Physics HWK Help please? (Electricity)

[BenW]

I really need to see my teacher about this. Thanks for all the help

[Byatt]

^^ not convinced that's right, Mike, you don't have 9V across that parallel component, and I'm not even certain that considering 9V across the whole 29k loop will give you the correct answer as parallel circuits are funny things...my method (right or wrong) follows.

So on another question i'm given this circuit:



I've done all but one question. I got to the last question and it says how much electrical energy is converted to heat by the red resistor in 7.5 mins?

I've based all my answers on nothing going through the right half of the circuit as restistance is greater than on the left

Whoa, no, just because resistance is more on the right doesn't mean there won't be no flow, it means there will be proportionally less!

Work out the total resistance of the circuit by considering the sum of the serial parts (10k + 1k) and the parallel bits (ends up to be ~6400Ohm), so total resistance of the circuit is 11kohm + ~6400Ohm = ~17400Ohm.

Use V=IR to get the current flowing through the serial bits (The bottom 10k and the top 1k resistor). turns out to be about .0005A

You can use the ratio of the resistances (So 10/28 of the current goes through the red bit) between the parallel parts to determine how much current flows through the right half of the parallel part (where our red resistor lies), then use P=VI=(I^2)R, and E = Pt (where t is in seconds).

Hope your teacher can give you a more intuitive way of dealing with this

[mike_w]

Hmm, thinking about it, you're right about the bit on the right not taking 9V! But the way I would do is to work out the resistance of the parallel bit (the two 10ks and two 4ks). You can then work out the potential difference that will flow through the entire parallel bits. From the potential difference that flows through the entire parallel bits, you can split this up between the three bits (4k, 4k and 10k). Then just use Power = V^2/R for the 4k component.

[Rave]

Thanks. just realised i made a silly error! Used V = I/R instead of V = IR!!

Maybe thats why i got 0.003Amps as one answer!

EDIT: my minds playing up....i did have it right. So are these the correct answers?

a. 2k + 1k = 3kΩ

b. I = V/R I = 9/3000 I = 0.003A

I haven't pulled a calculator out, but that looks right.

c. The 1k resister resists 0.001A so the 4k resistors have 0.002/2 going through them which is 0.001A

Resistor. Don't take the chance w/ typos in case you get an old school exam marker. Anyway- no. In any single loop circuit the current flowing is the same in every component unless they're in parallel. The current drawn from the battery is .003A; the current going through the 1K resistor is .003A; the current going through each of the 4k resistors, given that they're identical, is .0015A- half of .003A. Current is charge x time- I.E. the flow of electrons. The number of electrons coming out of the battery is the same as the number going back in. The power 'generated' is due to the potential difference, not because electrons are somehow disappearing. So, Anyway:

d: The two 4k resistors in parallel appear to the circuit as one 2k resistor. Given that W=IsquaredR (forgive my failure to do a proper squared symbol):

.003 x .003 x 1000 = .009W.

I think. I am drunk and this is taxing my brain. I'm pretty certain that my reasoning is correct at least. My mum is a physics teacher and I got an A in every module of my physics A-level. How soon age turns one's brain to mush.

As for e:- they want you to work all the values out again- to quantify what's going to happen, not to just tell them which way the values will go in a handwavy fashion. It's not hard though, just include the .6ohm in the current calculations. It's not going to make much difference in the grand scheme of things TBH, the battery is hardly being stressed by the circuit. If the total resistance of the circuit was something like 10 ohms, like in a radio control car, the internal resistance of the battery suddenly becomes a big factor. In this case it's negligible- but they still want you to do the sums again.

Edit: typo corrected

[herulach]

raves correct, i didnt spot that. youd never think i had a degree (nearlY) in this crapsome times