Why is the current reading the same in each lamp in a parallel circuit?

[phillyj]

Hi, I wasn't sure I should post this here.
We did a lab on series and parallel circuits and it went great until we got to the parallel circuit. The meters were connected in the correct way but for some reason, the current showed up on the meter as 12.8 (amps?) in each of the lamps.
In series, there was 1.7 amps in each of the lamps. There's no way that's 12 amps. Is it 1.28 amps? My TA doesn't know so any ideas?


http://img26.imageshack.us/img26/3258/92456738.jpg

That was my setup where X is the light bulb.

 

[berkeman]

Thread moved to Homework Help (homework/coursework/labwork belongs here)

The result seems a bit off, but comes close to being correct. What is the expected power supply current difference for the series and parallel cases (what is relative total impedance of 3 series lamps versus 3 parallel lamps)?

And what other correction factors should you consider? What is the impedance of the meter compared to the lamps? Were the lamps a lot brighter in the parallel configuration? Was the lamp where the meter was any dimmer than the other two lamps in the parallel configuration? What is the output impedance of your power supply?

Given that you measured 1.7A for the series case, what would you guess should be the current in the parallel case if the lamps were the same brightness as the series case, and the output impedance of the power supply were negligible? Would the parallel current be higher or lower for each of the correction factors I've mentioned?

 

[phillyj]

Sorry I didn't mention more.
IIRC, The power supply is rated at 5 V and 1.5 A. Yes, I'm sort of wondering why it said 1.7 A for series when the rating was 1.5A; maybe I changed the settings?

In series, the lights grew in brightness i.e Lamp1<Lamp2<Lamp3.
Parallel, they were the same.
This tells me I connected everything correctly. I'm not sure If my meter was set to the wrong setting. 12 amps makes no sense.

 

[berkeman]

Sorry I didn't mention more.
IIRC, The power supply is rated at 5 V and 1.5 A. Yes, I'm sort of wondering why it said 1.7 A for series when the rating was 1.5A; maybe I changed the settings?

In series, the lights grew in brightness i.e Lamp1<Lamp2<Lamp3.
Parallel, they were the same.
This tells me I connected everything correctly. I'm not sure If my meter was set to the wrong setting. 12 amps makes no sense.

Yeah, probably the wrong setting. And the lamps should not vary in brightness in either case, unless they are different types of lamps. Maybe in the series case they were barely turning on, so variations in manufacturing tolerances were more apparent than when they were all on full in the parallel case.