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Subelement T5
Electrical principles: math for electronics; electronic principles; Ohm's Law
Section T5D
Ohm's Law: formulas and usage
What formula is used to calculate current in a circuit?
  • Current (I) equals voltage (E) multiplied by resistance (R)
  • Current (I) equals voltage (E) divided by resistance (R)
  • Current (I) equals voltage (E) added to resistance (R)
  • Current (I) equals voltage (E) minus resistance (R)

Ohms law involves 3 variables - Voltage (\(E\), for Electromotive force) Resistance (\(R\)), and Current (\(I\)). In a recent webcast, Gordon West suggested a simple method for remembering their order. He suggests that you think of \(E\) as an Eagle, \(I\) as an Igloo, and \(R\) as a Rabbit.

Any time \(E\) is on the left side, \(I\) and \(R\) are on the right side; the Igloo and the Rabbit are both on the ground, so they go next to each other (multiplication, or \(E=I\times R\)). If \(E\) is on the right side, then the Eagle is always on top (in the air). So Resistance is \(\frac{E}{I}\), because the Eagle is always above the Igloo. Current (\(I\)) is \(\frac{E}{R}\), because the eagle is always above the Rabbit. (Remember that \(\frac{E}{R}\) means \(E\) divided by \(R\))

This might help you remember the formulae for Ohms Law.

Last edited by kd7bbc. Register to edit

Tags: ohm's law electrical current electromotive force (voltage) resistance arrl chapter 3 arrl module 5

What formula is used to calculate voltage in a circuit?
  • Voltage (E) equals current (I) multiplied by resistance (R)
  • Voltage (E) equals current (I) divided by resistance (R)
  • Voltage (E) equals current (I) added to resistance (R)
  • Voltage (E) equals current (I) minus resistance (R)

Ohms law involves 3 variables - Voltage (\(E\), for Electromotive force) Resistance (\(R\)), and Current (\(I\)). In a recent webcast, Gordon West suggested a simple method for remembering their order. He suggests that you think of \(E\) as an Eagle, \(I\) as an Igloo, and \(R\) as a Rabbit.

Any time \(E\) is on the left side, \(I\) and \(R\) are on the right side; the Igloo and the Rabbit are both on the ground, so they go next to each other (multiplication, or \(E=I\times R\)). If \(E\) is on the right side, then the Eagle is always on top (in the air). So Resistance is \(\frac{E}{I}\), because the Eagle is always above the Igloo. Current (\(I\)) is \(\frac{E}{R}\), because the eagle is always above the Rabbit. (Remember that \(\frac{E}{R}\) means \(E\) divided by \(R\))

This might help you remember the formulae for Ohms Law.

Last edited by kd7bbc. Register to edit

Tags: ohm's law electrical current electromotive force (voltage) resistance arrl chapter 3 arrl module 5

What formula is used to calculate resistance in a circuit?
  • Resistance (R) equals voltage (E) multiplied by current (I)
  • Resistance (R) equals voltage (E) divided by current (I)
  • Resistance (R) equals voltage (E) added to current (I)
  • Resistance (R) equals voltage (E) minus current (I)

Ohms law involves 3 variables - Voltage (\(E\), for Electromotive force) Resistance (\(R\)), and Current (\(I\)). In a recent webcast, Gordon West suggested a simple method for remembering their order. He suggests that you think of \(E\) as an Eagle, \(I\) as an Igloo, and \(R\) as a Rabbit.

Any time \(E\) is on the left side, \(I\) and \(R\) are on the right side; the Igloo and the Rabbit are both on the ground, so they go next to each other (multiplication, or \(E=I\times R\)). If \(E\) is on the right side, then the Eagle is always on top (in the air). So Resistance is \(\frac{E}{I}\), because the Eagle is always above the Igloo. Current (\(I\)) is \(\frac{E}{R}\), because the eagle is always above the Rabbit. (Remember that \(\frac{E}{R}\) means \(E\) divided by \(R\))

This might help you remember the formulae for Ohms Law.

Last edited by kd7bbc. Register to edit

Tags: ohm's law resistance electromotive force (voltage) electrical current arrl chapter 3 arrl module 5

What is the resistance of a circuit in which a current of 3 amperes flows through a resistor connected to 90 volts?
  • 3 ohms
  • 30 ohms
  • 93 ohms
  • 270 ohms

\(E = I \times R\)

\(R = \frac{E}{I}\) \(=\) \(\frac{90}{3}\) = 30 ohms

\(R\) = Resistance (ohms), \(E\) = Voltage (volts), \(I\) = Current (amperes)

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Tags: math ohm's law resistance electrical current electromotive force (voltage) arrl chapter 3 arrl module 5

What is the resistance in a circuit for which the applied voltage is 12 volts and the current flow is 1.5 amperes?
  • 18 ohms
  • 0.125 ohms
  • 8 ohms
  • 13.5 ohms

\(E = I \times R\)

\(R = \frac{E}{I}\) \(=\) \(\frac{12}{1.5}\) = \(8\) ohms

See Ohm's Law on Wikipedia

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Tags: math ohm's law resistance electromotive force (voltage) electrical current arrl chapter 3 arrl module 5

What is the resistance of a circuit that draws 4 amperes from a 12-volt source?
  • 3 ohms
  • 16 ohms
  • 48 ohms
  • 8 Ohms

\(E = I \times R\)

\(R = \frac{E}{I}\) \(=\) \(\frac{12}{4}\) \(=\) \(3\) ohms

See Ohm's Law on Wikipedia

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Tags: ohm's law math electromotive force (voltage) resistance arrl chapter 3 arrl module 5

What is the current flow in a circuit with an applied voltage of 120 volts and a resistance of 80 ohms?
  • 9600 amperes
  • 200 amperes
  • 0.667 amperes
  • 1.5 amperes

\(E = I \times R\)

\(I = \frac{E}{R}\) \(=\) $\frac{120}{80} $\(= 1.5\) amperes

See Ohm's Law on Wikipedia

E = Electromotive force (Volts) I = Intensity (Amperes) R = Resistance (Ohms)

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Tags: math ohm's law electromotive force (voltage) resistance electrical current arrl chapter 3 arrl module 5

What is the current flowing through a 100-ohm resistor connected across 200 volts?
  • 20,000 amperes
  • 0.5 amperes
  • 2 amperes
  • 100 amperes

Voltage = Current \(\times\) Resistance

Which we can write as: \[E = I \times R\]

Where:

  • \(E\) is the voltage applied to the circuit, in volts (V)
  • \(I\) is the current flowing in the circuit, in amperes (A)
  • \(R\) is the resistance in the circuit, in ohms (\(\Omega\))

In this case we have voltage and resistance and need to find current, so rearrange the equation: \begin{align} E &= I \times R\\ I &= \frac{E}{R}\\ \end{align}

Time to plug and chug! \begin{align} I &= \frac{200 \text{ V}}{100\ \Omega}\\ I &= 2 \text{ A}\\ \end{align}


See Ohm's Law on Wikipedia

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Tags: ohm's law electrical current math resistance electromotive force (voltage) arrl chapter 3 arrl module 5

What is the current flowing through a 24-ohm resistor connected across 240 volts?
  • 24,000 amperes
  • 0.1 amperes
  • 10 amperes
  • 216 amperes

\(E = I \times R\)

\(I = \frac{E}{R} = \frac{240}{24} = 10\) amperes

See Ohm's Law on Wikipedia

Last edited by kd7bbc. Register to edit

Tags: ohm's law math electrical current electromotive force (voltage) resistance arrl chapter 3 arrl module 5

What is the voltage across a 2-ohm resistor if a current of 0.5 amperes flows through it?
  • 1 volt
  • 0.25 volts
  • 2.5 volts
  • 1.5 volts

This is an Ohm's law question. Remember

\(E = I \times R\)

\(E = 0.5 \text{ A} \times 2\ \Omega = 1 \text{ V}\)

See Ohm's Law on Wikipedia

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Tags: ohm's law math electrical current electromotive force (voltage) resistance arrl chapter 3 arrl module 5

What is the voltage across a 10-ohm resistor if a current of 1 ampere flows through it?
  • 1 volt
  • 10 volts
  • 11 volts
  • 9 volts

Ohm's Law is the relationship between voltage, current, and the resistance in a DC circuit. It can be represented in the equation:

\(E = I \times R\)

Where \(E\) is the voltage/electromotive force (\(E\)), \(I\) is the current (\(A\)), and \(R\) is the resistance (Ω).

If you have any two values in the equation you can find the third:

  • \(E = I \times R\)
  • \(R = \frac{E}{I}\) (obtained by dividing both sides by I)
  • \(I = \frac{E}{R}\) (obtained by dividing both sides by R)

So for our equation:

  • \(E = ?\)
  • \(I = 1A\)
  • \(R = 10Ω\)

\(E = I \times R = 10Ω \times 1A = 10V\)

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Tags: ohm's law math electrical current electromotive force (voltage) resistance arrl chapter 3 arrl module 5

What is the voltage across a 10-ohm resistor if a current of 2 amperes flows through it?
  • 8 volts
  • 0.2 volts
  • 12 volts
  • 20 volts

\(E = I \times R = 2 \times 10 = 20\) volts

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Tags: ohm's law math electrical current electromotive force (voltage) resistance arrl chapter 3 arrl module 5

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