lightbulb experiment

[copper123]

I have a 120 volt, 100 watt lightbulb. When 120 volts A.C. of potential is between it, I get 100 watts of power. Also, when the bulb is working, I am pulling .83 amps of current. Also, this should have a resistance of 144 ohms.

Now if I hook my 100 watt bulb up to a 12 volt D.C. car battery, it should pull 8.3 amps of current and have a resistance of 1.44 ohms.
I did this little experment and I can not get the lightbulb to glow even a little bit. Even though its D.C. its still just a potential difference and its electrons. What part of this elementary experiment am I missing? Thanks in advance!

 

[jtester]

Re: lightbulb experiment

Your 100 watt bulb consumes that much power at 120 volts. The thing that is constant is the resistance 144 ohms, not the wattage.
When you connect it across a 12 volt battery you get less than .1 amps. That is why there is no light.
Jim T

 

[rattus]

Re: lightbulb experiment

I would point out that the filament resistance is strongly dependent on its temperature. The cold resistance is much less than the hot resistance, but you still need 120V to get 0.83A.

 

[physis]

Re: lightbulb experiment

And if you connect it to a 1.2 volt battery it should draw 83 amps?

You're not applying Ohm's law correctly.

Current is voltage divided by resistance.

V / R = I

You can rearrange this equation to say:

V / I = R and
I x R = V

Power (the 100 watts) comes from current times voltage.

I x V = P

You can rearrange this equation to say:

P / V = I and
P / I = V


You know the voltage is 120 and the power is 100.
So to find the current:

P / V = I or
100 / 120 = .83
I = .83 or the current is .83 amps.

Now you can find the resistance by:

V / I = R or
120 / .83 = 144
R = 144 or the resistance is 144 ohms.

Your mistake is in using the same wattage for 12 volts as you did with 120 volts. You have to use the voltage and the resistance to get the current and the power.

V / R = I
12 / 144 = .083
Current is .083 amps

I x V = P
.083 x 12 = .996 (it's actually 1. Lack of decimal places)
Power is 1 watt.

But this wont actually work right with a light bulb because the filiment temperature changes the resistance drasticly.