Why does a capacitor prevent the spark?

by arcnets
Tags: capacitor, prevent, spark, switch
 P: 513 I read this in Feynman's book 'Surely you are joking Mr.Feynman' but I didn't understand it. 'If you close an electrical switch, there will be a spark shortly before the contact is made. If you don't want any spark, just put a capacitor across the switch.' My question: Why does a capacitor prevent the spark? There's still the same voltage on the switch, isn't it?
 Emeritus Sci Advisor PF Gold P: 10,427 Capacitors force voltage changes to be continuous -- in other words, they ensure that the voltage won't go from 0V to +V instantaneously. The capacitor will have to charge from 0V to +V, which will take a period of time. - Warren
 Sci Advisor P: 875 I don't think this helps for closing a switch unless it is arranged so that the capacitor is linked to the switch. For example, a capacitor with a long charge time is rigged so that it bridges the switch just before the switch is closed. Njorl
 P: 513 Why does a capacitor prevent the spark? chroot - I think the capacitor is already charged before you close the switch. And if you short a charged capacitor, there will be a spark won't it? Njorl - do you think Feynman is wrong here?
 Sci Advisor P: 875 I have "Surely you must be joking..." at home. What page is it on? I'm not about to say Feynman's wrong lightly. Then again, he'd turn over in his grave if I didn't accept the possibility. Njorl
 Mentor P: 7,318 If a cap is placed parallel to the switch contacts, it will create a alternate current path for transient currents. Rather than jumping the closing gap of the switch contacts, current will take the path of least resistance into the capasitor. Consider that the cap will charge to circiut voltage while the switch is open, this provides a source of mobile electrons that are able to redistribute as the EM field changes around the closing switch contacts, essentially this will allow circiut current to flow BEFORE the switch contacts actually close. Thus eliminating any chance of a spark. Such capasitors are in common use in such places as old fashioned auto distributors (across the points) and in nearly every high voltage contactor circiut. Modern Solid State Relays (SSR) avoid this propblem in AC voltage switching by changing state only at a AC zero Volts crossing. So the switch changes state the circiut voltage is O, thus no arc is possible.
 P: 513 Thanks! I understand now.