# Homework Help: The reading of the ammeter and the reading of the voltmeter.

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1. Jun 8, 2017

1. The problem statement, all variables and given/known data
In this circuit

What will be the reading of the ammeter and that of the voltmeter when:
1. Key S1 and key S2 are open.
2. Key S1 and key S2 are closed.
3. Key S1 is closed and key S2 is open.
(Neglect the internal resistance of the source)
2. Relevant equations

3. The attempt at a solution
That might be an easy question, but my book doesn't give the solution, and I want to amke sure of my work.

Case 1.
i = 0
V = 2V

Case 2.
i = V/R =2/3 A
V = Vb - iR = 2 - ((2/3) * 3 = 0

Case 3.
Rt = 5+3 = 8 ohm
i = V/Rt = 2/8 = 0.25 A

V = iR = 0.25 * 5 = 1.25 V

2. Jun 8, 2017

### andrevdh

Yes, I think you have got all 3 right.

3. Jun 8, 2017

Ok, but I can't understand why in case two the voltage will be zero? I understand the maths but I don't understand the physics explanation.

4. Jun 8, 2017

### cnh1995

What does S2 do to the 5 ohm resistor?

5. Jun 8, 2017

S2 cancels the resistance 5 ohm.
But we do still have the resistance 3 ohm.

6. Jun 8, 2017

### cnh1995

In technical terms, S2 short-circuits the 5 ohm resistor. So the 5 ohm resistor is ineffective.

7. Jun 8, 2017

OK, and then what? more explanation please!

8. Jun 8, 2017

### cnh1995

What is the resistance of S2 and the wire that shorts the 5 ohm resistance?

9. Jun 8, 2017

It is zero so the voltage is zero.
Ok, I got it. But what's your opinion about the whole solution?

10. Jun 8, 2017

### cnh1995

11. Jun 9, 2017

### andrevdh

the current calculation (correctly) assumes the resistance of the parallel combination is zero and for the voltage it subtracts the voltage drop over the 3 ohm resistor from the power supply's voltage to obtain the voltage over the combination (the remaining part of the circuit) - a bit of a merry go round way of doing the calculation if you ask me. Words would have done better to provide the answer.