(EAM) Parallel Circuits Lesson
Parallel Circuits
Devices can also be arranged in a parallel configuration, such that if any bulbs go out, the circuit is still intact. When configured in parallel, lights and appliances can be turned on and off at will. Otherwise, if you turned one light off--or one burned out--all the other lights in the house would go off too. This is so because in parallel circuits the charges leaving the battery source have different paths they can follow to get back to the source.
In the design of this parallel circuit, each resistor (light bulb) is connected across the battery as if the other two resistors were not present. Remember that the current going through each resistor goes through only one resistor. Therefore, the voltage drop across each resistor must be equal to the total voltage drop through the circuit.
VT=V1=V2=V3
The total current passing through the circuit will be the sum of the individual currents passing through each resistor.
IT=I1+I2+I3
Consider the parallel circuit sketched below.
The voltage drop for the entire circuit is 90. V. Therefore, the voltage drop in each of the resistors is also 90. V.
In contrast to the series case, when we add resistors in parallel, we create more paths along which current can flow. By doing this we decrease the total resistance of the circuit!
Take a look at the diagrams below. On the left, we have the same circuit as in the previous section with a battery and a resistor. The ammeter shows a current of 1 A. On the right, we have added a second resistor in parallel to the first resistor. This has increased the number of paths (branches) the charge can take through the circuit - the total resistance has decreased. You can see that the current in the circuit has increased. Also, notice that the current in the different branches can be different. The total resistance of a number of parallel resistors is NOT the sum of the individual resistances as the overall resistance decreases with more paths for the current.
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