A newly charged 12V accumulator can easily start a car whereas eight new dry cells in series with a effective e.m.f. of 12V cannot start the same car because

Analyze the question: Both the accumulator (lead-acid battery) and the eight dry cells in series give 12 V. Yet only the accumulator can start the car. So the difference is not the voltage. It must be something else about how the two sources behave.

Concept: A car starter motor draws a very large current, often 100 A or more, for a short time. A source can supply this only if it has a low internal resistance.

• Accumulator: very low internal resistance, so it can deliver a large current without a big drop in terminal voltage.
• Dry cells: relatively high internal resistance, so they can only supply small currents.

The terminal voltage is V = E − Ir. With a large internal resistance r and a large current I, the "lost volts" Ir become so big that almost no useful voltage reaches the starter motor.

Solution:

1. The e.m.f. is the same (12 V) for both sources, so e.m.f. is not the reason.
2. The starter motor needs a large current.
3. The dry cells have a high internal resistance, so they cannot deliver that large current.
4. In other words, the dry cells have a low current capacity. They can supply only small currents, whereas the accumulator can supply large ones.

Checking the options:

• A. High current capacity: wrong, since that would let it start the car.
• B. Low current capacity: correct.
• C. Not rechargeable: true of dry cells, but irrelevant to starting the car.
• D. Difficult to connect: not a physical reason.

Answer: B

Exam tip: When two sources have the same e.m.f. but behave differently, think internal resistance. A low internal resistance means a large current can be supplied, which is why accumulators are used for starting cars. A high internal resistance means a small current, which is how dry cells behave.