The Student Room Group
Uni students - Complete our survey >>
Uni students - Complete our survey >>
Uni students - Complete our survey >>

Circuits help

Can someone please help me with these questions that are attached? I got 5a as 6V but that is the only one i can do!!!:frown:

Thank you for your help!!!!:tongue:
s.pptx166.0KB
Original post by jessicaallenn
Can someone please help me with these questions that are attached? I got 5a as 6V but that is the only one i can do!!!:frown:

Thank you for your help!!!!:tongue:


How did you get 6V?

V=I*R

Where is your difficulty in understanding how to solve the questions?

You need to know:

a) Ohms law and be able to rearrange for V, I or R as the subject.

b) Calculate resistances in series and in parallel.

c) Kirchoffs voltage and current laws.
(edited 10 years ago)
Reply 2
Avatar for disillusion
disillusion
OP
8
I kno that V=IR I got 6V because assumed that voltage is same everywhere???
I have been taught that but the question is confusing me and am not sure what figures to put in where
Original post by jessicaallenn
I kno that V=IR I got 6V because assumed that voltage is same everywhere???


OK first things:

The voltages are definitely NOT the same everywhere. Whoever told you that is incorrect and you must forget that immediately.

Look at the diagram carefully.

The 15 ohm resistor is shown with a current of 0.24A passing through it.

So the voltage developed across that resistor is given by V = I*R

Plug the values in and what do you get?

What can you then say about the voltage across the 4 ohm resistor?
Reply 4
Avatar for disillusion
disillusion
OP
8
Original post by uberteknik
OK first things:

The voltages are definitely NOT the same everywhere. Whoever told you that is incorrect and you must forget that immediately.

Look at the diagram carefully.

The 15 ohm resistor is shown with a current of 0.24A passing through it.

So the voltage developed across that resistor is given by V = I*R

Plug the values in and what do you get?

What can you then say about the voltage across the 4 ohm resistor?


I got 3.6V ???
Original post by jessicaallenn
I got 3.6V ???

3.6V is correct.

So label the diagram because you can see what's missing more easily.

Do you know Kirchoffs voltage law?

What can you now say about the voltage across the 4 ohms resistor?
Reply 6
Avatar for disillusion
disillusion
OP
8
Original post by uberteknik
3.6V is correct.

So label the diagram because you can see what's missing more easily.

Do you know Kirchoffs voltage law?

What can you now say about the voltage across the 4 ohms resistor?


erm is it that in a closed loop its got to equal 0 or something???
would the voltage 6-3.6=2.4V ?
Original post by jessicaallenn
erm is it that in a closed loop its got to equal 0 or something???
would the voltage 6-3.6=2.4V ?


Yes. 2.4V is correct.

The voltages dropped around the circuit must sum to the supply voltage.

Put that on the diagram also.

Now you can calculate I1 from I1 = V/R and you have part b) answered.

When you have done that, you need Kirchoff's current law to find the current through the unknown resistor.
Reply 8
Avatar for disillusion
disillusion
OP
8
Original post by uberteknik
Yes. 2.4V is correct.

The voltages dropped around the circuit must sum to the supply voltage.

Put that on the diagram also.

Now you can calculate I1 from I1 = V/R and you have part b) answered.

When you have done that, you need Kirchoff's current law to find the current through the unknown resistor.


So it's 0.6A??
Is kirchoff's current law stating that current in=current out so 0.6-0.24=0.36A???
Reply 9
Avatar for disillusion
disillusion
OP
8
Original post by uberteknik
Yes. 2.4V is correct.

The voltages dropped around the circuit must sum to the supply voltage.

Put that on the diagram also.

Now you can calculate I1 from I1 = V/R and you have part b) answered.

When you have done that, you need Kirchoff's current law to find the current through the unknown resistor.


sorry i don't get what value id use when calculating the resistance R for V
Original post by jessicaallenn
So it's 0.6A??
Is kirchoff's current law stating that current in=current out so 0.6-0.24=0.36A???


Yes and yes.

0.36A is correct.

So you now have all the information you need to complete the question and find the missing resistance. What is it?
Reply 11
Avatar for disillusion
disillusion
OP
8
Original post by uberteknik
Yes and yes.

0.36A is correct.

So you now have all the information you need to complete the question and find the missing resistance. What is it?


im not sure would it be 3.6/.36 so 0.1???
Original post by jessicaallenn
im not sure would it be 3.6/.36 so 0.1???


R = V/I

R = 3.6/0.36

is correct.

But the answer is not 0.1 ohms. Have another go using the same numbers.

What do you get?
Reply 13
Avatar for disillusion
disillusion
OP
8
Original post by uberteknik
R = V/I

R = 3.6/0.36

is correct.

But the answer is not 0.1 ohms. Have another go using the same numbers.

What do you get?


oh 10 ohms??
Original post by jessicaallenn
oh 10 ohms??


Yes. 10 ohms is correct. Simple error, lol. But you need to be accurate or you will lose marks.

Q6 is answered in the same way.

Start by looking at where you can add pieces of information from that you are already given.

You do not need anything more than we have aready covered in Q5.

Have a go yourself and post your answers here.:smile:
Reply 15
Avatar for disillusion
disillusion
OP
8
Original post by uberteknik
Yes. 10 ohms is correct. Simple error, lol. But you need to be accurate or you will lose marks.

Q6 is answered in the same way.

Start by looking at where you can add pieces of information from that you are already given.

You do not need anything more than we have aready covered in Q5.

Have a go yourself and post your answers here.:smile:


ok will do tomorrow have to revise for a maths test now!! thanks for your help!

Quick Reply

Related discussions

Latest

Trending

Trending

Articles for you