Question

Two battery having unequal emf:
A. cannot be connected in parallel.
B. cannot be connected in series.
C. can be connected in series only.
D. may be connected in parallel or series.

Answer 1

Hint: Charges have the property by which they can flow from a direction of low voltage to high voltage. And we should also know that, when a short circuit happens a very high amount of current flows.

Complete step by step solution:

When we connect two batteries with different emfs together in parallel they will act as a short circuit. This will result in a very large amount of current to flow, sometimes in terms of infinity. And the batteries are not always ideal, there is a small but significant amount of internal resistance present, probably at several ohms. Since the internal resistance is very small, the interconnecting wires will also be considered as resistances in series.
Now, in the case of small voltage difference, current will start flowing between the two batteries from higher voltage battery into lower voltage battery. And when the voltage gets balanced, the current flow will cease. But if there is a big difference in voltage there can be serious destruction, a large current may flow through the battery having lower voltage. It may overheat due the resistance inside and possibly explode.

Therefore, we cannot connect the two batteries in parallel and the correct option is (a).

Note: The exception is if the batteries are of the same type and voltage and charge state. In that case they will more or less match voltage source values and little or no current will flow.