27.12
Potentiomètre
Les mesures de tension et de courant à l'aide d'un voltmètre standard et d'un ampèremètre modifient le circuit mesuré soit en attirant soit en résista…
Lorsqu’un voltmètre est connecté à la batterie, il ne mesure que la tension aux bornes.
Pour des mesures précises des CEM, la résistance interne de la batterie doit être connue ou le courant délivré par la batterie doit être nul. Cependant, les voltmètres consomment peu de courant pour fonctionner.
Ainsi, les appareils de mesure nuls comme les potentiomètres sont plus précis que les voltmètres, car ils équilibrent la tension et ne permettent aucun flux de courant à travers l’appareil de mesure.
Il se compose d’une source de tension connectée à un long fil résistif uniforme.
La source de tension maintient un flux de courant constant et un gradient de potentiel uniforme à travers le fil.
La source de f.é.m. inconnue est reliée à un galvanomètre et à un jockey. Le point de contact est ajusté de telle sorte que la f.é.m. soit égale à la chute de potentiel à travers le segment de fil et que le galvanomètre affiche une déviation nulle.
Une source de f.é.m. standard remplace la source inconnue, et le point de contact est à nouveau ajusté pour obtenir une déviation nulle.
Le rapport des deux relations donne l’expression de la f.é.m. inconnue en termes de résistances, qui est proportionnelle aux longueurs équilibrées obtenues.
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Q1: Why is a potentiometer more accurate than a voltmeter for measuring emf?
A potentiometer is a null measurement device that balances voltages without allowing current to flow through the measuring device, eliminating circuit alterations. In contrast, voltmeters draw small currents to operate, which affects the circuit and introduces measurement errors. By achieving zero deflection on a galvanometer, the potentiometer measures the true electromotive force without distorting the circuit.
Q2: What components make up a potentiometer and what does each do?
A potentiometer consists of a voltage source connected to a long, uniform resistive wire of constantan or magnum, a rheostat to regulate current flow, a galvanometer to detect balance, and a jockey to adjust the contact point. The voltage source maintains a constant current and uniform potential gradient across the wire, while the galvanometer indicates when the unknown emf equals the potential drop.
Q3: How does the potentiometer achieve zero current flow during measurement?
The potentiometer balances the unknown emf against a known potential drop across a wire segment. When the contact point is adjusted so that these voltages are equal, the galvanometer shows zero deflection, indicating no current flows through the measuring circuit. This null measurement condition ensures the circuit remains unaltered during the measurement process.
Q4: What is the procedure for measuring an unknown emf with a potentiometer?
First, connect the unknown emf source with a galvanometer and jockey, then adjust the contact point until the galvanometer shows zero deflection and record this balanced length. Next, replace the unknown source with a standard emf source and adjust the contact point again for zero deflection. The ratio of the two balanced lengths gives the unknown emf in terms of the wire segment resistances.
Q5: Why must the potentiometer wire have a uniform potential gradient?
A uniform potential gradient ensures that the potential drop across any segment of wire is directly proportional to its length and resistance. This linear relationship allows the potentiometer to accurately calculate unknown emf values from the balanced lengths, making the measurement reliable and independent of absolute resistance values.
Q6: What measurements can a potentiometer perform beyond measuring unknown emf?
A potentiometer can measure the internal resistance of a cell by comparing the terminal voltage under load with the open-circuit emf. It can also compare the electromotive force of two voltage sources by balancing each against the same standard reference, providing accurate relative measurements without drawing significant current.
Q7: How does the potentiometer differ from standard voltmeters in handling internal resistance?
Standard voltmeters measure only terminal voltage and require knowledge of internal resistance for accurate emf determination. The potentiometer eliminates this requirement by achieving zero current flow, making the internal resistance irrelevant. This null measurement approach provides direct emf measurement without needing to account for voltage drops caused by the measuring device itself.