27.11
電圧計
電圧計は、2 点間の電位差または電圧を測定する電気装置です。 測定している回路要素と並列に接続されます。 並列接続が使用されるのは、並列の要素が同じ電位差を受けるためです。 電圧計は記号 "V " で表されます。
理想的な電圧計の抵抗は無限大であるため、回路内の 2 点間に接続しても電流は変わりませ…
電気回路では、回路素子と並列に接続された電圧計が、回路素子の2点間の電位差を測定します。
理想的な電圧計は、回路から電流を引き出さないように、無限の抵抗を持つ必要があります。ただし、実際の電圧計の抵抗は有限です。
電圧計を回路に接続しても測定された電位差が大きく変わらないように、抵抗の値を大きくすることが望ましいです。
どの検流計も、電流制限抵抗を直列に追加することで電圧計に変換できます。
検流計だけでフルスケールのたわみに必要な電位差はミリボルトです。
検流計の範囲は、コイルと直列に値の大きい抵抗を接続することで拡大できます。
この構成では、直列に接続された 2 つのコンポーネント間の電位差の合計が分散されます。その結果、コイル全体に現れる電位差は全体のほんの一部にすぎません。
この式から、フルスケール読み取り値を持つ電圧計の設計に必要な直列抵抗の値を決定できます。
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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.