power loss question

T

teeg123

Member
Location
Maine
Occupation
Journeyman Electrician
I have been trying to figure this out and have yet to get it.
what is power lost in R2

- R1 ----- R2-------R3
20ohms 10ohms 20ohms
voltage supplied 60 volts DC

the answer is 12 watts but I'll be darned if I can get that answer.......
power to R1 = v2/R =60*60/20= 3600/20=180
power to R2 360
power to R3 180
and this is where I get stuck, I'm sure i missed something and feel silly asking this question.
to be clear I rarely have to do any of this kind of calculations at my job. any help or guidance is appreciated.
 
Welcome to the forum.

The resistors are shown to be in series, so first calculate the circuit's current using 60 volts and 50 ohms.

Then calculate each resistors power loss/use using the above-calculated current and each resistor's value.
 
great
so, voltage/resistance 60/50=1.2

so would I use the equation of R*I squared
20*1.44=28.8
10*1.44=14.4
20*1.44=28.8
unfortunately I still don't see where the 12 watts comes from???
I really do appreciate your help!
 
What exactly is meant by “power lost?”
The power dissipated by R2 is 14.4W, but is it “lost?”

Poorly constructed question.
 
yes, this is a question from prov the company that provides the test. I'm sitting for my master's soon so I bought the practice exam. they have 12 watts but like i said I just can't get it.
 
The circuit current is 1.2 amps and since the resistors are in series that same current flows through each resistor. The voltage drop across each resistor is proportional to the resistance. Given the resistances, 20% of the voltage is dropped across the 10 ohm resistor, and 40% across each of the 20 ohm resistors. That gives a voltage drop 12 volts across the 10 ohm resistor, and 1.2 amps times 12 volts = 14.4 watts.
 
fantastic that's kind of what I thought but wasn't sure, so I thank you all for the assistance.
 
I also worked it backwards and if the R1 + R3 = 45 ohms then you would get 12.1W. Reasonably rounded down to 12W. But that would mean there is a typo in the Resister values.

60 V / 55 ohms = 1.1A;

1.1^2 * 10 = 12.1W

Either way, Prov is the worst.
 
retirede said:
What exactly is meant by “power lost?”
The power dissipated by R2 is 14.4W, but is it “lost?”

Poorly constructed question.
Click to expand...
It means "leaves the electrical domain". The energy still exists, it is just in another form, in this case heat. They just call it "power lost" to keep the wording simple.
 
Carultch said:
It means "leaves the electrical domain". The energy still exists, it is just in another form, in this case heat. They just call it "power lost" to keep the wording simple.
Click to expand...
To me, it means power that leaves the electrical domain other than that which is consumed for the intended use. Generally, “lost power” is undesirable.
Which is why it doesn’t make sense to me for this example.
 
The way i do it is like this
60 volts/50 ohms=1.2 amps that current is the same through all resistors

so 1.2 amps x 20 ohms=24 volts across R1
1.2 amps x 10 ohms=14.4 volts across R2
R3 same as R1

To find watts is amps x volts so R1= 1.2 amps x 24 volts=28.8 watts
R2=1.2 amps x 1.2 x 14.4 volts =17.28 watts
R3 same as R1 28.8 watts

To prove the answer take total watts 28.8 +17.28+28.8=74.88 watts/60 volts=1.2 amps
 
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