Unlike the idealized ammeter described in your text (Section 25.4), any real amm

Question

Unlike the idealized ammeter described in your text (Section 25.4), any real ammeter has a nonzero resistance. (a) An ammeter with resistance RA is connected in series with a resistor R and a battery of emf and internal resistance r. The current measured by the ammeter is IA. Find the current through the circuit if the ammeter is removed so that the battery and resistor from a complete circuit. Express your answer in terms of IA, r, RA, and R. The more "ideal" the ammeter, the smaller the difference between this current and the current IA. (b) If R= 4.50 ohms, E= 8.25 V, and r= 0.33 ohm, find the maximum value of the ammeter resistance RA so that IA is within 1.0% of the current in the circuit when the ammeter is absent. (c) Explain why your answer in part (b) represents a maximum value.

Explanation / Answer

Here,

As by inspection and ohm's law

Ia = E / (r + R + Ra)

now,

I = E /(r + R)

E = I * ( r+R)

Ia = I*(r +R) / ( r+R+Ra)

I = Ia ( r + R + Ra)/(R+r) = Ia (1 + Ra/(r + R) =

Ia = I if Ra << R + r

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