A through D When the lights of a car are switched on, an ammeter in series with

Question

A through D

When the lights of a car are switched on, an ammeter in series with them reads 9.75 A and a voltmeter connected across them reads 12.0 V (see the figure). When the electric starting motor is turned on, the armneter reading drops to 7.37 A and the lights drn somewhat. If the internal resistance o the battery IS 0.0513 and that of the ammeter is negligible, what are a) the emf of the battery and b) the current through the starting motor (in A) when the lights are on? Lights arting motor

Explanation / Answer

before motor is turned

voltmeter reading V1 = E - I*r

12 = E - (9.75*0.0513)

emf of the battery E = 12.5 v

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before motor is turned

voltage across bulb = v1 = I1*R

resistance of bulb R = V1/I1 = 12/9.75

after the motor is turned on

current through bulb I2 = 7.37 A

voltmeter reading V2 = I2*R = V*(I2/I1) = 12*(7.37/9.75) = 9.07 v

V2 = E - I*r

9.07 = 12.5 - (I*0.0513)

I = 66.8 A

current through motor I1 = I - I2 = 66.8 - 7.37 = 59.43 A <<<----ANSWER

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