# Homework Help: Add or Subtract?

1. Jan 16, 2008

### atwong713

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2. Jan 16, 2008

### malawi_glenn

Do you know any physical laws?

(you must give attempt to solution and some relevant equations/laws before you can get help, it is a part of the forum rules)

3. Jan 16, 2008

### atwong713

i know if its (-+)(-+) in series you add em n if its (-+)(+-) you subtract it right? but what if its like that? i'm guessing if they are 1.5V each then its 1.5v through the light bulb or since dey are in series den i add em so its 3V

Last edited: Jan 16, 2008
4. Jan 16, 2008

### malawi_glenn

Are you familiar with Kirchoffs loop law?

5. Jan 16, 2008

### atwong713

i'm not sure is it like if its a closed loop the net voltage has to equal zero or something? does that mean the answer is zero?

i dont really understand kirchoffs laws my professor didnt really explain it well i did some searches on this forum and it cleared up somethings but i'm still kinda confused about the signs of what the currents is suposed to be

Last edited: Jan 16, 2008
6. Jan 16, 2008

### Integral

Staff Emeritus

You are correct. Parallel sources do not add. There is 1.5V across the bulb.

7. Jan 16, 2008

### malawi_glenn

ok.

But do you know how to apply a voltage meter?

8. Jan 16, 2008

### atwong713

no whats that?

thanks
but could you explain it a little and clear it up for me what would happen if they weren't the same voltage

Last edited: Jan 16, 2008
9. Jan 16, 2008

### malawi_glenn

ok, then never mind;)

Your guesses was right as Integral stated.

A voltage meter is applied paralell (over) the circut, so if you apply it over one emf you get 1.5V and that means that you must have 1.5V over the battery (I could have done a picture for you)

Using Kirchofs laws, you would find that there are no current in the upper loop, but that there are current in the lower, so "only one" battery will contribute.

10. Jan 16, 2008

### atwong713

could you also help me on another problem and tell me if i got my facts straight

i say that it is moving away because if i say its positive going into element1 then since its negative its going out of element1 so its moving away from A

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11. Jan 16, 2008

### malawi_glenn

One thread for each problem, and use the template.

12. Jan 16, 2008