Answer: Diagrams of Ammeters: Finding Correct Current Measurement

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In summary: To find the total resistance, you need to add the reciprocals, not the fractions themselves.In summary, the question discusses the correct placement of an ammeter in a circuit and the correct answer depends on whether an ideal or non-ideal voltmeter is used. The conversation also delves into finding the order of brightness for lightbulbs in a circuit, using the rules for combining resistors in parallel and series to determine the total resistance.
  • #1
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Homework Statement



In which diagram(s) (labeled A - D) does the ammeter correctly measure the current through the battery?

187108B.jpg


Homework Equations



I don't believe there are any here, just to know that the ammeter is in series with the current that is passing through it and is measuring that current.


The Attempt at a Solution



The thing is I'm not sure where the current is coming out of, would I be correct in saying the emf is the source of the current throughout the whole of the battery? If that was the case I was thinking the only current that the ammeter is in series with with battery C but that answer was incorrect (masteringphysics), or would it be that the ammeter is in series with the potential difference measured by the voltmeters?

any help is greatly appreciated.
 
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  • #2
There is a correct answer here, and it's obvious. THink of the current as a continuously flowing loop of water through closed pipes, and the battery is the pump that moves the water. Remember that no current flows through the voltmeter.

Note: only two of these four circuits demonstrate correct placement of both meters. In one of them, one meter is misplaced; in another, both are misplaced.
 
  • #3
Remember that the voltmeters that they give are probably "ideal" meaning they have infinite resistance. From this, there are two that you can throw away, I believe.
 
  • #4
The question is, for which diagram is the ammeter correct?

If we assume an ideal (infinite resistance) voltmeter, there are two correct answers. This is probably not what the problem author had in mind, so I conclude that the voltmeter is non-ideal, and can draw some current.

If the voltmeter can draw current, there is just one correct answer for a correct ammeter reading.
 
  • #5
Redbelly98 said:
The question is, for which diagram is the ammeter correct?

If we assume an ideal (infinite resistance) voltmeter, there are two correct answers. This is probably not what the problem author had in mind, so I conclude that the voltmeter is non-ideal, and can draw some current.

If the voltmeter can draw current, there is just one correct answer for a correct ammeter reading.

no it is an ideal voltmeter so the answer is ammeters C & D, thanks for the help on this one guys, but unfortunately I do have another one that's a little harder:

15767_a.jpg


I am trying to find the order of brightness for the lightbulbs; I know that it will depend on the fraction of voltage divided among the lightbulbs and I know the voltage is divided across the each bulb depending on the resistance encountered in each pathway (with D and E in series with each other), but from there I'm not sure how to set up a fraction for the division of voltage, any help is greatly appreciated.
 
  • #6
clope023 said:
no it is an ideal voltmeter so the answer is ammeters C & D

I reread the problem statement, and see now that they are allowing for more than one answer. Thanks for clarifying.

New problem:
I am trying to find the order of brightness for the lightbulbs; I know that it will depend on the fraction of voltage divided among the lightbulbs and I know the voltage is divided across the each bulb depending on the resistance encountered in each pathway (with D and E in series with each other), but from there I'm not sure how to set up a fraction for the division of voltage, any help is greatly appreciated.

Let each lightbulb have a resistance of R. How is the voltage divided between the upper pair vs. the bottom three bulbs?
 
  • #7
Redbelly98 said:
I reread the problem statement, and see now that they are allowing for more than one answer. Thanks for clarifying.

New problem:


Let each lightbulb have a resistance of R. How is the voltage divided between the upper pair vs. the bottom three bulbs?

just looking at the diagram I would say 2/3 but I'm not sure that's correct, considering D and E are in series with each other.
 
  • #8
No. Use the rules for combining resistors in parallel & series.

What is the net resistance of the upper pair?
What is the net resistance of the lower 3-bulb combination?
 
  • #9
Redbelly98 said:
No. Use the rules for combining resistors in parallel & series.

What is the net resistance of the upper pair?
What is the net resistance of the lower 3-bulb combination?

well for D and E Rtot1 = Rd+Re

for the others that are in parallel, 1/Rtot = 1/Ra+1/Rb or I/Vab = 1/Ra+1/Rb

and C is in parallel with Rtot1 so 1/Rtot2 = 1/Rtot1+1/Rc or I/Vtot2 = 1/Rtot1+1/Rc

so Rtot for a and b is Ra+Rb/RaRb

and Rtot2 = Rtot1Rc/Rtot1Rc

so these would add up and give me the total resistance on each bulb?
 
  • #10
okay, I added up all the resistances and got this:
3/2R (the resistance of the 3 bulb combo) and 2/R (the upper pair) = 7/(2R) as the total resistance is this correct?
also, since I need to find the current through bulb c, so I'm thinking I just need the resistance of the 3bulb combo and they're potential difference between the bulb c and the bulbs d and e.
 
Last edited:
  • #11
clope023 said:
okay, I added up all the resistances and got this:
3/2R (the resistance of the 3 bulb combo) and 2/R (the upper pair) = 7/(2R) as the total resistance is this correct?

No.
3/(2R) is the reciprocal of the resistance of the 3-bulb combo.
2/R is the reciprocal of the resistance of the upper pair.
 

1. What is an ammeter and how does it work?

An ammeter is a device used to measure the amount of electric current flowing through a circuit. It works by connecting it in series with the circuit, which allows the current to flow through the ammeter and measure its value.

2. How do I know which ammeter to use for a specific circuit?

The ammeter you use should have a range that is equal to or greater than the maximum current that will flow through the circuit. If the current exceeds the range of the ammeter, it can damage the device.

3. Can an ammeter be connected in parallel with a circuit?

No, an ammeter should always be connected in series with the circuit. Connecting it in parallel can cause a short circuit and damage the ammeter.

4. How do I read the measurements on an ammeter?

An ammeter typically has a scale that indicates the units of measurement and a needle that points to the current value. Make sure to read the measurement at eye level and take note of the units being used.

5. What are some common errors when using an ammeter?

Some common errors when using an ammeter include connecting it in parallel with a circuit, using an ammeter with a range that is too small for the circuit, and not reading the measurement correctly. It is important to carefully follow instructions and double-check connections and measurements to avoid these errors.

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