Electronics > Beginners
On a 250V capacitor or a CRT, is the negative terminal -250V?
Brumby:
If they want to ask about that 125 litres of water, then you can tell them that's like all the electrons that are just sitting around doing nothing - waiting for something to move them.
Starglider:
Well they might ask: Why is it safe to touch when there are 125 in both buckets but not when there are 250?
Ysjoelfir:
(Noticed that you guys were much faster than me.... but hey, never the less this could be another way to see it :) )
If you short circuit any charge holding device, let it be a capacitor, a battery or your CRT, you basically introduce a second element into the equation: A resistor.
If you use a screwdriver to short out the energy container, like it is often done with CRTs, or if you just use a plain piece of wire, this resistor is very, very small.
Never the less it is there and closes the circuit.
Since - as you/the 10-year-old already noted - energy can't escape (=can't evaporate into nothing), we have to find a way for the energy to "get out" of the energy container and to the surrounding world, leaving the container empty.
This is where said resistor comes into the game. Energy can't be destroyed but it can be changed into another form of energy, in this example the energy is converted by the resistor into heat. If you use a smaller resistor you can absolutely feel it heating up.
If you use the screwdriver or a jumper wire you can't feel it - just because the thermal mass of those "resistors" are so massively huge that the converted energy just doesn't affect your "resistor" in any noticeable way - except to the points where you touched the capacitor: you will most likely see and hear some sparks while touching (because of the high resistance of the insulating air, potential dirt, grease,... on the "resistor" in the moment very very close to the point of it making contact) and see some black burnmarks = produced heat. And that is, where your stored energy went :)
Starglider:
--- Quote from: Ysjoelfir on May 31, 2019, 05:49:46 am ---(Noticed that you guys were much faster than me.... but hey, never the less this could be another way to see it :) )
If you short circuit any charge holding device, let it be a capacitor, a battery or your CRT, you basically introduce a second element into the equation: A resistor.
If you use a screwdriver to short out the energy container, like it is often done with CRTs, or if you just use a plain piece of wire, this resistor is very, very small.
Never the less it is there and closes the circuit.
Since - as you/the 10-year-old already noted - energy can't escape (=can't evaporate into nothing), we have to find a way for the energy to "get out" of the energy container and to the surrounding world, leaving the container empty.
This is where said resistor comes into the game. Energy can't be destroyed but it can be changed into another form of energy, in this example the energy is converted by the resistor into heat. If you use a smaller resistor you can absolutely feel it heating up.
If you use the screwdriver or a jumper wire you can't feel it - just because the thermal mass of those "resistors" are so massively huge that the converted energy just doesn't affect your "resistor" in any noticeable way - except to the points where you touched the capacitor: you will most likely see and hear some sparks while touching (because of the high resistance of the insulating air, potential dirt, grease,... on the "resistor" in the moment very very close to the point of it making contact) and see some black burnmarks = produced heat. And that is, where your stored energy went :)
--- End quote ---
Funny, I asked someone else that and they said that the Sparks were just a side effects. I specifically asked if the 250 volts was being converted into heat and light and dissipating into The ether and was told no. So I guess that is correct after all? does the full 250 volts disappear as a spark?
This assumes that there was 250 volts sitting inside the CRT to begin with, which I know is unlikely after you hit the power especially if it's there is a discharge resistor on the flyback transformer.
Brumby:
--- Quote from: Starglider on May 31, 2019, 05:47:55 am ---Well they might ask: Why is it safe to touch when there are 125 in both buckets but not when there are 250?
--- End quote ---
Because it's only dangerous when there is a difference If you've got the same in both buckets, then there's no reason for any water to move.
It's the same sort of thing as why things at 27ºC don't feel hot or cold - even though they are at 300ºC above absolute zero. It's because they are the same temperature that we feel comfortable.
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