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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?
4 megohms.
• 2 megohms.
• 400 kilohms.
• 200 kilohms.

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?

4 megohms.

From wp2ahg: Ohm's Law:

$R=\frac{E}{I}$ $Ohms=\frac{Volts}{Amps}$

$R=\frac{200\text{ Volts}}{50\text{ Microamps}}$

$\frac{200\text{ Volts}}{0.00005\text{ Amperes}} = 4,000,000\text{ Ohms}$

To convert into megaohms:

1 M = 10 6 or 1,000,000

$\frac{4,000,000\text{ Ohms}}{1,000,000\text{ mega}} = 4\text{ Megaohms}$

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Which of the following Ohms Law formulas is incorrect?
• I = E / R
I = R / E
• E = I x R
• R = E / I

Which of the following Ohms Law formulas is incorrect?

I = R / E

From crzy_ray:

V = voltage is Electromotive force or E
I = current in Amps
R = resistance in Ohms

${\text{I}=\frac{\text{R}}{E}}$

Thus, the current equals resistance divided by volts or electromotive force.

When you see Ohm's law incorrect, it should raise your "IRE"

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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.
100 volts.

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

100 volts.

From wp2ahg:

Ohm's Law:

$E = I ∗ R$ $Volts = Amps ∗ Ohms$ $E = (2\text{ amps}) ∗ (50\text{ ohms}) = 100\text{ volts}$

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

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

2 amperes.

From wp2ahg:

Ohm's Law:

$R=\frac{E}{I}$ $Ohms=\frac{Volts}{Amps}$

$\frac{200\text{ Amps}}{100\text{ Ohms}} = 2\text{ Amps}$

V = voltage is Electromotive force or E
I = current in Amps
R = resistance in Ohms

${\text{I}=\frac{\text{R}}{E}}$

thus, Current equals resistance divided by volts or electromotive force.

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

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

30 ohms.

From wp2ahg:

Ohm's Law:

$R=\frac{E}{I}$ $Ohms=\frac{Volts}{Amps}$

$\frac{90\text{ Volts}}{3\text{ Amperes}} = 30\text{ Ohms}$

V = voltage is Electromotive force or E
I = current in Amps
R = resistance in Ohms

${\text{R}=\frac{\text{E}}{I}}$

thus, resistance equals volts divided by amperes or electromotive force.

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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.
380 ohms.
• 470 ohms.

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?

380 ohms.

From wp2ahg:

Ohm's Law:

$R=\frac{E}{I}$ $Ohms=\frac{Volts}{Amps}$

Mili (m) is 10-3 or 1/1,000
thus, 125 mA is 0.125 A

$\frac{110\text{ Volts}}{0.125\text{ Amperes}} = 880\text{ Ohms}$

Since the relay coil has 500 Ohms contributing to the circuit, then $880\text{ Ohms} - 500\text{ Ohms} = 380\text{ Ohms}$

V = voltage is Electromotive force or E
I = current in Amps
R = resistance in Ohms

${\text{R}=\frac{\text{E}}{I}}$

thus, resistance equals volts divided by amperes or electromotive force.