# Double-grounded circuit, find I across one of two resistors

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1. Apr 3, 2016

### kamhogo

1. The problem statement, all variables and given/known data
What is the current in the 2 ohms resistor?

2. Relevant equations
Kirchhoff's loop law

3. The attempt at a solution
Grounding does not affect how the circuit behaves, so I ignored the ground in and applied the loop law (clockwise from the negative terminal of the 9V battery ):

ΔV(loop) = 9V -(2 ohms)*I - 3V - (4 ohms)*I =0
6V = (6 ohms)*I
I = 1 A

Correct?

2. Apr 3, 2016

### drvrm

well i do not work with circuits much- but if you have grounded both terminals of a load -can you expect a current to flow in the lower 4 ohms resistance.
grounding a circuit defines the potential to be zero (definitely)
further i failed to understand -what you wish to convey through this expt.?

3. Apr 3, 2016

### Orodruin

Staff Emeritus
Incorrect. The system is grounded at two different points, indicating that those points must have the same potential.

4. Apr 3, 2016

### epenguin

^^...So you have the same simplification as in the last problem. After which...?

5. Apr 3, 2016

### kamhogo

So I can ignore the 4 ohms resistor in my calculations? Then the current through the 2 ohms resistor would be given by:

9V - (2 ohms)*I - 3V =0
6V / 2 ohms = I = 3A

6. Apr 3, 2016

### kamhogo

From what I understand now, the 4 ohms resistor might as well not be there....it's between 2 points of zero potential and so no current flows through it. Right?

7. Apr 3, 2016

### Orodruin

Staff Emeritus
Right. You need a potential difference between its ends to provide a current.

In this respect, grounding the two points is like short-circuiting them.

8. Apr 3, 2016

### drvrm

When connecting in Parallel you are doubling the capacity (amp hours) of the battery while maintaining the voltage of one of the individual batteries. Use a jumper wire between the positives of both batteries and another jumper wire between the negatives of both batteries. Connect your positive and negative wires to the same battery to run to your application.

9. Apr 3, 2016

### Orodruin

Staff Emeritus
This does not add any insight to the current problem at hand. Please stay on topic.

10. Apr 3, 2016

### phyzguy

This is correct.

11. Apr 3, 2016

### kamhogo

This is confusing.....what's a jumper wire? Why connect the terminals?

12. Apr 3, 2016

### SammyS

Staff Emeritus
Ignore the reply of @drvrm !

13. Apr 3, 2016

### CWatters

This is not relevant to this thread.

In addition it is wrong and potentially dangerous. In this thread the batteries are different voltages and batteries with different voltages should never be connected in parallel.

14. Apr 4, 2016

### kamhogo

I'm getting better at this circuit analysis thing!

15. Apr 4, 2016

### SammyS

Staff Emeritus

etc.

That seems correct.

The image is hard to read. I had to zoom way in.

In the future it's best to start a new tread for a new problem.

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16. Apr 4, 2016

### epenguin

And to use an app like DocScanHD to make the images more visible and less unpleasant to look at.

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