If I consider a voltmeter and connect it across a battery , which is in such a circuit, that no current is flowing in that branch (Before Voltmeter is connected), will the voltmeter, show the EMF of the battery or the potential difference? Considering both ideal and non-ideal cases.
$\begingroup$
$\endgroup$
Add a comment
|
2 Answers
$\begingroup$
$\endgroup$
An ideal voltmeter has infinite input impedance and therefore will draw no current and the reading will be the battery emf.
A real voltmeter has a finite impedance. So it will draw current. It will not read the battery emf. But the greater the ratio of the voltmeter impedance to the battery internal resistance, the closer the reading will be to the battery emf.
The circuit diagram below shows a battery with internal resistance (a real battery) and the equivalent circuit diagram of a voltmeter that uses a galvanometer to provide an analog reading of voltage. In order for the voltmeter to work, a minimum small amount of current is needed to operate the galvanometer. The galvanometer coil has a very low resistance. The current limiting resistor of the voltmeter limits the amount of current to enough to operate the galvanometer. The greater that resistance is, the lower the current drawn, and the lower the voltage drop across the battery internal resistance. That makes the voltage across the battery terminals approximately equal to the battery's internal EMF.
Hope this helps.
$\begingroup$
$\endgroup$
An ideal voltmeter has an infinite resistance.therefore no current will be flowing through it.since there is no current flowing through the branch of the circuit to which it is connected across then none of the energy given to electrons due to chemical reaction in the cell will be used to overcome the internal resistance of the cell as the electrons are not really moving in the circuit.
But when considering a non ideal voltmeter the resistance is not infinite and therefore there will be a current in the closed loop circuit involving the cell and the voltmeter.Now if we consider that the cell has no internal resistance still the potential difference across the cell will be its e.m.f and voltmeter will be indicating the e.m.f as well .
But if the cell has an internal resistance then the reading of voltmeter will be lesser than the e.m.f as energy of electrons is also consumed when moving through the cell.
