parallel circuits

[olygt]

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Adding a resistor in parallel to an existing resistor (R1) in a circuit would (also has capacitor attached to circuit)?
A) increase current available for charging up plates
B) decrease maximum charge on the capcitor plates
C) decrease the power drained by the resistors during charging

I chose C b/c intuitively I though that adding a resistor would increase the overall resistance and decrease the current which decreases the power. The book is saying that adding the extra resistor would decrease the overall resistance which increases the current available. How does an extra resistor decrease the overall resistance? Does the equation: 1/R = 1/R1 + 1/R2 have anything to do with this?

 

[BloodySurgeon]

yepp exactly.. the answer therefore is A. For example if you have a 4 ohm resistor the overall resistance is 4 ohm. However if you have two 4 ohm resistors in parallel the overall resistance is 2 ohms (plug in formula and you'll see). Then if V=IR and the voltage is the same between parallel resistors, then when the resistance is lowered, the current will increase.

 

[BloodySurgeon]

remember the formula of resistors are r = pL/A... therefore increase the length or add a resistor is series increase r and increase area or add a resistor in parallel will decrease r.

 

[spo01]

How does an extra resistor decrease the overall resistance?

a good analogy is to think of a toll bridge. if you only have one toll booth open that means the cars have to go through one by one and soon the cars are piled back and it takes forever to get across the bridge. however, if you were to open up more toll booths (parallel to each other) this would fix the problem and cars could pass through easier.

 

[olygt]

thanks guys, that helped a lot

 

[Big Sneez]

a good analogy is to think of a toll bridge. if you only have one toll booth open that means the cars have to go through one by one and soon the cars are piled back and it takes forever to get across the bridge. however, if you were to open up more toll booths (parallel to each other) this would fix the problem and cars could pass through easier.

I always like to analogize it to water-flow. Think of a barrel of water which is discharging its contents through a hose of resistance X. Tapping a second hose into the barrel will now reduce the resistance to X/2.


>S