Help...Calculate the power dissipated by resistor 2 in the circuit here

[L Plate]

Can anyone pls help how u gonna calculate this , I can't see the answer in the choices..

Calculate the power dissipated by resistor 2 in the circuit here

R1 = 5 ohms

R2 = 10 ohms

and 20v in series connection

the choices answer are.

A) 19.9 watts

B)10 watts

C)17.8 watts

D) 14.4 watts

cheers and have a nice evening

 

[NozSpark]

Lplate,

You have the voltage and the resistance,, now you need to work out the current and power in the circuit

Then as you have a 10ohms and 5ohms resistances in series they will draw power in a ratio.. I.e. The 5ohms resistor will draw twice as much power as the 10ohms one

HTH

---------- Post Auto-Merged at 23:35 ---------- Previous post was made at 23:34 ----------

Oh,,, and the answer is there!

 

[Samage89]

I'm getting 26.6w for some reason...

Resistors in series you add up, making 15ohms, then just do V(2) / R

 

[NozSpark]

Nearly there!

 

[L Plate]

thanks guys

I did tried the power law which is P= IXV , since the question is , Calculate the power dissipated by resistor 2 in the circuit

so if I tried 10x20= 200V

I've tried the 3,4,5 triangle so ( 5,10,15 ) that means , 15ohms/20V =0.75

Ive tried the Rt .. ( 1/5+1/10 ) = 0.3 so, 0.3ohms/20v = 0.015 >>> oh mannnnn confused.com LOL , I hope i am using the right formula..

BTW here is the original question >> LEVEL 3 - UNIT 1 - MOCK EXAMINATION (60 minutes 40 questions) Application of health and safety and electrical principles

 

[NozSpark]

Lplate,,

I see your problem...

You are trying to work out the power dissipated by R2 straight away,,, you have to work out the power dissipated by the whole circuit and then split it down to individual components

The long way...

Work out the total current draw of the circuit

Calculate the total power dissipation of the circuit

Calculate the power dissipated by individual components... This bit is the inverse ratio of the resistor sizes (smaller resistances dissipate more power!)

 

[L Plate]

ok thanks Noz i will work it out and post my calculation tom ... in the mean time I will skip to other part of the exams and go back to that question after... THANKS A LOT NOZ...

 

[ADS]

L Plate,

The formulas you want are:

R1 + R2 = ...........this gives you total resistance.

then,

Voltage / Total Resistance V/Rt = .........this gives you the current in the circuit

then,

You can either use P = I x I x R2 .........this will give you the answer,

or,

calculate the voltage across R2 .......V(R2) = I x R2 then,

get your answer from P = V(R2) x I .........which is essentially the same thing

 

[L Plate]

thank you ADZ. I will surely try to find out the answer and thanks for the info .

 

[NozSpark]

I'm getting 26.6w for some reason...Resistors in series you add up, making 15ohms, then just do V(2) / R

Yes,,, but that's the total power dissipation of the circuit!

 

[chrishaworth]

ADS is bang on. I=V/R so current through the whole circuit is 20/(10 + 5) which equals 1.33333. Power dissipated by a resistor (R2, 10 ohms) is I squared R, so 1.33333 * 1.33333 * 10 = 17.77777 or 17.8 as in the answer.

Cheers, Chris

 

[L Plate]

Hi chris,

at first I was using the ohms law, yea you are right but after that >> Power dissipated by a resistor (R2, 10 ) is I squared R, so 1.33333 * 1.33333 * 10 = 17.77777 or 17.8 as in the answer. >> pls can you tell me which formula is that , I might missed that one during the lecture ...

cheerssss

 

[NozSpark]

Seeing as the cats been let out the bag as it were...

The formula is P=I(squared)R...... Cant do the squared symbol!

you can apply this to the individual resistors, But your I is determined by both resistors

 

[L Plate]

thanks Noz..

this is really funny because we have this last Monday and even our instructor can't figure it out that is why I brought this in the forum .. we know that the answer in the question was 17.8ohms but non of us have worked it out ... tsk tsk tsk .... anyway .. thanks guys for your help ..

have a nice evening ....

Lplate

 

[ADS]

L Plate,

There are three formulas that you can use for power in resistive circuits (depending what information you have available)

If you know the voltage and the current, use P = V x I

If you know the current and the resistance, use P = I

 

[L Plate]

thanks a lot ADZ