# Homework Help: Power dissipated

1. May 22, 2015

### estike

1. The problem statement, all variables and given/known data

What is the power dissipated by a 60 Watt (rated at 120 V) light bulb if it is connected to a 20 V power source (assume the resistance of the bulb is constant)?

2. Relevant equations

3. The attempt at a solution

If it

2. May 22, 2015

### BvU

Hello E, welcome to PF !

You were saying ?

3. May 23, 2015

### estike

I dont know where to start i know that the power dissipated is in Joules but i dont know what equation to use

4. May 23, 2015

### CWatters

Power is actually measured in Watts.

You initially have a 60W bulb running on 120V. What equation relates Power, Voltage and Current?

5. May 23, 2015

### estike

Ohm´s law V=RI

6. May 23, 2015

### estike

W = VC right ?

7. May 23, 2015

### CWatters

Yes Ok but better to write it as P = I * V.

Now using that and Ohms law write a new equation relating Power, Voltage and Resistance.

8. May 23, 2015

### estike

if 60 = 120 * I = 0.5 A

I use this current and know that is 120V*0.5A = W right ? and W is 60 right

9. May 23, 2015

### CWatters

Yes Ok the current is 0.5A.

You actually need to calculate the resistance as well.

10. May 23, 2015

### estike

hum ok i understand know so if V= RI since 220 = R *0,5A = so the resistence is 110 ohms and since is constant

20 = 110*i = so I =0.182 A

So P = I*V = 0.182 A*20 = 3.6 Watts ? Am i right ?

11. May 23, 2015

### CWatters

should be 120V.

12. May 23, 2015

### estike

so is 120 = R * 0.5A = 60 ohms

And then 20 = 60*I = 1/3 A

so W = 1/3A*20 V = 6,66 watts right

13. May 23, 2015

### CWatters

Back later.

14. May 23, 2015

### estike

but the answers are 60W , 1.67 W , 20W or 120W i really dont understand then :x

15. May 23, 2015

### estike

i know what i did wrong it was 240 ohms i made a stupid error

W = 1/12A*20V and then it is 1,67 W

16. May 23, 2015

### CWatters

Sorry I got called away..

Ok you got to the right answer but there is an easier route that I was trying to lead you down in post #7. I think you're substituting values too soon. There is no need to work out the current in either circuit. Here is how I would do it...

P = I * V
V = I * R

Substitute to eliminate I..

P = V2/R (Aside: I memorised that one and also P = I2 * R)

Rearrange to give R...

R = V2/P

Substitute values..

= 14400/60
= 240 Ohms

For second circuit R is the same. V changes to 20V....

P = V2/R
= 202/240
= 1.7W

17. May 23, 2015

### CWatters

You can also do it without working out R....

P1 = V12/R
P2 = V22/R

P2 / P1 = V22 / V12

P2 = P1 * V22 / V12

= 60 * 202/1202
= 1.7W

18. May 24, 2015

### theodoros.mihos

@estike you are right, the value is 5/3 = 1.666...