Internal Resistance of Ammeter

[jdido09]

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Why should an ammeter have low internal resistance in order to have the least effect on the current flowing through a resistor in series?

Wouldn't the ammeter and resistor have the same current passing through since they are in series?

 

[costales]

The current in the circuit resistor is not the same current seen by the ammeter's internal resistance. It is the sum of two currents: the current going through the internal resistance (i.e. galvanometer which can carry only a small current), and the current going through a shunt resistor (in parallel with the galvanometer - this shunt takes up most of the current). (Note: MCAT takers shouldn't worry about these details)
A resistor's job is to resist current. To get an accurate measurement, you want the internal resistance to be very small - ideally zero.

 

[muhali3]

The current going through a circuit is dependent upon the total resistance (equivalent resistance) of the circuit.

V = I x Req

By adding an ammeter of very large resistance, you would increase Req, decreasing the current coming from the voltage source. So the measured current would be less.

You are correct about elements in series having the same current going through them. But that doesn't mean the current won't change if you add another resistor. The current will decrease for the entire circuit. So, let's say you add another resistor, and the current in the circuit decreases from 3A to 2A. Now all elements in series would have 2A going through them. This is less current than if you didn't add the resistor.

Hope that made sense.