# How could this possible?

1. Dec 4, 2011

### otomanb

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?

2. Dec 4, 2011

### gnurf

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?

3. Dec 4, 2011

### Dickfore

If the internal resistance of the voltmeter is r, then, the voltage drop across it is:

$$V = \frac{(220 \, \mathrm{V}) \, r}{1 \, \mathrm{k \Omega} + 1 \, \mathrm{k \Omega} + 1 \, \mathrm{k \Omega} + r}$$

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:

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

So, the internal resistance of the voltmeter is very high.

Last edited: Dec 4, 2011
4. Dec 4, 2011

### otomanb

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?

5. Dec 4, 2011

### gnurf

The Machine is right, human.

6. Dec 4, 2011

### otomanb

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.
plz help!

7. Dec 4, 2011

8. Dec 4, 2011

### otomanb

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 !

9. Dec 4, 2011

### Dickfore

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. Dec 5, 2011

### otomanb

thank you sir!

11. Dec 6, 2011

### Nevertamed

it means the electrons are just 'waiting' for the continuity of the circuit be stablish in the ends of the 'break'