FCC Commercial Element 3 pool, section 9

Subelement B

Electrical Math

Section 9

Ohm’s Law-1

39B1

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
A.
4 megohms.
B.
2 megohms.
C.
400 kilohms.
D.
200 kilohms.

39B1

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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39B2

Which of the following Ohms Law formulas is incorrect?

A.
I = E / R
Correct Answer
B.
I = R / E
C.
E = I x R
D.
R = E / I

39B2

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39B3

If a current of 2 amperes flows through a 50-ohm resistor, what is the voltage across the resistor?

A.
25 volts.
B.
52 volts.
C.
200 volts.
Correct Answer
D.
100 volts.

39B3

Ohm's Law:

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

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

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39B4

If a 100-ohm resistor is connected across 200 volts, what is the current through the resistor?

Correct Answer
A.
2 amperes.
B.
1 ampere.
C.
300 amperes.
D.
20,000 amperes.

39B4

Ohm's Law:

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

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

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39B5

If a current of 3 amperes flows through a resistor connected to 90 volts, what is the resistance?

A.
3 ohms.
Correct Answer
B.
30 ohms.
C.
93 ohms.
D.
270 ohms.

39B5

Ohm's Law:

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

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

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39B6

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?

A.
150 ohms.
B.
220 ohms.
Correct Answer
C.
380 ohms.
D.
470 ohms.

39B6

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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