# Power Dissipated by 60W Bulb in 20V Source[/B]

• estike
In summary, the power dissipated by a 60 Watt light bulb connected to a 20 V power source is 1.7 Watts.
estike

## Homework Statement

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)?

## The Attempt at a Solution

If it [/B]

Hello E, welcome to PF !

You were saying ?

I don't know where to start i know that the power dissipated is in Joules but i don't know what equation to use

Power is actually measured in Watts.

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

Ohm´s law V=RI

W = VC right ?

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.

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

Yes Ok the current is 0.5A.

You actually need to calculate the resistance as well.

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 ?

hum ok i understand know so if V= RI since 220= R *0,5A

should be 120V.

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

Back later.

but the answers are 60W , 1.67 W , 20W or 120W i really don't understand then :x

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

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

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

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

## What is the power dissipated by a 60W bulb in a 20V source?

The power dissipated by a 60W bulb in a 20V source can be calculated using the formula P = V^2/R, where P is power in watts, V is voltage in volts, and R is resistance in ohms. In this case, the resistance of the bulb is 20V/60W = 0.33 ohms. Plugging this value into the formula, we get P = (20V)^2/0.33 ohms = 120W. Therefore, the power dissipated by the bulb is 120 watts.

## How is the power dissipated by a 60W bulb affected by the voltage of the source?

The power dissipated by a 60W bulb is directly proportional to the voltage of the source. This means that as the voltage increases, the power dissipated by the bulb also increases. Conversely, if the voltage decreases, the power dissipated by the bulb will also decrease.

## What is the resistance of a 60W bulb in a 20V source?

The resistance of a 60W bulb in a 20V source can be calculated using the formula R = V^2/P, where R is resistance in ohms, V is voltage in volts, and P is power in watts. In this case, the power of the bulb is 60 watts and the voltage of the source is 20V. Plugging these values into the formula, we get R = (20V)^2/60 watts = 0.33 ohms. Therefore, the resistance of the bulb is 0.33 ohms.

## How does the power dissipated by a 60W bulb change over time?

The power dissipated by a 60W bulb remains constant over time as long as the voltage and resistance of the bulb remain constant. However, over time, the resistance of the bulb may change due to factors such as temperature or wear and tear, which can affect the power dissipated by the bulb.

## What factors can affect the power dissipated by a 60W bulb in a 20V source?

The power dissipated by a 60W bulb can be affected by factors such as the voltage and resistance of the bulb, as well as external factors such as temperature and wear and tear. Changes in these factors can result in a change in the power dissipated by the bulb. Additionally, using a different type of bulb with a different wattage or a different voltage source can also affect the power dissipated by the bulb.

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