Calculating Current and Voltage in a Circuit with Multiple Resistors

[donpacino]

you don't know what the voltage across the resistors are because you have not drawn it properly!

have you followed the step-by-step instructions I gave you?

ie have you looked at my drawing and drawn your own?
- unless you have the mental insight of a genius its essential to always - always - draw the problem clearly and neatly. Once this is done, you will find the problem virtually solves itself. The scribble you provided is creating you more problems than it is solving!

william at this point simply drawing it out is not going to solve the problem, as Op has your drawing to look at

 

[donpacino]

no, i have followed the steps and redrew it but i still don't get it

check post 29

 

[nothing909]

what is the relationship between the 5 ohm resistor and the four ohm resistor. that should give you some insiight into the relation between the four ohm resistor and the (1 & 2) ohm resistors.

the

check post 29

the 4 ohm resistor is in parallel with the 5, yeah, so are all 3 of them in parallel with each other?

 

[donpacino]

the

the 4 ohm resistor is in parallel with the 5, yeah, so are all 3 of them in parallel with each other?

yup! if you have any doubts, look up the definition of parallel resistors. eventually it will be intuitive

 

[William White]

no, i have followed the steps and redrew it but i still don't get it

okay, so you know the total currentif you have drawn the circuit properly, you will see that the current is going through three parallel paths, top middle bottom

do you know the rules for dividing the current through the top middle and bottom paths?if the top path is 3ohms
the middle path is 4 ohms
and the bottom path is 5 ohms

and the total current is 14A

what current is going though each path? The lower the resistance the MORE current, so expect the largest amount to go through the top path, and the least in the bottom path.

when you know these currents, tell me what they are.

 

[nothing909]

okay, thank you, but i feel like an idiot still asking question when u guys have explained all this like 10 times.

if the voltage for the 4 ohm resistor is 17.09v, that means the voltage across the 5 resistor and the 1 ohm resistor are all 17.09, yeah? so what's the voltage across the 2 ohm resistor?

 

[nothing909]

okay, so you know the total currentif you have drawn the circuit properly, you will see that the current is going through three parallel paths, top middle bottom

do you know the rules for dividing the current through the top middle and bottom paths?if the top path is 3ohms
the middle path is 4 ohms
and the bottom path is 5 ohms

and the total current is 14A

what current is going though each path? The lower the resistance the MORE current, so expect the largest amount to go through the top path, and the least in the bottom path.

when you know these currents, tell me what they are.

i don't understand how to work it out. do i need to just do something like voltage division but instead with current?

 

[donpacino]

okay, thank you, but i feel like an idiot still asking question when u guys have explained all this like 10 times.

if the voltage for the 4 ohm resistor is 17.09v, that means the voltage across the 5 resistor and the 1 ohm resistor are all 17.09, yeah? so what's the voltage across the 2 ohm resistor?

close but no... that 19.09 V is across the series combination of the 1 and 2 ohm resistors.

and again don't feel bad for asking questions

 

[nothing909]

so 17.09 is the Vin? so i just do 17.09 x 1/3 and 17.09 x 2/3 and that'll give me the voltages for both of the individual resistors?

 

[William White]

i don't understand how to work it out. do i need to just do something like voltage division but instead with current?

so, this is the key to solving the problem;

the current through the resistor is inversly proportional: less resistance more current.

you have 14A

If you have a 2+1 = 3 ohm resistance in the top path. 4 ohm resistor in the middle
5 ohm in the bottom

can you show that the currents are
6A
4.5A
3.5A

?

 

[nothing909]

so, this is the key to solving the problem;

the current through the resistor is inversly proportional: less resistance more current.

you have 14A

If you have a 2+1 = 3 ohm resistance in the top path. 4 ohm resistor in the middle
5 ohm in the bottom

can you show that the currents are
6A
4.5A
3.5A

?

look at my above post. is that right what i said?

 

[William White]

yes, the voltage across the three parallel paths is 17.9 V...

so you know the resistances...so what are the currents through each path?

 

[nothing909]

yes, the voltage across the three parallel paths is 17.9 V...

so you know the resistances...so what are the currents through each path?

The currents are just I=V/R after I've worked out the voltages, yeah?

 

[donpacino]

so 17.09 is the Vin? so i just do 17.09 x 1/3 and 17.09 x 2/3 and that'll give me the voltages for both of the individual resistors?

exactly

 

[William White]

Yes,

and you have told me the voltage drop across the three paths - it is 17.9 V

so now tell me the current through each resistor...

 

[nothing909]

the current across the 4 ohm resistor is 71.4
the current across the 5 ohm resistor is 89.55
the current across the 1 ohm resistor is 5.97
the current across the 2 ohm resistor is 5.97

 

[William White]

you have already told us the correct TOTAL current, which is 14A

all your currents must add up to this! you have multiplied rather than divided! i = v / R

you have 17.9V across a 5 ohm resistor...that is 3.58 A

what are the others?

 

[nothing909]

wait, i don't understand. is my answer for the 1 and 2 ohm resistors correct?

i did for the 1 ohm resistor: 17.91x1/3, then I=5.97/1

for the 2 ohm resistor : 17.91x2/3, then I=11.94/2

 

[William White]

yes;

The 1 and 2 ohm resistors are in series - the current through them is the same.

therefore they are equivalent to a 3 ohm resistor

17.9 V / 3 ohm = 6A

 

[nothing909]

yeah sorry

for the 4 ohm it's 4.48
for the 5 ohm it's 3.58

 

[nothing909]

4.48 + 3.58 + 5.97 = 14.03

 

[donpacino]

looks good man...
my recommendation, work a few more practice problems like this!

 

[nothing909]

k, wait, just one more question before i go. how did will work out the total current was 14A?

 

[William White]

4.48 + 3.58 + 5.97 = 14.03

yes!

now, you can see the key is to have a good drawing!

10 minutes on a good drawing saves you hours of work!

 

[William White]

k, wait, just one more question before i go. how did will work out the total current was 14A?

you work out the total resistance of the circuit

look at my drawing

you have

3ohms (the resistor in series) +
1 / ( 1/(1+2) + 1/4 + 1/5 ) = 1.3 ohms (this is the rule for resistors in parallel ... you remember learn this one!)

total R = 3 + 1.3 = 4.3 ohms

total current = voltage / total R
= 60 / 4.3
= 14A

 

[nothing909]

omg, that was such a silly question, i worked that out myself right at the very start...

anyway thanks for all the help, that simple question was giving me a lot of stress.