Question

When we touch Positive and negative potential wires with each other, it give spark! Why?

Answer 1

In a simple atmospheric pressure spark gap, as the potential difference (voltage) between the electrodes is increased (or as the distance between the electrodes is decreased), electrons begin to be emitted by the cathode (negative electrode) and travel to the anode (positive electrode). As they travel, some of the electrons will collide with gas molecules, knocking electrons loose and forming cations and more free electrons. Near the electrodes, where the concentration of traveling electrons is highest, a faint glow caused by the recombination of ions and electrons will become visible. This glow is called a corona discharge and removes energy from the ionized gas at a high enough rate to prevent the formation of plasma. At higher voltages (or smaller gaps) the energy put into the molecules by electron collision exceeds the ability of the corona discharge to dissipate the energy and a plasma is formed. The electric field between the electrodes will then separate the cations and electrons. The electrons will flow towards the anode, while the cations will flow towards the cathode. When the cations impact the cathode, they recombine with electrons from the surface of the cathode, completing the electric circuit. In an atmospheric pressure spark gap, the flow of cations and electrons tends to be confined to a fairly narrow channel which is called a spark.