Question

What is the resistance of an ideal voltmeter and an ammeter?
(A). Infinite and One
(B). Fixed and Infinite
(C). Zero and Infinite
(D). Infinite and Zero

Answer 1

- Hint: An ideal voltmeter should have no current passing through it. An ideal ammeter should let the entire current pass without any hindrance. And we know current is inversely proportional to resistance, that means the more the resistance less current passes through it and vise versa.

Complete step-by-step solution -
A voltmeter is connected in parallel to the current carry wire, to measure the potential difference between two points. Since it is connected to the wire, a finite amount of current will pass through it. This will lead to the change in actual flow of current, and hence, change in the actual potential difference. In order to minimize this, the resistance to a voltmeter is kept high enough such that the current passing through it is minimum. However, this would still cause some imperfections in the reading. In an ideal situation, where the voltmeter is able to measure the actual potential difference across two points, there should be no current passing through it, and hence its resistance should be infinite.
An ammeter is connected in series with a current carrying wire, to measure current passing through it. Since it is connected to the wire and has a finite amount of resistance, there will be some potential difference across it. This will lead to the change in actual flow of current. In order to minimize this, the resistance to an ammeter is kept minimum such that the voltage drop across it is minimum. However, this would still cause some imperfections in the reading. In an ideal situation, where the ammeter is able to measure the actual current passing through a wire, there should be no voltage drop across it, and hence its resistance should be zero.
Hence, the correct option is D.

Note: Looking at the other options, with infinite resistance, an ideal voltmeter would work fine but with one-ohm resistance, the ammeter would work but won’t be an ideal one. With infinite resistance, the ammeter would stop the flow of current in the circuit and hence won’t work at all. While zero or finite resistance, the voltmeter would draw all or significant amounts of current and thus won’t work. Hence, the second and third option isn’t correct. The available ammeters and voltmeters are not ideal ones, hence the reading we get isn’t the right one (approximately equal to it).