Why is the Voltmeter Showing 220V in a Broken Circuit?

  • Thread starter Thread starter otomanb
  • Start date Start date
AI Thread Summary
The voltmeter shows 220V in a broken circuit due to its infinite internal resistance, preventing current flow and resulting in no voltage drop across the resistors. The high internal resistance allows the voltmeter to read the source voltage without registering any changes, as its precision rounds values above 219.995V to 220V. Despite the circuit being open, the voltmeter effectively measures the potential difference at the source. The discussion highlights a common misconception regarding voltage readings in open circuits and emphasizes the voltmeter's role in parallel connections. Ultimately, the circuit's lack of current flow leads to the observed voltage reading without any actual voltage drop across the components.
otomanb
Messages
59
Reaction score
0
As every body knows in series voltage drops and voltmeter is applied in parallel but look at this image . as the circuit is broken and voltmeter is applied but still Vmeter is showing 220V?
how?


52jkvo.jpg
 
Physics news on Phys.org
The voltmeter has infinite internal resistance, so there is no current flowing in the circuit. What's the drop over the resistors? What did you expect?
 
If the internal resistance of the voltmeter is r, then, the voltage drop across it is:

<br /> V = \frac{(220 \, \mathrm{V}) \, r}{1 \, \mathrm{k \Omega} + 1 \, \mathrm{k \Omega} + 1 \, \mathrm{k \Omega} + r}<br />

If the voltmeter is digital and the precision is 0.01 V, it means that everything above 219.995 V will be rounded up to 220 V, so:

<br /> \frac{(220 \, \mathrm{V}) \, r}{1 \, \mathrm{k \Omega} + 1 \, \mathrm{k \Omega} + 1 \, \mathrm{k \Omega} + r} &gt; 219.995 \, \mathrm{V}<br />
<br /> 220 \, r &gt; (3 \, \mathrm{k \Omega} + r) \, 219.995<br />
<br /> 0.005 \, r &gt; 659.985 \, \mathrm{k\Omega}<br />
<br /> r &gt; 1.32 \times 10^5 \, \mathrm{k\Omega} = 132 \, \mathrm{M\Omega}<br />

So, the internal resistance of the voltmeter is very high.
 
Last edited:
actually this question was asked by our teacher in exams i answered 0 volts but when i checked it with a software livewire 1.11 it answers 220 V who is rite?
 
The Machine is right, human.
 
but the circuit is open so there is no current flowing
v=IR
I=0A so
v=0*any Resistance = 0volt
but software is saying 200.
it's contrary with the theories that we read before in book.
on thing more voltmeter is always applied in parallel with the load here in pic the voltmeter is not applied to parallel even though it's not parallel the resistors or voltage source.
please help!
 
otomanb said:
but the circuit is open so there is no current flowing
v=IR
I=0A so
v=0*any Resistance = 0volt
but software is saying 200.
it's contrary with the theories that we read before in book.
on thing more voltmeter is always applied in parallel with the load here in pic the voltmeter is not applied to parallel even though it's not parallel the resistors or voltage source.
please help!

read my post.
 
ohhhh! got that one!
you mean there is no current flowing so voltmeter is telling the source voltages 220. and there is no voltage drop?

got that one! thank you !
 
No, I meant the voltage drop on the remaining 3 resistors is smaller than 0.005 V and the voltmeter cannot register any difference with its sensitivity.
 
  • #10
thank you sir!
 
  • #11
it means the electrons are just 'waiting' for the continuity of the circuit be stablish in the ends of the 'break'
 

Similar threads

Replies
6
Views
5K
Replies
5
Views
4K
Replies
33
Views
2K
Replies
5
Views
3K
Replies
3
Views
2K
Replies
3
Views
1K
Back
Top