# Stupid Physics Question

Discussion in 'Physics' started by crazyengineer, Jun 3, 2011.

1. ### crazyengineer Thread Starter Member

Dec 29, 2010
156
2
Hello Everyone,

I just wanted to check to make sure my understanding is correct. Let say I have a simple 1.5 volt battery with a positive and negative terminals. Then I take each terminal and connect it to it's own sheet of metal. Does that mean one piece of metal has a positive 1.5v and one has a negative 1.5v. If so, what rudimentary physics equation can I use to find the charge on each sheet of metal?

2. ### beenthere Retired Moderator

Apr 20, 2004
15,815
282
No, it means there is a difference in potential of 1.5 volts between the plates.

Jul 7, 2009
1,585
141
Actually, it's not a stupid question. You'd find the charge with the equation for a capacitor Q = CV. However, you haven't specified the size of the plates nor their orientation, so the problem has an indeterminate answer because we don't know the capacitance C (but read more here). And, except for some simple geometries, it's not going to be trivial to calculate the resulting capacitance.

However, it's an experiment you can do yourself and, without a heck of a lot of work, you could make a circuit that would integrate the current from one of the plates to ground, letting you actually measure the charge that was put on the plate when you connected the battery. It might be kind of a fun experiment to do.

So there are lots of things that could be learned from it.

4. ### ErnieM AAC Fanatic!

Apr 24, 2011
7,395
1,607
I believe you have fallen victim to a common beginner's misunderstanding, that a voltage can exist at a point. It is a frequent stumbling point as you have probably heard many many times people say "the voltage on the widget is (some) volts." and they are clearly talking about one point where the voltage is.

In reality, a voltage only exists between two points, and there is no such thing as a voltage at a point (or if there is that voltage is always zero).

When people speak of a voltage at some point, it is a shorthand abbreviation of the voltage at the named point and some standard pre-agreed upon reference point.

In many many cases that reference point is called the "ground" of the circuit. Keep in mind that "ground" can mean many many things from an isolated terminal of a battery to a black terminal on a power supply to literally the earth itself.

Say your entire world consists of that battery, the two plates, and of course a meter to measure voltage. If you put the + of the meter on the positive terminal the only other place to put the other meter lead is the negative terminal, and you will read +1.5V

If on the other hand you put the + of the meter on the negative terminal the only other place to put the other meter lead is the positive terminal, and you will indeed read -1.5V.

But all you have done there is reversed the leads of the meter. The 1.5V exists between the terminals.

5. ### victorhugo289 Member

Aug 24, 2010
49
3
I don't know exactly what you're asking but when it comes to batteries I always try to keep this in mind:

For an AA battery:

----------------------Voltage: 1.5 V
---------Internal resistance: 0.5 ohm (average)
---------Amp-hour capacity: 2 amp-hours @ 50mA (for AA only)
----Estimated working time: 40 hours
-Total charge in Coulombs: 7200 Coulombs

Now:

7200 Coulombs * 6.25x10^18 electrons/Coulomb = 45 zetta-electrons.