Why sparks are produced

[sofia]

Hi
Why sparks are produced when a wire or a switch carrying current (specially high current) is disconnected. Whats the theory behind sparks ??

 

[IanP]

Here you can find interesting explenations on sparks and switches:
https://www-personal.umich.edu/~reginald/opensw.html

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. At the point on the cathode where the spark connects, a large amount of heat is generated by the impacting cations, damaging the electrode surface (more current = more damage). Electrons impacting the anode surface do not cause much damage, since they are thousands of times lighter than the cations and thus have much less kinetic energy.

 

[itheman]

Hi,
From a circuit point of view, I suspect that is because of the inductive nature of the relays(or switches) and the normal wires. One of the basic equation is you cannot change current flowing thru an inductor suddenly.

v = Ldi/dt. A sudden change in I because of opening of a relay(for eg.) will mean that you will change the current from a very high value to zero in a very small time. This means the voltage will go very high(to oppose the change in current). Once the voltage across two terminals goes very high(two terminals being the now opened switch teminals.,), you have a spark-gap.,and there will be arcing between the terminals. (The physics explanation for the arcing is given by Ian below). Hope that helps..:

 

[Enrique15]

Hello.

Well, in a non-electrical view, I always thought of "sparks" as if you were driving a car at high speed, and suddenly you STOP. Of course, the car won't stop immediately, but it'll move some meters until it really stops. That all because of Inertia.

So, to understand SPARKS in a childish way , I think of electrons moving through a wire when suddenly you disconnect the circuit. The electrons, for an instant, will try to continue moving because they can't stop immediately (the same as the car).

Thinking a little more "electrically", you can think one reason is the "space" between the contacts you are separating: in the begining of the circuit opening, while the gap between contacts is increasing, for a short time there's a small space between wires, resulting in a little space of AIR, so there's enough electric force to break air impedance, enabling electrons to flow in that short time.
Of course, this is in the case of high current values (or high voltage).

Complete theory was given previously.

Good-bye.

 

[AndrzejM]

Hello.

Well, in a non-electrical view, I always thought of "sparks" as if you were driving a car at high speed, and suddenly you STOP. Of course, the car won't stop immediately, but it'll move some meters until it really stops. That all because of Inertia.

I can add that the mechanical and electrical phenomena are described by the same math equations:
F = m * a = m (dv / dt) (force = mass x velocity increase per second)
velocity increase per second = acceleration
E= m v^2 /2 (energy = mass x velocity to second power)

voltage U - force F , mass - m - inductance - L , velocity v - current -I :

U = L ( dI / dt)
E = I^2 L /2

 

[gupta]

when you creat high potetential difference between very small air gap, air in the gap becomes ionize and began to pass electricity.
same happens when you switch on or off a circuit it begins to conduct before formation of intimate contact and passes current according to power of load connected.
thus it creat sparks

 

[Big Boy]

Spikes, when disconnecting a wire or fliping off a switch, are created because of the inductive property of the circuits.

In an inductor (or inductive circuit), the currect can not change instently. This is the opposite of a capacitor (or capacitive circuit) where the voltage can not change instently.

So, imagine a circuit where you have current flowing and you disconnect the wire. At the moment the wire is disconnected (or the switch is flipped off), some current will continue to flow in the wire for a brief moment. Since there is no power source feeding the circuit, but that electrons continue to flow, the lead of the wire you disconnect, or the pole of the switch you turn off, will get at a very high voltage (can be in the order of a few thousands volts depending on the current and the inductance of the curcuit). It is this voltage that create a spark. The bigger the current and inductance, the bigger the spark. For example, unpluging a big motor while it's running can fuse the connector leads.

This effect is also present in relays and in switching power-supply (to name a few) and have to be taken into account (or else, the relay of power supply would be destroyed). In this case though, a diode is used to limit the voltage to safe level (and let the current loop through the diode when the circuit is open).