27.11
Voltmetro
Un voltmetro è un dispositivo elettrico che misura la differenza di potenziale o la tensione tra due punti. Viene collegato in parallelo all'elemento…
In un circuito elettrico, un voltmetro collegato in parallelo con l'elemento del circuito misura la differenza di potenziale tra i due punti dell'elemento del circuito.
Un voltmetro ideale dovrebbe avere una resistenza infinita in modo da non assorbire corrente dal circuito. Tuttavia, i voltmetri reali hanno una resistenza finita.
È auspicabile avere un valore elevato per la resistenza in modo che il collegamento del voltmetro a un circuito non alteri in modo apprezzabile la differenza di potenziale misurata.
Qualsiasi galvanometro può essere convertito in voltmetro aggiungendo in serie un resistore di limitazione della corrente.
La differenza di potenziale richiesta per la deflessione a fondo scala per il solo galvanometro è in millivolt.
La portata del galvanometro può essere aumentata collegando una resistenza di alto valore in serie con la bobina.
Questa configurazione distribuisce in serie la differenza di potenziale totale tra i due componenti. Di conseguenza, solo una frazione della differenza di potenziale totale appare attraverso la bobina.
Da questa espressione, è possibile determinare il valore del resistore in serie richiesto per la progettazione di un voltmetro con lettura del fondo scala.
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Q1: Why is a voltmeter connected in parallel with a circuit element?
A voltmeter is connected in parallel because elements in parallel experience the same potential difference. This configuration allows the voltmeter to measure the voltage across the circuit element without being in series with the current path. Parallel connection ensures accurate measurement of the potential difference between two points.
Q2: What is the difference between an ideal and real voltmeter?
An ideal voltmeter has infinite resistance, so it draws no current from the circuit. Real voltmeters have finite resistance but should have high enough resistance to avoid significantly altering circuit currents. Inexpensive voltmeters have resistances around 10 megaohms, while high-precision voltmeters reach 10 gigaohms.
Q3: How can a galvanometer be converted into a voltmeter?
A galvanometer converts to a voltmeter by adding a high-value resistor in series with the coil. This series resistor limits current and extends the measurement range. The total potential difference distributes between the resistor and coil, so only a fraction appears across the galvanometer coil at full-scale deflection.
Q4: What role does the series resistor play in voltmeter design?
The series resistor in a voltmeter acts as a current-limiting component that extends the measurement range. It distributes the total potential difference between itself and the galvanometer coil. By calculating the appropriate series resistor value, designers can achieve full-scale deflection readings for specific voltage ranges.
Q5: Why do voltmeters need high resistance values?
High resistance in a voltmeter minimizes current draw from the circuit being measured. This prevents the voltmeter from appreciably altering the circuit's behavior or the measured potential difference. The higher the voltmeter's resistance, the less it disturbs the circuit and the more accurate the voltage measurement becomes.
Q6: What is electromyography and how does it use a voltmeter?
Electromyography is a medical diagnostic technique that uses a sensitive voltmeter to measure electrical activity in muscles. A fine needle with two electrodes is inserted into muscle tissue, and the voltmeter measures the potential difference between the electrodes. Physicians use these readings to diagnose neurological and neuromuscular diseases.
Q7: How does voltmeter resistance affect circuit measurements?
Voltmeter resistance directly impacts measurement accuracy. If resistance is too low, the voltmeter draws significant current and alters the circuit's behavior, leading to inaccurate readings. Sufficient resistance ensures the voltmeter measures the true potential difference without substantially changing the currents in the circuit being analyzed.