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How to Calculate Resistive Current

A resistive current in a DC circuit is calculated with Ohm's Law, I=E/R, where I is Current in units of amps, E is Voltage in units of volts and R is Resistance in units of ohms. The resistive current is calculated by substituting the Voltage and Resistance values for the variables in Ohm's equation. Alternating circuits, however, may have a complex number of Impedance, Z, made up of a real number of Resistance, R, and an imaginary number of Reactance, X, all expressed in units of ohms. Because of the nature of alternating circuits, Ohm's Law is rewritten for AC circuits as I=E/Z, where Z is calculated as the square root of the sum of (R squared plus X squared).

Things You'll Need

Volt meter
Resistor color codes
Calculator

Instructions

Calculating the Resistive Current in a DC Circuit
- 1
  Measure the voltage in the circuit with a volt meter.
- 2
  Find the value of the resistor. A resistor's value can be calculated by decoding the colored rings on its body.
- 3
  Substitute the values of the voltage and the resistance into Ohm's equation and calculate the circuit's resistive current. Example: In a simple circuit with a 12-volt battery and a 6-ohm resistor, the resistive current is calculated as I = E/R; I = 12/6; I = 2 amps.
Calculating the Resistive Current in an AC Circuit
- 4
  Find the resistance of the circuit. Example: 60 ohms.
- 5
  Calculate the inductive reactance in the circuit and add it to the capacitive reactance. The sum will be the circuit's net reactance. Example: Inductive reactance is j30 ohms and capacitive reactance is --j10 ohms for a net reactance of j20 ohms.
- 6
  Combine the resistance's real number with the reactance's imaginary number to arrive at the impedance's imaginary number. Example: R=60 ohms; X= j20 ohm; therefore Z=60+j20 ohms.
- 7
  Measure the AC circuit's voltage. Example: 120 volts rms. The rms stands for "root-mean-square" and is used for alternating volts and currents.
- 8
  Substitute the values of the voltage and the impedance into Ohm's equation and calculate the circuit's resistive current. Example: In a circuit with 120 volts rms and an impedance of 60+j20 ohms, the resistive current is calculated as I = E/Z where Z equals the square root of (60 squared plus j20 squared) which equals Z=63.25 ohms. The resistive current in an AC circuit is therefore calculated as 120/63.25, which equals 1.9 amps rms.

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