physics Q

[eyeloveu]

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does anyone have a trick to figuring out questions relating to circuits? For example, finding the amount of current flowing into a resistor or capacitor in a given circuit? I know the formula for resistors and capacitors in parallel or in series, but I am having a hard time putting those equations to use and how to recognize when and where to use them. If you have a trick to tackling these questions, please let me know. Thanks!

 

[jvelos3]

V=IR
Voltage = Current x Resistance

Resistor:
when in series: Add resistors, then find I through formula above. Current through each resistor is the same. Then you can find the voltage drop at each resistor by taking the current x resistor = voltage.

when in parallel: 1/R + 1/R = 1/R etc.
Current flows through the path of least resistance. This means that the resistor with lower resistance will have more current. It is also at an exact ratio. Also, the current that diverges is additive...meaning the Current through both resistors = the current through resistor 1 + current through resistor 2.

- so for example if you have a 12V system with a 2ohm and 3ohm resistor in parallel the current will be as follows...

1/2 + 1/3 = 1/(6/5) so the total resistance is 6/5ohm.

The total current will be 12 = I(6/5) so I = 10.

Now...the 2ohm resistor will have more current flow than the 3ohm resistor in a ratio of 3:2. so the 2 ohm resistor will have a current of 6 and the 3ohm resistor will have a current of 4.

So remember that with resistors...
In series: Current is same, Voltage is different for each resistor.
In parallel: Voltage is same, Current is different for each resistor.

I am not so sure about capacitors...maybe someone else can chime in...

 

[eyeloveu]

OK, I have a question about the math that you did. Isn't 1/2 +1/3=5/6? This would change the rest of the problem b/c the total current that you used wouldn't add up. Right?

 

[jvelos3]

yes 1/2 + 1/3 = 5/6 BUT the answer must be in 1/R form. 5/6 = 1/(6/5) so R is 6/5

 

[eyeloveu]

Got it! Thanks so much!