27.11
电压表
电压表是一种测量两点之间电位差或电压的电气设备。 它与所测量的电路元件并联。 使用并联连接是因为并联的元件具有相同的电势差。 电压表用符号“V”表示。
理想的电压表具有无穷大的电阻，因此将其连接在电路中的两点之间不会改变任何电流。 真正的电压表总是具有有限的电阻，但电压表应该具有足够的电阻，以便将其…
在电路中，与电路元件并联连接的电压表用于测量该电路元件两端之间的电势差。
理想的电压表应具有无限大的电阻，以便不会从电路中汲取电流。然而，实际的电压表具有有限的电阻。
希望电阻值较大，这样将电压表连接到电路时，不会显著改变所测得的电势差。
通过串联一个限流电阻，任何检流计都可以改装成电压表。
仅使检流计达到满量程偏转所需电势差为毫伏级别。
通过在线圈上串联一个高阻值电阻，可以扩大检流计的量程。
这种配置将两个串联组件之间的总电势差进行分配。因此，只有一部分总电势差施加在线圈两端。
根据此表达式，可确定设计具有满量程读数的电压表所需的串联电阻值。
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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.