voltmeter and resistance!?

Question

A voltmeter is to measure voltage without affecting current, so with an infinite resistance, no current will flow through it.

So now current is not affected…but then since current does not flow through it, how is the voltmeter still able to detect and measure the voltage WITHOUT any current flow??

And since it’s resistance is so high, once the voltage comes into contact with it to be measured, wouldn’t A LOT of voltage be used up instead due to the high resistance? So in the end affecting the voltage by lowering it by a lot?

Please tell me if my concept is wrong!!

Answer 1

A votometer is measuring the difference in potential between the two poles, or pos & neg. It is not measuring how much current is flowing in a circuit - you can have a voltage without having current flow, and you can also have voltage present without external resistance. But you must have all three if there is to be current flowing in the circuit.

You are really discussing an impossibility - Infinite resistance. i say that becasue of the characteristics of components that no component is " Perfect." But for infinite resistance you are implying a perfect resistor, which is impossible in the real world - this is only a theoretical model that is not possible. No conductor is a perfect conductor - there is some resistance present. No insulatro is perfect - there is some leakage present. No resistor is perfect there is some current flow, and no diode is perfect it has some leakage also.

So knowing that, the rest don't even matter. But if infinite resistance were possible it don't mean you would need infinite voltage to power it. In fact, that may only generate a greater voltage drop across the resistor since the resistance is porprotional to the curent flow ( current entering the resistor).

I think you are a bit confused thinking voltage comes in contact with resistance. That is not true, you are measureing different things here. One is a measure of potential the other is the resistance preventing short circuit current travel.
It is not like when you are measuring the resistance that part of it is voltage or anything, it is not like the voltage is pushing the resistance higher. The voltage is only the force or energy of force driving the electrons. ... think water pressure.
Amperage is the amount of current in motion .. think water flow.

Resistance is like the force trying to stop current - think the size of the pipe limits how much can sqeeze through.
the fact is, in order to have infinitely large resistance you'd need infinitely small current - but don't need infinite voltage

R = E / I
10 = 10 / 1
20 = 10 / .5
40 = 10 / .25
soooo......

infinitely large = E / (Infinitely small)

This means, as resistance would increase to an infinitely large number the current would decrease to an infinitely small number but the voltage does not have to even change to drive this circuit.

Answer 2

An analogue meter does require current flow since its current that drives the meter and it will have an effect on the circuit.

A digital meter, however, reads voltage. Yes there will be an extremely small current flow as the wires are connected and everything becomes charged and there will be some leakage depending on the scaling circuits but as a percentage of the reading the effects to small to care about.

Voltage is the potential difference and cannot be "used up" Use ohms law to do some examples to see what happens.

Didn't some well known person say that is is impossible to measure something without affecting its measurement?

Answer 3

In theory, the resistance of the Voltmeter is infinite, in reality, the resistance is large. Thus the voltmeter will only draw a small amount of Current to give a reading (V=IR, with R is large).

The small amount of Current draw is insignificant to the total Current flowing in the circuit. Hence potential difference across a body measured will not decrease significantly to the theoretical value.

Answer 4

An excellent question. Consider that voltage is actually electrical pressure (at battery terminals?) similar to water pressure. Water pressure can be measured using a pressure gage (Bourdon tube) by allowing the water to enter the gage and expand the tube moving its needle but no steady water flow (current) is allowed because the tube is a 'perfect' resistor. A voltmeter may experience a transient action as 'electric pressure' enters it to deflect the indicator needle but no significant steady state flow (current) is permitted by the high resistance of the instrument. See Fig. 8.13 of the first link.

Answer 5

It's like tasting the big curry pot, you will only use very small amount to taste it, consider no curry has lost.