Basic Circuit Analysis
Parallel Circuits

Section 2-3 Kirchhoff's Current Law

 Important Fact The voltage in a parallel circuit is the same across component and equal to the source voltage.VT = V1 = V2 = V3 = ... = Vn

 Circuit 1 Circuit 2

This is an example of a very general type of parallel circuitnot just a battery and three resistors connected in series. We don't know exactly what the Source is. It might be a battery, an electrical generator, or any other source of electrical energy. Likewise the loads aren't necessarily resistors. They might be lamps, electronic equipment, or any other appliance that presents a fixed resistive load.

The important point is that the Source is providing 24V. This is a parallel circuit, so you can be confident that the source is providing 24V to each of the loads.

 Important Facts The sum of currents through each resistor in a parallel circuit is equal to the total total.IT = I1 + I2 +I3 + ... + In

 The source voltage in this example is 24V. You can see that the same 24-volt level appears across all the resistors that are connected in parallel with it.The source current happens to be 70mA. That is also the circuit's total current. The important thing to see here is that the branch currents add up to the source, or total, current.

Finding the Total Current of a Parallel Resistor Circuit

Example

 Here is a simple 2-resistor parallel circuit. Let: VT = 24V R1 = 100W R2 = 300W Given those values for total voltage and each resistor: Determine the total resistance -- RT Determine the total current -- IT Determine the voltage across each resistor -- VR1, VR2 Determine the current through each resistor -- IR1, IR2

When solving parallel resistor circuits, you are most often given the voltage of the source (VT) and the value of each resistor.

Given that information, you can use the equations for parallel resistance to calculate the total resistance (RT) of the circuit.

Knowing the total voltage and resistance, you can use Ohm's Law to find the total current:  IT = VT/RT

 Procedure Given the source voltage (VT) and values of all   Find the total resistance (RT) Calculate the total current IT = VT/RT