Science Buddies: "Ask an Expert"

I have a battery operated DC motor which gives me an output of 2.5 V D.C when I rotate the axle of the motor. I need to connect a capacitor to the O/P of the motor so that the charge gets stored in the capacitor and will power a small bulb for about a few seconds. DO u know what capacitor should I use?

Short answer: No. There is enough information to answer your question.

What grade are you in? Do you know what logrithms are from math? Have you had calculus?

Have you tried searching for Resistor Capacitor circuits in a physics or Electrical Engineering text boot? They usually talk about RC time constants. Here is a reference: http://tpub.com/neets/book2/3d.htm that you be able to understand without understanding the more complicated math required to describe these curves.

Which "small bulb"? It matters! Typical small tungsten light bulbs act like non-linear resisters. When the filament is cold, they have a higher resistance and as they start to glow, their resistance decreases with temperature. Their specifications typically have a recommended operating voltage and list a corresponding milliamp current rating. From this you can use ohm's law to estimate the resistance at this specific operating point. Unfortunately, that operating point might not correspond to the voltage in your circuit. For example a PR2 bulb a 2.85 V draws 500 mA.

Have you considered using a LED instead? They are much more efficient so you have to store less energy. A 5mm Red high output LED takes only 20 mA to light.

Spell check isn't enough! That should be "text book" not "text boot".