Need some electrical help

[Will24]

Ok, in my basic electricity class, we have a teacher that doesn't know shiite from shinola and he's trying to teach the class from another teacher's curriculum, well he has me big time confused...





He was discussing an alternative method for finding parallel circuit total resistance, I'll try to explain





he's saying that you add the total resistance of the circuit, so let's say...



R1 + R2 + R3 + R4 = X



Well, he's saying to add them all up then take the reciprocal... .



so have have 1/X now...





Then he's saying take the total current draw on the circuit, and make it the numerator of the fraction...







So, let's say we have a simple circuit with 10 amps as our total current draw



2ohms + 3 ohms + 5 ohms + 10 ohms = 20 ohms



He's saying,



Put 10 amps / 20 ohms



So in essence, he saying that makes the total system resistance . 5 ohms





Can someone PLEASE explain to me how this works?

We have a test on it Monday, and I sincerely don't understand, he wants us to use this technique...



Thanks guys!





-Will

 

[dresslered]

He knows "shiite from shinola".



The total resistance of a parallel circuit is given by: 1/[(1/R1)+(1/R2)+1/(R3)..... ] = 1/[(1/2)+(1/3)+(1/5)+(1/10)] = . 88235 ohms.



Then, 1/R does equal I/V. However, even in your last statement, it should be very clear that amps/ohms does not equal ohms. Hopefully he is not grading your test!

 

[evldsl]

Just remember PIE,EIR.

p=POWER,

i=CURRENT

e=VOLTS

r=RESISTANCE



Always remember the basic's.

 

[Rman]

man that guy is way out there on this one. dresslered is right on the money. One trick I remember...



if you have 2 resistors in parallel, use the product over the sum



so if r1 is 10 ohms and r2 is 20 ohms



(10*20)/(10+20)= 6. 667 (rT)



It's been a while but I pulled that one out of my fogged memory.



I'm a EE and actually went to school for this so if you need more help shoot me an email.

 

[Rman]

man i looked at that for a few minutes... I cannot for the life of me figure out what the hell that guy is thinking. There is just no way to get ohms out of amps. If you stick amps in there you get your potential (volts)... but in no way will that correlate to ohms.



post this again when wonderboy realizes his mistakes.

 

[klenger]

I think what the instructor was suggesting is to calculate the current through each resistor using the formula I = E / R. You could use any voltage. Let's assume 10 volts.

Current IR1 = 10/R1

Current IR2 = 10/R2

...



Now add up the total current through all the resistors. Let's assume this is 5 amps.



Now calculate the parallel network resistance:

R = E / I

R = 10 / 5

R = 2 ohms



Using some algebra with this method will undoubtably yeild the same formula that dresslered quoted above. The reason that the voltage doesn't matter with method I explained is because it is factored back out of the calculations on the last step. Try the process with 100 volts and you will get the same result.



Boy, this is dusting the cob webs off the memory cells. Shall we try some AC circuit theory next?

 

[Rman]

wait till we get to FM diodes... zeners for the PC people. Freakin Magic is generally the explaination i give for most electron theory!

 

[evldsl]

" Man", it's the capacitors that make magic .

 

[DMcClellan]

It's all "FM" to me. Just don't let the smoke out.

 

[Rams-n-Hogs]

Just don't let the smoke out.

Just like these trucks, that's the FUN part.



Jim

 

[ETOBICOKE]

Resistors in parallel

There are two ways of calculating the total resistance of a circuit, with resistors in parallel. If all the resistors are all of equal value, divide the value of one resistor by the number of them in the circuit. If they are of different values, add the reciprocal of each of the values together and then take the reciprocal of this sum. This will be the total resistance in the circuit.





Neil

 

[PJereb]

Will did you figure it out yet?





It sounds like you have two diffrent things mixed up/ combined.





Finding the total resistance in a parallel circut is pretty easy.



Total resistance =



1

__________________

1 + 1 + 1 +etc

--- --- ---

R1 R2 R3





So with your #'s of

2ohms + 3 ohms + 5 ohms + 10 ohms



It would look llike



1

______________________

1 + 1 + 1 + 1

--- --- --- ---

2 3 5 10



Which is... ... ...





1

______________

34

------- (peform this first)

30



Which is... ... .....



1

______________________

1. 133333333333333333



And the answer is



. 882 ohms



So the more resistors you add up in a parallel circut, the less resistance you have (the closer it is to a perfect short circut)











Below I put the formula for a series circut.



Total resistance in a series circut is... ... .

Rt = R1 + R2 + R3 + etc

 

[Will24]

Thanks everyone for the replies, especially PJereb... .





That's what he was trying to explain, but he screwed up somewhere... my first post was my notes from class, copied verbatim from the board...







-Will

 

[KBennett]

There are some good replies here and I'll add my $. 02.



When you are looking at a circuit you have three basic parts:

1. Electricity(supply)

2. Load

3. Wires or traces that connect them together.



After you have a good idea of how each component works together you will be able to troubleshoot a circuit. In a parallel or series circuit the resistors share the load, and the sum of the voltage drops will always equal the supply. I hope your school gives you an opportunity to check out the theory you have learned in a hands-on lab environment. When you breadboard a couple resistors together, apply power to the circuit, and then poke around with a voltmeter it will all start to make sense.

 

[Will24]

Well, it all made perfect sense right up until he tried to teach the class the alternative to the product over sum method... I had 4 years of power technologies in high school, and the first quarter each year all we ever did was series/parallel problems with plugging in various numbers...





We do have a lab that goes with the class, but most of my class is, let's say, slow. Teacher gave us three hours to wire three resistors in series and do 3 experiments with them...





When he walked by my bench at the beginning of the 2nd hour, he looked at me funny... some kids were struggling to wire the three resistors in series, but on my bench was an accurate mockup of a complete truck light harness, complete with a relay for all switching operations, and a functional flasher for the turn signals...



I had parts from 7 different lab kits on my bench