# questions about what actually happens

Discussion in 'Physics' started by stuff, Mar 10, 2008.

1. ### stuff Thread Starter New Member

Mar 10, 2008
1
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I have several basic questions here (stuff I don't understand).
1) current: Say you have some voltage source, eg. a capacitor (that seperates the charges who-knows how. Unfortunately I haven't been explained that. THAT would be good to know). If the voltage source is connected to a circuit, is there an electric field in the wire? In a real wire, does internal resistance start to negate the simplification where all the voltage is supposedly across resistors/capacitors/other devices?
2) What does the magnetic field come from? When I learn magnetism, I admit it starts to sound like circular logic: there is a magnetic force's effect on a particle-some equation-some contrived magnetic field-the field's effect on a particle. Hopefully the answer is not part of that.

2. ### saidin New Member

Mar 10, 2008
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1) a charged capacitor has a voltage across it. When connected to a circuit it will discharge. There is always an electric field around a wire.

This Wikipedia article explains how the magnetic field around a wire is affected by current and the properties of the wire in question.

3. ### studiot AAC Fanatic!

Nov 9, 2007
5,003
523
Some notes that might help.

Voltage Sources

These come in two flavours.

Primary
A Primary Voltage Source is capable of generating electricity from another energy source. For example a battery generates electricity from chemical energy.

Secondary
A Secondary Voltage Source is not capable of generating electricity on its own but can convert electrical energy from a primary source to another (still electrical) form. For example a capacitor stores energy supplied to it.

Magnetic Fields

These are generated by electricity in motion (current flowing). There is no magnetic field associated with a charged capacitor until it is connected to a circuit and current flows.

Charged particles (usually electrons) whizzing around constitute current flow if they are all moving in the same direction. Movement at the sub atomic level causes the magnetism of materials.

Electric Fields

These occur whether there is current flow or not.
They appear in the space between any two points where is is a difference (separation) of charge.
So there is an electric field within a wire that is connected to a battery or charged capacitor.
There is also an electric field around the wire because of the difference in charge between the space around the wire and the charges (current) moving along it.
We measure this difference as potential difference.

There is no electric field in or around a wire that is in free space and not connected to any circuit.
Temporary instantaneous fields appear and disappear at random, but the net effect cancels to zero.

Hope this helps.

4. ### omnispace Member

Jul 25, 2007
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1) The capacitor separates charges because it has an insulator in-between two conductors. There is no electric field inside an ideal wire. A real wire would have a very small electric field on the inside, but the simplification still works for the most part. You just have to make sure your wire gauge can handle the current you want to put through it.

Most of the electric field is on the surface of the wire, pointing outward. This is true whenever the wire has a voltage relative to another wire, whether there is a current in the wire or not.

2) Every electron has some mysterious property known as spin. The magnetic field comes from the spin of electrons that are either moving in a wire or aligned in a magnet. I think when the electrons are all pulled in one direction, their spins all have to be going the same direction as well.

You can also look at the magnetic field as the curling of another mystery field, the vector potential, which for some reason points in the same direction as the current.

5. ### thingmaker3 Retired Moderator

May 16, 2005
5,073
8
An eloquent explaination of capacitors and how they work may be found here: http://www.allaboutcircuits.com/vol_1/chpt_13/index.html

Magnetism works by Pure Freaking Magic. Fortunately, it is a Hermetic magic and therefore always obeys the formulas provided in the textbooks.

6. ### Dave Retired Moderator

Nov 17, 2003
6,960
172
Further to the recommended reading in the AAC e-book, I would also suggest that if you get a chance have a look at Serway and Beichner's book on Physics for Engineers. The sections on electrostatics will cover this in (understandable) detail.

There is also the excellent Electricity and Magnetism e-book (free) - check the Useful Websites for Electronics thread stuck to the top of the Electronics Resources forum for the download link.

Dave