27.11
Voltmeter
A voltmeter is an electrical device that measures the potential difference or voltage between two points. It is connected in parallel with the circuit…
In an electric circuit, a voltmeter connected in parallel with the circuit element measures the potential difference between the two points of the circuit element.
An ideal voltmeter should have infinite resistance so that it does not draw current from the circuit. However, real voltmeters have finite resistance.
It is desirable to have a large value for resistance so that connecting the voltmeter to a circuit does not appreciably alter the measured potential difference.
Any galvanometer can be converted into a voltmeter by adding a current-limiting resistor in series.
The potential difference required for full-scale deflection for just the galvanometer is in millivolts.
The galvanometer's range can be increased by connecting a high-value resistor in series with the coil.
This configuration distributes the total potential difference between the two components in series. As a result, only a fraction of the total potential difference appears across the coil.
From this expression, the value of the series resistor required for designing a voltmeter with full scale reading can be determined.
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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.