Suppose an old flashlight battery shows a voltage of 1.45 V when the probes of a

Question

Suppose an old flashlight battery shows a voltage of 1.45 V when the probes of a voltmeter are placed across it. However when a resistor of 250 ohms is hooked across the terminals, the voltage across the terminals (which is the same as the voltage across the resistor) reads 1.25 V.

A. We claim that in the first case, the voltmeter reads the true emf (electro-motive force) of the battery. How do we know this? (Hint:voltmeters have extremely high resistance.)

B. What is the current when the resistor is hooked up? What is the internal voltage drop, inside teh abttery?

C. What is the battery's internal resistance?

Explanation / Answer

1.45 - 1.45r/(r + 250) = 1.25 Solving internal resistance = 40 Ohms Current = 1.45/(40 + 250)= 0.005 A

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