Electricity Physics - Support needed on Q

total R across parallel resistors is 50 ohms
I = 0.24A

Q is what is the current in 60ohms? answer is 1/3 I = 0.008A

but why? greater resistor = lower current. Yes that makes sense as greater resistance opposes current. But the equation R=VI confuses me sometimes thinking that more current = more resistance which is not true.

Reply 1

Joinedup

but R=V/I

you can use an Ohms law triangle to help you remember

you put your finger over the variable you're trying to find
put your finger over V and you can still see IR... V=IR
put your finger over I and you can still see V over R... I=V/R
put your finger over R and you can still see V over I... R=V/I

Original post by Batman2k1

Reply 2

Batman2k1

oh okay, that makes lt easler, what about the question? how wud u do that? 1/3 of the whole current.

Would you say the question is alike to a potental divider? if my understanding is correct then, when a current goes around a potentometer/potental divider, which basically means, the voltage is split from the components, as in a series, this is a common property found.

So one of the resistance out of the whole resistance packaged together in a series is a voltage for that resistance, which is a fraction of the emf.

But here its parallel, so V will have to be the same? But current is obviously different, we want to know the current. So that means, if 60 ohms s bigger, current would be the smallest? makes sense for 1/3, then 60/90 * total current = 0.08A?
no wait higher resistance means lower current, but thts just weird, it would hve to be 30/90 for the bigger resistor, is a simple way/knowledge of knowng ths?

Original post by Joinedup

but R=V/I

you can use an Ohms law triangle to help you remember

Reply 3

Joinedup

You can think of parallel resistances as a 'current divider' which is analogous to series resistors being a potential divider.

for the 30 ohm and 60 ohm in parallel, the 30 must carry double the current of the 60.

so if the total current is known the 30 must carry 2/3 rds of the total and the 60 must carry 1/3rd.
cos 2/3rds is double 1/3rd

Reply 4

Batman2k1

How do you know that, 60 ohms carries 1/3 i? Define rds?

i think if i look at the triangle, then v stays the same, but if 60ohms = x2 of 30,
then i = 1/2 of the current in 30 ohms
hmm confused myself, yeah how did u get that? thanks!!!

Original post by Joinedup

Reply 5

Joinedup

rds is a suffix for thirds... I think it's actually unnecessary so you can ignore it.

you are correct that the PD is the same across both resistors in the parallel pair
because for each individual resistor in the parallel pair I=V/R you can see that the current through the 30Ω must be twice the current through the 60Ω

This similar to dividing a whole pizza with no pieces left over between two people so that one person gets twice as much as the other person... the only way it's possible is if one person gets 1/3 of the total pizza and the other person gets 2/3 of the whole pizza.
1/3 + 2/3 = 3/3 = 1 so the slices add up to the total amount of pizza
1/3 multiplied by 2 = 2/3 so one person is getting twice the amount of pizza as the other one

what this means for the proportion of the total current going through each resistor is that the 30Ω takes 2/3 of the total current and the 60Ω takes 1/3 of the total current.

so when the total current is known, the current in the 60Ω is I/3

Reply 6

Batman2k1

great analogy, so it is like the 60 ohms is the fat guy who needs to have a lower proportion of pizza so, it takes less of the whole current. The current must be shared, and equality matters in all, in any science based topic.

whats confusing me is the formula for potentometer with finding Vout from 1 resistor
In a series
Vout = R1/Rtotal x Vin

But with current, if the bigger resistor has a bigger voltage, then it must take less of the current.
I know that resistors follow ohms law at constant temperature. In this case would the resistor follow that? Half the voltage/ half the resistance gives double current
thats inversely proportionality,
can you elaborate on this part? - you seem to know this more solidly

Original post by Joinedup

Reply 7

Exotic Ghoul

firstly, its 0.08 amps not 0.008. This is because voltage is different in series, so find voltage in the 20 ohm resistor combination which is 4.8 as I = 0.24 = constant in series, then do 4.8/60=0.08 :-)

Original post by Batman2k1

Reply 8

Batman2k1

got it, so find the voltage through the whole lot. As its parallel it's split, so you take a fraction of the voltage got it. This method is neat, okay, simple enough now.

In R1, the resistance is 50 ohms. So total I = v/r = 24/100 = 0.24A

R1 has a current of 0.24A because the current is same in the series regardless of anything?

so 0.24A*50 = 12V that is right

Original post by Exotic Ghoul

Reply 9

Batman2k1

this is off topic question, but with sig figs. If least sf is 2.... but Question 3b) has sf of 1, then do you leave sf from the whole q or just the subq?
sometimes, i see paper answers, and i observe that the person doesn't even bother adding 500.344..... just leaves it as 500
im confused on what to do here appropiately?

Original post by Exotic Ghoul

Reply 10

Exotic Ghoul

do not round until the final calculation, Store your calculations on your calculator and use the A B C buttons to perform the final calculation, in this case it is the subq

Original post by Batman2k1

Reply 11

Batman2k1

l use just the ans button as a 1 store thing... hmm good idea, maybe i look into it, probably good for multistep.
i guess its lowest sf each sub

Original post by Exotic Ghoul

do not round until the final calculation, Store your calculations on your calculator and use the A B C buttons to perform the final calculation, in this case it is the subq

Reply 12

Batman2k1

i am also stuck on this moments question, i was wondering if u know how to get C? b easy bar is balanced, so 0.4 but c is darn confusing, not gettin soemthing here