When a car\'s starter is in use, it draws a large current. Thecar\'s lights draw

Question

When a car's starter is in use, it draws a large current. Thecar's lights draw much less current. As a certain car is starting,the current through the battery is 57.6 Aand the potential difference across the battery terminals is8.19 V. When only the car's lights areused, the current through the battery is 2.10 A and the terminal potential difference is10.9 V. Find the battery's emf.

V   (This answer is right,only need help with internal resistance)

Find the internal resistance.

When a car's starter is in use, it draws a large current. Thecar's lights draw much less current. As a certain car is starting,the current through the battery is 57.6 Aand the potential difference across the battery terminals is8.19 V. When only the car's lights areused, the current through the battery is 2.10 A and the terminal potential difference is10.9 V. Find the battery's emf.

V   (This answer is right,only need help with internal resistance)

Find the internal resistance.

Explanation / Answer

   Battery emf (E), potential difference (terminal voltage ) (V), current (I), internal resistance (r) andexternal resistance (R) are related as    E   =   V   +   I* r    When starter and bulb are in use                      E   =   8.19   +  57.6 *r               ----------(1)    When only bulb is in use                      E   =   10.9   +   2.10*r               -------------(2)    Subtracting equation (2) from equation(1)                55.5*r   -   2.71   =   0       r   =   2.71/ 55.5             =   0.0488       

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