Why does and ammeter has zero internal resistance and a voltmeter has an infinitely large resistance?

Question

I thought the voltage value was dependant on the current V=IR so how can the voltmeter measure the voltage with little current running through it. Same for the ammeter to measure I (current) without potential difference(voltage) between the terminals. I jsut don't get that part. Trying to get ready for my Physics II test. I know I'm looking at it the wrong way. Help me!

Answer 1

Dear asker

Let me draw and analogy to help understand the concept of voltage and current.

voltage is the potential diference between two points ( this is equivalent to the pressure in the water pipe.) In order to measure the pressure the tap has to be closed to see how much is the pressure which is being exerted by the water.

In order to measure this pressure or the Voltage the voltmeter is used, we do not want to have any current flow otherwise the pressure ( voltage) will drop. so the voltmeter should have infinitely large resistance.


Current it like the flow of the water which can be measured when the water is actually runnng. If the tap is closed slightly the water( current ) will reduce. Therefore the Ammeter which is used to measure the current flow should have zero internal resistance, as as not to obstruct the current flow.


Hope it makes it easy to remember. Good Luck with your test.

Answer 2

Ampere meter does not have zero resistance. It has very LOW resistance. Voltage meter does not have infinately large resistance. It has relatively HIGH resistance. There is a big difference between what I said and what you said.

Amp meter has low resistance because when current flows over this small resistance, it generate small voltage across it, which you will (in effect) measure with the meter. The voltage generated across this small resistance will be V=IR. To be more precise, the meter is connected in parallel with the small divider resister. The current flows mostly to this resister and only very small amount will actually flow through the meter giving you an indication. The value of the resister has to be sufficiently small, compared to the internal resistance of the meter, so that only small amount will flow though the meter. (in order of 100 micro amps)

Volt meter has high resistance because you will connect it in series with the voltage. Unless you have a large resistance, it will draw too much current and burn up. In this configuration, the divider resister is connected in series with the meter. The total current that flow though the meter and the resister (which are the same) will be i = e/r. This small current, you will measure by the meter to get an indication of the value.

To help you understand....

Recognize that volt meter and amp meter is actually two parts. Divider resister and the meter. See how they are connected and visualize the way current will flow, and the voltage is induced.

Answer 3

When you read voltage in a circuit, the volt meter is placed in parallel with the component or source being tested - and in order to get an accurate reading, the volt meter must not significantly change the component being tested. After all, we're looking for the voltage drop across the component (it's current dependent) and not the voltage drop across the component and a low ohm meter in parallel with it. The higher the impedance of the meter, the less effect it has on the circuit being tested - hence, the more accurate it is in testing the actual circuit values.
With an amp meter, any appreciable resistance in the meter when it is placed in series on the circuit being tested would lower the current being read and give a lower amp draw than the circuit is actually drawing when the meter is removed and the circuit is reconnected with no resistance.
In short, when measuring amps - series measurement - we want to add no resistance to the circuit. When measuring voltage - parallel measurement - we don't want to draw any more current in the circuit as this will cause a greater voltage drop across the component(s)being tested.

Answer 4

Voltmeters measure potential, if they allowed current to pass through, that would reduce the potential and skew the results.

An ammeter measures active current flow, if it provided any resistance the current flow would be diminished and the results similarly tainted.

Answer 5

One way to think of it is current (Amps) is the amount of water that flows through a hose. Voltage is the pressure of the water. You can measure pressure without running water through the hose.

Resistance is largely the size of the hose, less resistance can cause pressure to drop (if the pump isn't big enough).