# Circuits Question

Hey guys, need help with the following circuits question:

1) In these circuits the cells are identical and their internal resistances can be ignored (diagram of both circuits attached).

Indicate whether each of the statements below is true of false for the circuits shown.

i) When switch S1 is closed, the reading on voltmeter V1 will increase.
ii) When switch S1 is closed, the reading on ammeter A1 will increase.
iii) When switch S2 is closed, the reading on ammeter A2 will increase.
iv) When switch S2 is closed, the reading on ammeter A3 will decrease.

- Is it true, true, false, false?

Thanks
Original post by Nator
Hey guys, need help with the following circuits question:

1) In these circuits the cells are identical and their internal resistances can be ignored (diagram of both circuits attached).

Indicate whether each of the statements below is true of false for the circuits shown.

i) When switch S1 is closed, the reading on voltmeter V1 will increase.
ii) When switch S1 is closed, the reading on ammeter A1 will increase.
iii) When switch S2 is closed, the reading on ammeter A2 will increase.
iv) When switch S2 is closed, the reading on ammeter A3 will decrease.

- Is it true, true, false, false?

Thanks

There are 3 false and 1 true.
Original post by Stonebridge
There are 3 false and 1 true.

False, False, True, False?
Original post by Nator
False, False, True, False?

It's important you aren't randomly guessing here.
It will become like a game of mastermind.

It would help me to know why you think they are true or false.
Only then can I see where you are going wrong.

More volts = more current.
Original post by Stonebridge
It's important you aren't randomly guessing here.
It will become like a game of mastermind.

It would help me to know why you think they are true or false.
Only then can I see where you are going wrong.

More volts = more current.

Haha right

And ok, so the first will be less because there will be a current through the bulb, second will be false because some current flows through the voltmeter, the third will be true because more current flowing through A2, and last is false because more current will flow through. Now how far am I off?
Original post by Nator
Haha right

And ok, so the first will be less because there will be a current through the bulb, second will be false because some current flows through the voltmeter, the third will be true because more current flowing through A2, and last is false because more current will flow through. Now how far am I off?

Let's have a closer look...
The cells all have the same voltage/emf and no internal resistance.

Circuit 1
S1 open
pd across the voltmeter is V
pd across bulb is V
all in parallel
current in bulb and ammeter (shown) I=V/R
Close S1 and you have the same voltage V still across the bulb and voltmeter because they are all in parallel.
So what happens to I=V/R (The resistance of the bulb doesn't change)
Can you do a) and b) now with reasoning?

Circuit 2
S2 S3 open
only the current shown flows in the circuit.
Close S2 and you short circuit the terminals of the cell.
The result is a large current flows through the top part of the circuit.
What happens to the current that was originally in that middle part?
(edited 12 years ago)
Original post by Stonebridge
Let's have a closer look...
The calls all have the same voltage/emf and no internal resistance.

Circuit 1
S1 open
pd across the voltmeter is V
pd across bulb is V
all in parallel
current in bulb and ammeter (shown) I=V/R
Close S1 and you have the same voltage V still across the bulb and voltmeter because they are all in parallel.
So what happens to I=V/R (The resistance of the bulb doesn't change)
Can you do a) and b) now with reasoning?

Circuit 2
S2 S3 open
only the current shown flows in the circuit.
Close S2 and you short circuit the terminals of the cell.
The result is a large current flows through the top part of the circuit.
What happens to the current that was originally in that middle part?

So for the first part there is no change, as they are parallel so have equal voltages, hence there is no change in either of the 2 after closing the circuit? Also sorry just a bit confused here, but originally I thought that as the switch is open, the voltmeter would have no reading as no current could pass through to it as the switch is open, so how does it get a reading without having any current flowing through it?

And the middle part would have less current, therefore the first part is third statement is false and the last one is true.
1st circuit
The voltmeter gets its reading V from the lower cell, initially. It's across the terminals of that cell.
When you close switch S1 you just connect in parallel another cell of the same voltage. So the meter still reads V.
Same voltage across the bulb (V) means the same current as before.
So you are correct that there is no change and the 1st two are false.
By the way, it is important that the internal resistance of the cells is negligible (ideally zero) for this to be true.

Circuit 2.
Correct.
The readings on both ammeters will decrease.
Original post by Stonebridge
1st circuit
The voltmeter gets its reading V from the lower cell, initially. It's across the terminals of that cell.
When you close switch S1 you just connect in parallel another cell of the same voltage. So the meter still reads V.
Same voltage across the bulb (V) means the same current as before.
So you are correct that there is no change and the 1st two are false.
By the way, it is important that the internal resistance of the cells is negligible (ideally zero) for this to be true.

Circuit 2.
Correct.
The readings on both ammeters will decrease.

Haha silly me, of course Good stuff, thanks a lot for explaining!
Original post by Stonebridge
1st circuit
The voltmeter gets its reading V from the lower cell, initially. It's across the terminals of that cell.
When you close switch S1 you just connect in parallel another cell of the same voltage. So the meter still reads V.
Same voltage across the bulb (V) means the same current as before.
So you are correct that there is no change and the 1st two are false.
By the way, it is important that the internal resistance of the cells is negligible (ideally zero) for this to be true.

Circuit 2.
Correct.
The readings on both ammeters will decrease.

Hello why will A 3 decrease?