Question

An ammeter is showing the reading as 3.5 mA. What will be the value of current in microampere?​

Answer 1

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If the answer you are hoping for is 2A, you will be sorely mistaken. As given, your question has no simple answer. The real answer has to do with how you are modeling the voltage source and the ammeter.

An ideal voltage source will produce the same voltage regardless of the load (“stuff in the circuit”). However, since real voltage sources (a battery, for example) will not do this. In the case of a battery, the maximum possible current is related to the maximum chemical reaction rate that is going on inside of the battery (or — perhaps a better name for this example — electro-chemical cell).

An ideal ammeter will measure the current, but have no resistance. However, practically speaking, all real ammeters have an internal resistance. Think about the ammeter below (source ):

The green thing is a spring which you can adjust to set the calibration of the device. The red is the wire that connects to the leads that the current you wish to measure flows through. As the current flows through the (red) wire, a magnetic field forms, interacting with the magnetic field produced by the external magnet (N and S poles pictured above). This causes the knob to twist (depending on how much current is flowing), and the marks allow you to read the current. (Cool story, bro, but what does this have to do with internal resistance?) I’m SO GLAD you asked. :) That red wire wraps a whole bunch of times. When you have LOTS of wire, you get a resistance.

So let’s break this down into the cases that we could have:

Ideal battery, Ideal ammeter. Current measured is infinite. It is a bare wire connected across the battery terminals.

Real battery, Ideal ammeter. It will be the max current of the voltage source.

Ideal battery, Real ammeter. We need the internal resistance of the ammeter to figure it out. As it is connected in parallel across the battery, the current measured will just be VbattRammeter .

Real battery, Real ammeter. Well, it might be the same as above, as long as this current isn’t near the max current of the voltage source (battery). If not, it gets more complicated. The current will be smaller than the max battery current, but how much smaller depends on how the ammeter resistance compares to the 10 ohm resistor that you have included.