27.11
Voltímetro
Um voltímetro é um dispositivo elétrico que mede a diferença de potencial, ou tensão entre dois pontos. Ele é conectado em paralelo com o elemento do…
Em um circuito elétrico, um voltímetro conectado em paralelo com o elemento do circuito mede a diferença de potencial entre os dois pontos do elemento do circuito.
Um voltímetro ideal deve ter resistência infinita para que não extraia corrente do circuito. No entanto, os voltímetros reais têm resistência finita.
É desejável ter um grande valor de resistência para que a conexão do voltímetro a um circuito não altere sensivelmente a diferença de potencial medida.
Qualquer galvanômetro pode ser convertido em um voltímetro adicionando um resistor limitador de corrente em série.
A diferença de potencial necessária para a deflexão em escala real apenas para o galvanômetro é em milivolts.
O alcance do galvanômetro pode ser aumentado conectando um resistor de alto valor em série com a bobina.
Essa configuração distribui a diferença de potencial total entre os dois componentes em série. Como resultado, apenas uma fração da diferença de potencial total aparece na bobina.
A partir desta expressão, o valor do resistor em série necessário para projetar um voltímetro com leitura de escala total pode ser determinado.
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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.