Correct Answer
    2 amperes.
  • 1 ampere.
  • 300 amperes.
  • 20,000 amperes.

Ohm's Law:

I = E / R (or Amps = Volts / Ohms)

I = (200 amps) / (100 ohms) = 2 amps

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If a current of 3 amperes flows through a resistor connected to 90 volts, what is the resistance?
  • 3 ohms.
  • Correct Answer
    30 ohms.
  • 93 ohms.
  • 270 ohms.

Ohm's Law:

R = E / I (or Ohms = Volts / Amps)

R = (90 volts) / (3 amps) = 30 ohms

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A relay coil has 500 ohms resistance, and operates on 125 mA. What value of resistance should be connected in series with it to operate from 110 V DC?
  • 150 ohms.
  • 220 ohms.
  • Correct Answer
    380 ohms.
  • 470 ohms.

Ohm's Law:

R = E / I (or Ohms = Volts / Amps)

R = ((110 volts) / (0.125 amps)) - (500 ohms already in the circuit) = 380 ohms

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