# Homework Help: Find current in circuit

1. Aug 27, 2015

### nothing909

How do I find the current across the 5Ω resistor?

I worked out the supply current by simplifying the circuit down.
I then went on to work out the voltage across the 3Ω resistor using the supply current.
I then used that voltage to then work out the voltage across the 4Ω resistor.

Now I'm stuck. How do I work out the voltages of the other 3 resistors and get the current across the 5Ω resistor?

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2. Aug 27, 2015

### donpacino

do you know how current division work?
or you can use ohms law to find the voltage drop across the first resistor

3. Aug 27, 2015

### nothing909

I don't know how current division work, and what do you mean by using ohms law to find find the voltage across the first resistor?

4. Aug 27, 2015

### donpacino

you know the current through the 3 ohm resistor, so you should be able to calculate the voltage drop across it. then use the supply voltage to figure out the voltage across the 5 ohm resistor.

5. Aug 27, 2015

### nothing909

I calculated the voltage across the 3Ω resistor which I show in the last picture. After I worked out the voltage across the 3Ω resistor, I subtracted it from the supply voltage to work out the 4Ω resistor.

Can you explain how I work out the current and voltages for the rest of the resistors?

6. Aug 27, 2015

### donpacino

if you know the voltage across the 4 ohm resistor, then you know the voltage across the 5 ohm resistor. you also know the voltage across the series combination of the 2 and 1 ohm resistors.

7. Aug 27, 2015

### nothing909

I've just started my engineering course in and I've not did this kind of stuff for a long time. Can you tell me the voltage across 5Ω resistor so I can understand what you mean.

8. Aug 27, 2015

### donpacino

the 5 ohm resistor and the four ohm resistor are in parallel....

9. Aug 27, 2015

### nothing909

So the voltage across the 4Ω resistor is the same as the 5Ω resistor?

10. Aug 27, 2015

### donpacino

yes, due to the fact that they are in paralell.

edit: disregaurd what i said , you skipped a step in your work and i thought u made an error

11. Aug 27, 2015

### nothing909

I'm sorry for going on and maybe I'm asking stupid questions, but I really need to know this

I don't understand what you mean when you say that I've made a calculation error. Where did I do this? Which picture shows this?

I know that after working out the voltage of the 5Ω resistor, it allows me to .work out the current and that's the question answered, but can you please explain how I then work out the voltages and currents of the 2Ω and 1Ω resistors.

12. Aug 27, 2015

### donpacino

thats fine, no need to apologize....

you did not make an error. I thought you added 1/Rt and 3 ohm.

so the 2 and 1 ohm resistors are in parallel with the 5 ohm resistor. SO you can either use voltage division, or use the same process you did before

13. Aug 27, 2015

### nothing909

How do I do voltage division?

14. Aug 27, 2015

### donpacino

https://en.wikipedia.org/wiki/Voltage_divider

its a shortcut for finding the voltage across individual resistors in a series chain.

lets say you have 2 resistors R1 and R2 in series and a voltage Vin across them

the current is Vin/(R1+R2)
then to get the voltage across either of the resistors, you multiply the current by the resistance.

so Vr1=Vin*R1/(R1+R2)

the same goes for R2.
make sense?

15. Aug 27, 2015

### donpacino

current division, which I talked about earlier, can be derived in a similar way...

two resistors R1 and R2 in parralel with current Iin going into them

the voltage is
V=Iin*R1*R2/(R1+R2)

then voltage over resistance gives you current

so IR1=Iin*R2/(R1+R2)

16. Aug 27, 2015

### nothing909

Okay, maybe a dumb question, but I understand voltage division now, but what would be the value of Vin for the 1Ω and 2 Ω resistor?

17. Aug 27, 2015

### William White

tip

draw circuits using only horizontal and vertical lines.

another tip

draw circuits going left to right (rather than as a loop) (this is a very common, commonsense way to draw circuits)

start on the left with the positive terminal of the battery. imagine walking along the wire. thats a straight line. when you get to a junction you can turn left, right, or keep going. draw like this until you get to the negative terminal of the battery which is on the far right of your page. Now, what is in parallel and what is in series very clearly defined. This method is very useful as circuits become more complex and it is essential to know what is in parallel with what just by glancing at the diagram.

this way you can get a much clearer picture of what is going on, something like this, but draw it for yourself.

when it is like this you can quikly work out the total current; and the current flowing through each resistor

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18. Aug 27, 2015

### donpacino

vin is the voltage across the two resistors.... so what is the voltage across the two resistors?

19. Aug 27, 2015

### nothing909

Is the voltage across the two resistors 42.09v? or is it just 60v like the supply voltage. i dont really know

20. Aug 27, 2015

### donpacino

21. Aug 27, 2015

### donpacino

so the 60V goes across the 3 ohm resistor as well as the 1 and 2 ohm resistors.
you want to use the voltage that is DIRECTLY across the 1 and 2 ohm resistors.

22. Aug 27, 2015

### nothing909

Okay, I'm sorry, I don't understand what you mean. Can you please just tell me the voltage across the two resistors so I can understand?

Also, is the 4Ω, 5Ω and 1Ω resistor all in parallel with each other? If so, does that mean the 1Ω resistor has a voltage of 17.09 like the rest in parallel?

23. Aug 27, 2015

### William White

1) Draw the diagram properly - so it looks like something I have done. Do it very carefully and neatly. If you do it properly, you will know, just by looking what is in series and what is in parallel!

2) You can then replace all the resistors with a single resistor. There are rules for working out equivalent resistances, which you need to know. So you will have two drawings, one with all your resistors (from step one) and one with a single resistor. The circuit that contains ONE resistor that has the same resitance as all your resistors in the other circuit. They are the same as far as the battery is concerned.

3) Now you can work out the total current through this single resistor using ohms law. This current is the current leaving your battery in your original drawing.

4) Now go back to your original drawing, with several resistors.

4) Work out the volt drop (using ohms law) across each resistor.

5) Done.

Try to do this step by step. When you get stuck on a step, ask.

24. Aug 27, 2015

### nothing909

Okay I've did all that. I reduced all the resistors into one single resistor and then worked out the supply current using ohms law as you can see in the working.

I understand how to work out the voltages for the 3Ω, 4Ω and 5Ω resistor, but I don't understand how to work out the voltages for the 2Ω and 1Ω resistor.

25. Aug 27, 2015

### donpacino

you can work it out the SAME way you've been doing it before. look at the drawing William produced. look up the definition of series and parallel and tell me what relationship those resistor have with the four and 5 ohm resistors.