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Yesterday I was playing around with a small DC Motor ,(ones which you can find inside a toy) and found something interesting. I took a cell and fixed one terminal of the cell to a terminal of the Motor with the help of wires and kept the other terminal of the cell open, waiting to be connected. When I touched the open terminals of the cell and the motor, the motor would obviously start rotating, but also along with sparks ,due to the second law of thermodynamics. After I disconnected one of the wire, the current flow stops ,but the motor keeps on rotating due to inertia. When the motor completely stops, I repeatated the process and the same thing occured again. BUT, when I retouched the open terminals while the motor was still rotating due to inertia, the sparks would not come out.The sparks started to appear again when the motor (rotating in inertia), slowed down. But the intensity of the sparks would be lower than when the motor was not rotating at all. I repeated this experiment many times and the same result came out. I repeated the experiment by reversing the terminals and as intuition told, it showed the same result. I came up with some explanation for it, but I'm not sure of anything so far.

What is the possible reason of this phenomenon?

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The effective resistance (more precisely, impedance) of a motor is low while at rest or at low speed, and increases with speed.

When you connect your voltage source to motor at rest, the winding in the motor acts as a shorting wire, so you are shorting the terminals of the source, hence sparks (great current runs through the winding which means when disconnected, leads to sparks).

When you connect or disconnect your voltage source while motor is running fast, the motor has great impedance so it is not so good a shorting wire; the running current is much lower and therefore sparks on contacts smaller as well, or none.

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  • $\begingroup$ I'm not quite sure what you mean by the "shorting of the terminals" and moreover why does it happen? Also, why is the impedance low while at rest? $\endgroup$ – NightKruger Feb 9 at 20:30
  • $\begingroup$ Shorting terminals means you take your voltage source (such as battery) and connect plus and minus with wire (don't do it). Very large current would run which would damage the battery and you would see sparks. Sparks happen due to large concentration of electric charge on the wires which gets created if large enough current is temporarily stopped; this will break the air and current will jump over the air gap, which you can see as spark. $\endgroup$ – Ján Lalinský Feb 9 at 21:38
  • $\begingroup$ By the way, where did you see the sparks coming from? I assumed above that they come from the motor terminal, but there can be also some sparking inside, if it has brushes inside. $\endgroup$ – Ján Lalinský Feb 9 at 21:39
  • $\begingroup$ I can only see sparks from the terminals . It should be happening inside too , but i can't see them. $\endgroup$ – NightKruger Feb 10 at 5:58

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