When the lights of an automobile are switched on, an ammeter in series with them

Question

When the lights of an automobile are switched on, an ammeter in series with them reads 9.90 A and a voltmeter connected across them reads 11.80 V. When the electric starting motor is also turned on, the ammeter reading drops to 8.10 A and the lights dim somewhat. If the internal resistance of the battery is 49 m? and that of the ammeter is negligible, what is the emf of the battery?

What does the voltmeter read when the starting motor is turned on?

What is the current through the starting motor?

Explanation / Answer

emf= ,Current=I ,voltmeter read=V'

=V+ir
1m=106
= 11.80+(9.90*49*106)
   = 485100011.8V(emf)

now = V'+ (.Imotor+ 8.00 A)r
where V' =IARlight
= (8.10 A) (11.8 V/9.90A)
V'=9.65 V(voltmeter read)
Imotor= -V'/r -8.10
        = 485100011.8-9.65/49*106-8.10
         = 9.90-8.10
    Imotor=1.80A(current )

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