Physics 2012 JAMB Past Questions - Question 38: A newly charged 12V accumulator can easily start a car whereas eight new dry cells in series with an effective e.m.f of 12V cannot start the same car because

Correct Answer

B

Explanation

The new dry cells have higher internal resistance than the newly charged accumulator.Since I = E/R+rThe current capacity is low when r is high.
the effective e.m.f of the eight new dry cells in series is 12V, which is the same as the 12V accumulator. However, there are a few reasons why the dry cells may not be able to start the car:

1. Internal Resistance: Dry cells have a higher internal resistance compared to accumulators. This means that when a high current is drawn from the cells to start the car, the voltage across the cells drops significantly due to the internal resistance. As a result, the voltage available to start the car may be insufficient.

2. Capacity: Accumulators typically have a higher capacity compared to dry cells. This means that they can deliver a larger amount of charge over a longer period of time. Starting a car requires a significant amount of energy, and the dry cells may not have enough capacity to provide the necessary power.

3. Discharge Characteristics: Accumulators have a more stable discharge characteristic compared to dry cells. This means that the voltage of the accumulator remains relatively constant during the discharge process. On the other hand, the voltage of dry cells decreases more rapidly as they discharge. This rapid voltage drop may not provide enough power to start the car.

Overall, while the effective e.m.f of the dry cells is the same as the accumulator, the internal resistance, capacity, and discharge characteristics of the dry cells may prevent them from delivering enough power to start the car.