Series vs. Parallel Circuits Physics Mrs. Tobler

Simple Circuit What are the requirements to make a light bulb light up? –Need a power source or battery (voltage difference) –Need wires (conductors) –Need light bulb (electrical load = something to use the electricity) –Must contain at least 1 pathway to connect negative terminal to a positive one on power source.

Four devices are commonly used in the laboratory to study Ohm’s law and Circuits: the battery, the voltmeter, the ammeter and an electrical load. Ammeter measures current through the battery, the filament, and itself. Current must pass through it so must be in the circuit. Low Resistance. Voltmeter measures voltage drop across a battery, filament or resistor. The current does not travel through a voltmeter, has high resistance.

The following symbols are used in electric circuits:

Color Coding Circuits A technique to show how the voltage changes in a circuit When the current encounters a resistor or an electrical load the voltage will either step up or step down. Remember for current to flow there must be a voltage difference Higher voltage will be on the positive terminal and decrease around until the negative terminal –Highest voltage = Blue or Purple –Lowest voltage = Red –Use the other ROYGBIV colors to show how the voltage decreases as the current moves through the circuit

Example #1 The longer line of the battery is the positive terminal and has the highest voltage. Start color coding to see how the voltage changes for this circuit with 3 resistors. Continue with the same color until you encounter a resistor, then change to a lower energy color. This circuit has 3 different voltages for each resistor based on the color coding. There is also only one pathway so this is a Series Circuit

Example #2 Now try color coding this circuit with multiple pathways This circuit has only 1 voltage difference at each of the resisters, so they have the same voltage as the battery at each resister. There are multiple pathways for the current to flow so that makes this circuit a Parallel Circuit

Series Circuit Key characteristics –Has only one pathway –Current is the same throughout the circuit –V Total = V 1 + V 2 + V 3 + … –R Total = R 1 + R 2 + R 3 + … –Follows Ohm’s law I = V/R –If one device in the circuit fails then none will work

Serie Example Find the total resistance of the three resistors connected in series. 12 Ω 4 Ω 6 Ω

What if? Find the current for this circuit if it has a 12 V battery? 12 Ω 4 Ω 6 Ω 12 V

Parallel Circuit Key characteristics –At least 2 pathways –If one device fails the others still work –V Total = V 1 = V 2 = V 3 = … –I Total = I 1 + I 2 + I –Follows Ohm’s law for each branch separately –Resistance decreases as # of parallel branches increases

Example Find the total resistance of the same three resistors now connected in parallel. 12 Ω4 Ω6 Ω

How about this? Find the current across the 6Ω resistor if it is connected to a 12V power source. 12 Ω4 Ω6 Ω 12V

Overloading Since parallel circuits have lower overall resistance they allow a greater amount of total current to flow. –Overloading can occur if there is more current than is safe Results in over heating that can melt the insulation and then start a fire –Protection Fuses that are connected in series –Fuse will burn out if current is too high and break the circuit so the current won’t flow. Circuit breakers act as a switch that will open if too much current flows through –More modern safety feature –Don’t need to replace burned out fuses only flip a switch

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