Question

You wish to measure both the current through and the potential difference across some component of a circuit. It is not possible to do this simultaneously and accurately with ordinary voltmeters and ammeters. Explain why not.

Short answer

Answer: It is not possible to simultaneously and accurately measure the current through and the potential difference across a component in a circuit using ordinary voltmeters and ammeters because both instruments impact the circuit's resistance when connected. An ammeter adds some resistance to the circuit when connected in series, causing an underestimate of the actual current, while a voltmeter alters the total resistance of the circuit when connected in parallel, leading to a change in the current and an inaccurate measurement of the potential difference. This makes it difficult to obtain accurate and simultaneous measurements for both the current through and the potential difference across a component using these instruments.

Step-by-step solution

Understanding Ammeters and Voltmeters

Ammeters and voltmeters are devices used to measure current and potential difference (voltage) in an electrical circuit, respectively. An ammeter is connected in series with the component to measure the current flowing through it, while a voltmeter is connected in parallel to the component to measure the potential difference across it.

Ammeter Impact on Circuit Resistance

An ideal ammeter should have zero resistance to minimize the disturbance it creates in the circuit. However, in reality, every ammeter has some internal resistance. When connected in series with the component, it adds to the total resistance of the circuit, affecting the current flowing through it. This causes an underestimate of the actual current.

Voltmeter Impact on Circuit Resistance

A voltmeter should ideally have infinite resistance to avoid any current flow through itself, ensuring it measures the true potential difference across the component. But every voltmeter has a finite, though high, resistance. When connected in parallel to the component, it forms a parallel resistance combination, which alters the total resistance of the circuit. This leads to a change in the current flowing through the component, causing the voltmeter to measure an inaccurate potential difference across the component.

Simultaneous Measurements with Ammeters and Voltmeters

As seen from the previous discussions, both ammeter and voltmeter have an impact on the circuit's resistance when connected, leading to changes in the current and voltage in the circuit. When attempting to measure both current and voltage simultaneously with ordinary ammeters and voltmeters, the disturbances created in the circuit affect the actual values of both the current and voltage. Thus, it becomes difficult to obtain accurate and simultaneous measurements for the current through and potential difference across a component using these instruments.