Connect the same two dry batteries in series or in parallel, and then connect the same small bulb. Is the brightness of the small bulb the same?

Connect the same two dry batteries in series or in parallel, and then connect the same small bulb. Is the brightness of the small bulb the same?

Of course not, because the voltage is not the same, the current is not the same

Which is the most powerful of two dry batteries in series or in parallel

It's the same size
From the practical point of view, series will reduce the line current, more commonly used

Can the new and old dry batteries be used in series or in parallel? Why?

Generally, batteries are not used in series. Although batteries in series can increase the voltage, because the resistance values of new and old batteries are different, the use of batteries in series will consume too much energy of new batteries and shorten the service life of batteries. Batteries can be used in parallel. Batteries in parallel can increase the amount of current, and the formed circuits are also different, causing less damage to batteries

Is there a small LED bulb that can be powered by two dry batteries?
It is required that the screw diameter is about 1cm, within 3V, and the power is 3W. Is there any LED bulb that meets the requirements?

There is no screw, to their own modification, there are 1w3w5w, voltage 2.8v-3.3v