When the lights of a car are switched on, an ammeter in series with them reads 1
ID: 3163919 • Letter: W
Question
When the lights of a car are switched on, an ammeter in series with them reads 10.1 A and a voltmeter connected across them reads 11.90 V. When the electric starting motor is turned on, the ammeter reading drops to 7.0 A and the lights dim somwhat. If the internal resistance of the battery is 0.040 and that of the ammeter is negligible, a. what is the emf of the battery when the lights are on? b.What is the current through the starting motor when the lights are on?
S Starting Lights Ill ge Lights motor RExplanation / Answer
a) The emf is gven by
= V + i r = 11.9+ 10.1(0.04)
= 12.3 V
b)The voltage of the light is
V ' = i Rlight= (7)(11.9/ 10.1) = 8.25 volts
The emf is
= V ' + ( i motor + 7) r
imotar = ( - V ' / r) - 7
= (11.9- 8.25 / 0.04) - 7
= 84.25 A
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