BMAT thread 2009

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Reply 1780

Sarah182

thegreatest

is the answer B?

Yes!

how did you get that??

Reply 1781

carcinoma

Woody.

The fact that I can't do this question depresses me. I really need to practice exactly these types of questions and learn about them, does anyone know of a good resource? I mean, I = V/R V = 24. I think series, Rtotal = 1/R1 + 1/R2 and in parallel it's R1 + R2 etc.

So going by that... 1/6 + 1/12 + 8 = 8.25

24/8.25 = ~3

No.

1/6 + 1/12 = 1/4 = Total Resistance in parallel = 4 Ohms

4 + 8 = 12 Ohms

24/12 = 2 Amps

Therefore answers, C, D and E are eliminated.

Next we know that current takes the path of least resistance.

So more of the 2 amp current goes through the 6ohm resistor in the parallel section. Therefore the only answer possible is B.

You dont need to do AS physics calculations to get the answer.

Reply 1782

carcinoma

thegreatest

is the answer B?

Sarah182

Yes!

how did you get that??

The answer is B and how you get it is in my post above.

Reply 1783

loveisjustasong

vas876

Hmmm, I got to 2 Amps. Why wouldn't it be A?

Reply 1784

thegreatest

Sarah182

Yes!

how did you get that??

Its hard to explain on here...but here goes...

The total resisitence in parallel is 4.......therefore voltage is splits in the ratio 4:8 ....which means 8:16 ....Voltage across paralel is therefore 8....So 8 volts goes to each resistor ....V=IR ..so thru the 6 ohm resistor......I=8/6=1.3333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333333

sorry its a crap explantion but thats how I did it

Reply 1785

carcinoma

Get Cape.Wear Cape.Fly.

Hmmm, I got to 2 Amps. Why wouldn't it be A?

2ampts is the TOTAL current, the question asks for the current across the 6ohm resisor.

It can be A because most of the total current will flow through the 6
Ohm resistor with a small amount flowing through the 12 ohm resistor.
More current flows at lower resistance.

Reply 1786

Sarah182

vas876

No.

1/6 + 1/12 = 1/4 = Total Resistance in parallel = 4 Ohms

I dont understand this bit... where did 1/6 and 1/12 come from?

Reply 1787

carcinoma

thegreatest

That works but its not the actual method. The actual method is not needed, two calculations then a dedication gets the answer as explained above.

Your method fails when the resistors are not 2:1.

Reply 1788

Woody.

vas876

Sorry, I don't understand the first part
~~1/6 + 1/12 = 1/4, how did you get that the total r = 4?~~ EDIT: Nevermind, I understand that now

And I know it's not AS level, that's why I'm depressed at my inability to calculate it.

Reply 1789

carcinoma

Sarah182

I dont understand this bit... where did 1/6 and 1/12 come from?

Ahh **** i forgot you dont do physics (im sure this was done at GCSE)

To calculate resistance of a parallel circuit:

1/Total resistance of parallel circuit = 1/R1 + 1/R2 ....... ect.

Reply 1790

carcinoma

Woody.

To calculate total resistance of the parallel resistors, you find the reciprocal of the sum of the reciprocals of each resistor.

1/Total Resistance in parallel = 1/R1 + 1/R2 ect.

Seen edit.

Reply 1791

Woody.

vas876

To calculate total resistance of the parallel resistors, you find the reciprocal of the sum of the reciprocals of each resistor.

1/Total Resistance in parallel = 1/R1 + 1/R2 ect.

Yep, thanks for explaining that, I just remembered that it was 1/ rather than just the sum.

Reply 1792

Sarah182

vas876

Ahh **** i forgot you dont do physics (im sure this was done at GCSE)

To calculate resistance of a parallel circuit:

1/Total resistance of parallel circuit = 1/R1 + 1/R2 ....... ect.

ahhhh thankyou! it makes sense now :smile:

Yeah I swear I've never done that at GCSE though ¬¬
pffft gcse syllabus my ass. :p:

Reply 1793

loveisjustasong

Sarah182

ahhhh thankyou! it makes sense now :smile:

Yeah I swear I've never done that at GCSE though ¬¬
pffft gcse syllabus my ass. :p:

It is actually GCSE, just not covered by all boards. I bought one of those fat GCSE Physics (all boards) book and it has everything on the spec :yep:

Reply 1794

Sarah182

Get Cape.Wear Cape.Fly.

It is actually GCSE, just not covered by all boards. I bought one of those fat GCSE Physics (all boards) book and it has everything on the spec :yep:

ohhhh okay, that would make sense I guess.. :rolleyes:

Oh well, I've decided Im going to miss out all the physics questions at first anyway, and then just do the few that I might be able to do when I've had a go at the rest.. :smile:

Reply 1795

thegreatest

vas876

That works but its not the actual method. The actual method is not needed, two calculations then a dedication gets the answer as explained above.

Your method fails when the resistors are not 2:1.

Actually it doesnt. The voltage is split thru whatever ratio the resistors are at in series. Its not rocket science to work out the ratio or to work out the voltage thru each resistor. My method therefore does not fail.

Reply 1796

Onychophagia

thegreatest

This is the way I did it. The other way is little more than an educated guess.

Reply 1797

Woody.

thegreatest

I appreciate it's hard to explain, but I'm still slightly confused. I understand that the resistance in parallel is 4, and it's 8 in series, so 4:8. Why is it then 8:16?