Question from a New Member

[Majid786]

Hi,

Greeting to all at electricain forum .

I'm currently taking the City & Guilds 2330 level 2 based in Lancashire, and I will be taking the 202 exam on Tuesday .

I would like to ask anyone could have a go at this question which is a similar one we will get in the exam it is from the book I brought on electrical calculations. I have also asked Imran and we seem to be a bit stumped with this one.

Internal Resistance :

Arrangement as follows : To resistors in series one value is unknown and the other is 20 ohms . Followed by one resistor in series with value of 5 ohms .

Supply : 30V

Current going to Parallel resistors 0.5 A

Find the value of the unknown resistor.

Let me know how you work this one out as the answer given by the book are as follows:

2.86 ohms

Thanks

 

[Doc Hudson]

Hello Majid786, welcome to the forum. I am not sure you have written your question down clear enough? It only appears to mention resistors in series, 'Unknown', '20ohm' then '5ohm'. But then refers to current through the parallel resistance? Do you have a drawing with this question? Or how many resistors are there in total and which ones are the parallel ones?

Doc H.

 

[Majid786]

Hi Doc,

Apologies for that I shall make it clearer:

The arrangement is as follows:

There are two resistors placed in parallel one value is not known and the other resistor has a value of 20 ohms.

There current that flow to the parallel resistors is 0.5A

Then there is also a single resistor connected in series which has a value of ohms .

The voltage supplied to the circuit is 30v .

The question is to find the value of the unknown resistor which is connected in parallel with the 20Ohm resistor.

The answer given: 2.86 ohms

Ref: book Electrical Installations Calculations: Basic by A.J Watkins , C Kitcher

I hope this is a bit clearer.

Many thanks

 

[Majid786]

N.B the resistor is series has a value of 5 ohms

Thanks

 

[NozSpark]

I think one of your numbers is still wrong....

I can't work it out.

If you work it backwards with the figures they give..

Parallel resistance = (20*2.86)/(20+2.86) = 2.5 ohms

+ the series resistor (5ohms) =7.5ohms

Now if you take the voltage and divide by resistance you get

30/7.5 = 4A

which is not what the question gives

Now if the series resistor was 50ohms then that would be alot closer!!

 

[Majid786]

Thanks Noz,

I appreciate the help I'm still working on this togther with the rest of the revision for the 202 ..

Thanks again

 

[androiduk2002]

Hey Majid, Im on 202 as well but im evening classes so we wont have our exam till june or july, but was told most of the failure from the full timers have been down to S.I. units and parallel and serious resistor calculation errors.

 

[SPECIAL LOCATION]

To resistors in series one value is unknown and the other is 20 ohms . Followed by one resistor in series with value of 5 ohms .

Supply : 30V

Current going to Parallel resistors 0.5 A

I THINK THE ABOVE LINE SHOULD READ THROUGH THE 20OHM RESISTOR!

Find the value of the unknown resistor.

Let me know how you work this one out as the answer given by the book are as follows: 2.86 ohms

There are two resistors placed in parallel one value is not known and the other resistor has a value of 20 ohms.

There current that flow to the parallel resistors is 0.5A

Then there is also a single resistor connected in series which has a value of ohms .

The voltage supplied to the circuit is 30v .

The question is to find the value of the unknown resistor which is connected in parallel with the 20Ohm resistor.

The answer given: 2.86 ohms

Right Majid....

I think you have written your question down slightly wrong...

I reckon the line about the 0.5amps should be the current through the parallel 20ohm resistor.

as the current through the two parallel resistors will be different if resistances are different.

Remember

Resistors in series: Voltage divides, Current stays constant.

Resistors in parallel: Voltage stays constant, Current divides.

So:-

If 0.5a through the 20ohm resistor IxR=V 0.5A x 20ohm = 10V

If 10 of our volts are across the parallel resistors then the remaining 20v

(30v -10v), must be across the 5ohm resistor.

20V / 5ohm = 4A.

So 4amps total drawn by circuit,

0.5A is going down the 20ohm parallel path

so (4A - 0.5A) 3.5A are going through the unknown resistor.

And

the unknown resistor has 10V across it.

R = V/I 10v / 3.5A = 2.857ohms.

Therefore...

Unknown resistor equals 2.86 ohms!



coffeeput the kettle on