Actually I was trying to make a capacitor and used Aluminium foil and paper to do this. Then i went for testing it. But I found that the capacitor was not working. Firstly I charged it with a battery of 4V and then took off the capacitor to light up a 3V led light. I also tested the for the correct terminals but still the led was not lighting up.
I think that capacitor's negative terminal is fastly loosing its charge to paper from where electrons move to positive terminal.
So how can i make it work? Also how to increase time for about 8 to 10 sec.
Best Answer
Unless you were using a capacitor with a relatively large capacitance, I am not surprised that your LED did not light up.
As described your experiment failed on one or both of the two counts.
Newspaper is quite a good conductor because it absorbs water from the atmosphere and so becomes a reasonable conductor. The mention of waxed paper by @RobTristram was he suggesting that you stop the absorption of water by the paper by first carefully heating it to drive out the water and then coating it with wax to ensure that the water did not return.
In fact, you are much better off using plastic sheet dried with a fan heater, to remove surface moisture, and I have found that thin acetate sheep of the type used with ohps (overheat projectors) or a bin bag work well.
However, that is not the end of the story. Suppose that you have an LED with a rating of $3\,\rm V$ and $10\, \rm mA$ and want it to be on for $10$ seconds.
The charge which a capacitor must deliver would be $10\times 10 = 100\,\rm mC$.
If the voltage across the capacitor was about $3\,\rm V$ then you would need a $100/3 \approx 30\,\rm mF$ capacitor.
I now come to the second count which is the capacitance of your capacitor.
Suppose the paper has a thickness of $0.1\,\rm mm$, a dielectric constant of about $4$ and the capacitor has a side length of $50\,\rm cm$.
The capacitance of such a capacitor with there being no air gaps will be about $20\,\rm \color{red}{nano}farads$ and thus much too small to store sufficient charge at the voltages you are wanting to use.
There is another thing which you must consider and that is to do with the rate of discharge of the capacitor.
When an LED is conducting it presents a fairly resistance to the circuit and this will mean that if you had just a capacitor and an LED in the circuit the time constant of the circuit, $CR$, might be fairly small and so the LED would not be on for very long.
Large value capacitors, $\approx F$, are available or so perhaps one of those with an appropriate series resistor and a higher initial charging voltage for the capacitor should enable you to light up your LED.
Perhaps, if nothing else, your failure has shown you how difficult it is to store a large amount of electrical energy, eg to power an electric car!