Electrical Circuit - Maths question

Electrical Circuit - Maths question
Homepage: http://www.thephysicsteacher.ie/
Blog: http://ozymandias1.wordpress.com/

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Howto & Style

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• Leaving
• Cert
• Physics
• Electricity
• Electric
• Circuits

[nomoreheroesanymore]

Just asking, not criticising as such, but why do you not use the formula for R's in parallel - (R1*R2)/(R1+R2) ?

Seems easier to me than remembering to invert 11/30.

And why is a recurring 3 such a big argument against decimals in favour of fractions ?

But thanks, and credit to you for putting this up.

[yerrahyouknow]

Would you believe I have never come across this option (R1*R2)/(R1+R2) before! I reckon I could suggest both methods and let students pick the one they like, or perhaps would be familiar with. Thanks

The issue with the recurring 3 is that it gets rounded down, which - if repeated a lot - can introduce avoidable errors in the final answer.

Feel free to correct me, and thanks for taking the trouble to comment.

[Tilex1990]

hmm.. we always say U=I*R and not V=I*R O,o

[yerrahyouknow]

Why U?

[blackcurtain84]

which level is this? university?

[yerrahyouknow]

Irish Secondary School - similar to final year of high school?

[Joetheman416]

He can still use the assumption of conventional flow (positive to negative) since the math works out regardless. (I think, anyway).

[nomoreheroesanymore]

Have you not heard of the terms 'conventional current flow', versus 'electron theory' ? ? ?

[srkh28]

11/30? 107/11? what the! - welcome to 1963.

[UniversalExpanse]

Why not teach the correct way, which is from negative to positive (electron flow theory)? How can you teach people the wrong way and expect them to learn properly?

[xbeerxpressurex]

@winwutwut In your example the Total Resistance depends in the resistors disposal on the circuit. You only demonstrated the equivalent resistance for R1 and R2 (in parallel) and the equivalent resistance for R3 and R4 (in parallel too). Now, if you have them in series you should add them. If they're in parallel you apply the rule: [ (R1//R2)//(R3//R4) ]

[winwutwut]

@xbeerxpressurex

This is easier for meto use when calculating many parallel resistors: i need to calculate the Rtotal for R1, R2, R3. R4= (R1*R2)/(R1+R2) and then: Rt= (R3*R4)/(R3+R4)

[nomoreheroesanymore]

@ xbeerxpressurex

Point taken, but I did explicitly say that it is a formula for resistors in parallel. For me it's merely academically interesting to point it out. I'm in no position to criticise a professional teacher. Of course if I were a teacher, I'd do as you say and use one formula for all situations.

Best wishes.

[xbeerxpressurex]

@nomoreheroesanymore That only works for a pair of resistors in parallel, don't forget that. If you use the formula demonstrated in the video you'll never get the result wrong.

[magnamino]

@yerrahyouknow That formula: Rt= (R1*R2)/(R1+R2) is only good for 2 resistors in parallel. The inverse of the inverse: 1/Rt = 1/R1 + 1/R2 + 1/R3 ...etc... is for more than 2 resistors in parallel. the first formula will, of course, Not work for more than 2 resistors in a parallel circuit... Nice analogy with the water flow. I use similar ones when instructing: i.e., diodes can be compared to check valves, etc.

Jim, (23+ years electrical experience)

[mufc99ok]

@tels2006  hes good but confusing just keep it simple eh

[mufc99ok]

this reminds me of phase 4 in ringsend intence shit