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Electrical charges on plates

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LAN_403

Plasma

Sun Feb 25 2018, 05:22AM

Registered Member #61406 Joined: Thu Jan 05 2017, 11:31PM
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Posts: 268

Hi, I'm just wondering about some things. Say you have two plates one is +2Q the other is - 1Q,when you connect them what would the value be.
the second question if you have two plates both are +1Q and connect them what would the charge be.
Cheers

Dr. Slack

Sun Feb 25 2018, 07:15AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

You need to think carefully about exactly what Q is. I'm assuming that it's a measure of the quantity of charge, the number of coulombs with respect to the body being electrically neutral. Making some simple assumptions ...

If you connect two bodies that had initially +2Q and -1Q, the total new charge would +1Q.

If you connect two bodies that had initially +1Q and +1Q, the total new charge would be +2Q.

I'm struggling to think behind the question to figure out why you don't think the answer is obvious, otherwise you wouldn't have asked.

In the second case, one might be tempted to take the average rather than the sum, but then one would be thinking of voltage rather than charge. With two bodies, the capacitance has doubled, so the charge per voltage has doubled.

Notice I've stuck to the terminology 'bodies' rather than 'plates'. The latter implies some extra geometry which could confuse the situation, given the response of charges to electric fields. Your question is woefully short on detail, so only by going to 'bodies', with implied capacitance to infinity and not to each other, is it possible to make some assumptions and get to a simple answer. Sometimes, depending on geometry, it's possible to configure bodies such that the charge won't flow when they're connected, see how to charge an electrophorous

for example. This uses electric fields to move the charge around at constant voltage. However, whether the charge moves or not, two connected bodies will have the same total charge as it's a simple book-keeping exercise to add up the total charge wherever residing on both.

Plasma

Sun Feb 25 2018, 01:33PM

Registered Member #61406 Joined: Thu Jan 05 2017, 11:31PM
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Posts: 268

Thanks I wasn't quite sure, I thought maybe question one could be +0.5Q as maybe the +1Q would get shared over two objects.
the second question I thought if you charge up a plates on a capacitor then remove one plate and replace it with another, you could have more charge by then connecting the two plates.
Thank you

johnf

Mon Feb 26 2018, 06:12AM

Registered Member #230 Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284

Plasma
time to mess with your head
make a two plate capacitor with a sheet of teflon or mylar or whatever you have to put between the plates make it bigger so you can remove it without touching the metal plates

make a second set plates no teflon or mylar but spacing the same as the first.
Charge up the first set of plates carefully remove the plastic sheet and put it between the second set of plates.
now measure the voltage on the second set of plates.

This now should destroy everything any of you have leart about capacitors distributing charge over their plates.

Further reading to be done under the CATT anamoly

Plasma

Wed Feb 28 2018, 07:39AM

Registered Member #61406 Joined: Thu Jan 05 2017, 11:31PM
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Posts: 268

Johnf I think I understand it, the delitic forms a dipole of positive and negative, the plates pick that up.
what if you charge up two insulator and stage them on top of the over. What would the voltage be if they were indunval charged to 12volt.
I can't test this as all the settings on the multimeter read out of bounds.
cheers

Sulaiman

Wed Feb 28 2018, 09:02PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

johnf wrote ...

Plasma
time to mess with your head
make a two plate capacitor with a sheet of teflon or mylar or whatever you have to put between the plates make it bigger so you can remove it without touching the metal plates

make a second set plates no teflon or mylar but spacing the same as the first.
Charge up the first set of plates carefully remove the plastic sheet and put it between the second set of plates.
now measure the voltage on the second set of plates.

This now should destroy everything any of you have leart about capacitors distributing charge over their plates.

Further reading to be done under the CATT anamoly

From your description, I'm guessing that you have not done the dielectric/plates thing yourself, just read about it ?
................
If you consider each part of the circuit vs. 'the universe' and the starting conditions, there seems to be no CATT anomaly, just an obfuscating question syntax.
Understand why v < c and the rest is obvious. (typ. v = 0.6c to 0.9c)

Plasma

Thu Mar 01 2018, 06:37AM

Registered Member #61406 Joined: Thu Jan 05 2017, 11:31PM
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Posts: 268

Got a multi, the charge is on the metal plates,maybe... More testing

Plasma

Sat Mar 03 2018, 07:52PM

Registered Member #61406 Joined: Thu Jan 05 2017, 11:31PM
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Suliman how did you get the 0.6-0.9 figures.
the capatance seems to lower the voltage read. Is it that or the dielectric constant of the material, as plasma has one of the highest, should it be worth testing that.
Thanks

Sulaiman

Sat Mar 03 2018, 08:27PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Plasma wrote ...

Suliman how did you get the 0.6-0.9 figures.
the capatance seems to lower the voltage read. Is it that or the dielectric constant of the material, as plasma has one of the highest, should it be worth testing that.
Thanks

Frankly, I plucked them out of the air,
electromagnetic signals travel along co-ax cables typ. 0.66c to 0.8c,
a parallel pair of wires in space, or one wire against a plane/the earth, forms a transmission line, with equivalent series inductance and parallel capacitance, an impedance.
I think c0.95c for a single wire above earth.
(for a wire I use 1uH/m and 5 to 10 pF/m as a rule-of-thumb)
So the numbers are not precise, but representative.
I'm sure numbers <0.6c and >0.95c are common under other circumstances.

In reality, when you transfer charge from one object to another,
the connection path has 'inductance', and the bodies have 'capacitance',
this 'tuned circuit' will oscillate.
A little more complex than the simple theoretical model
- which would require infinite current flow for an infinitesimal time.

The thing to remember about inductance and capacitance is that they are simplified representations of quantum stuff that confuses me, so if engineering models are used at extremes, they need modifying.

+Q and -Q are relative to what ?
normally it is implied relative to earth, the body must have capacitance and voltage to hold a charge (dQ=C.dV etc.)
so the bodies by definition start at different potentials .....

Plasma

Sun Mar 04 2018, 11:44AM

Registered Member #61406 Joined: Thu Jan 05 2017, 11:31PM
Location:
Posts: 268

I measure the ac voltage and subtract it from the dc voltage. How would you minimise the leakage current, would that be apart of the ac part.
I got it wrong before, stacking the same charge plates together increase the voltage registered.

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