Question

why is the longer lead of LED is always connected to the positive terminal and shorter to the negetive terminal of the battery?

Answer 1

If you connect an LED on its own to your battery, the battery will heat up the LED and destroy the LED. Try connecting one LED to the battery alone by plugging it right across the battery voltage. If you connect it one way, nothing happens, because the LED won't let electrons through that way. Connect it the other way around. Now it should shine and get warm. The correct way is with the long lead on the Positive Terminal and the short lead on the Negative Terminal. The LEDs will not explode. 

Answer 2

Explanation:

Current always flows from the anode to the cathode. ... it is always necesssary to make sure that longer lead of LED is always connected to the positive terminal and shorter to the negetive terminal of the battery because current only flows from anode to cathode not from cathode to anode