# General electricity

1. Jun 20, 2004

### Alkatran

You'll excuse me for putting such a basic question here, but my high-school exam is tomorrow and I just want to make sure I have things straight! (I'm not very worried here..)

The constant is 9.0* 10^9 right? (nm the units)

1: Current is charge per second (*speed?) (the amount of electrons passing through per second). The reason resistance lowers current is it slows down the electrons (thus the "per second")

2: Voltage is basicly potential gravity. constant*charge1*charge2/distance.

3: R=IV, (series) R = R+R... , I=I=I, V=V+V, (parralel), 1/R = 1/R + 1/R.., I = I+I, V=V=V

4: Magnetic fields. The force around a wire is constant*current/distance (more current means more force, more distance means less force, distance to power 1 since it's a cylinder and not a sphere)

5: Charges are exactly like gravity, except with the possibility of a repulsion. (kinoof 2, here)

6: The equation for the magnetic field between two plates is: Field = Voltage/Distance (between plates). The field is uniform across the entire area.

uhm... (keeps thinking...) I think that's all. I wish I still had my physics book and I didn't have to "study from memory" (you know, go over it so you don't need to spend that downtiem during the exam).

2. Jun 20, 2004

### Gza

A magnetic field is NOT a force.

I think the better analogy would be between charge and GRAVITATIONAL MASS, not gravity itself.

You have described electric potential energy of the system of charges, charge 1 and charge 2, not voltage. Voltage is the work per unit charge to get from point A to point B.

3. Jun 20, 2004

### Alkatran

My bad. I meant the "strength".

Yep, thanks.

This one's got me worried, because I was always mixing up the ones with two charges with ones with oen charge (field vs other charge) and the /r^2 with /r (I am no good with terms, I work on ideas). Also, I take all of this in french, so !!. V = J/C, yes, I knew that, I remember now. One volt is when it takes 1 J to move 1C from infinity to the point you're checking, correct?

4. Jun 20, 2004

### chroot

Staff Emeritus
There seem to be a lot of problems here actually...

What constant are you talking about? And don't even say "nevermind" about units -- units are your best friend and will help you more times than you can imagine, if you'll let them. Don't ever forget units.

Current is the number of coulombs of charge passing a point in a given amount of time.

Well, it's not gravity -- but the equation is correct. Potential goes as the inverse of the distance.

Ohm's Law is V = IR, not R = IV.

This is the sort of place where units will help your memory. Electric fields are commonly represented in units of volts/meter, or units of newtons/coulomb. Keeping track of the units will make your life loads and loads easier.

- Warren

5. Jun 21, 2004

### HallsofIvy

Staff Emeritus
"The constant is 9.0* 10^9 right? (nm the units)"

I thought he meant that the units were "Newton-meters"! (i.e. work)

6. Jun 21, 2004

### Alkatran

I don't need to know the constants because I know the formula it is in:

F = k*c1*c2/r^2

since the units of c1*c2/r^2 are C^2/M^2 and the units of F are Kg*m/s^2, u must have the units needed to convert from one to the other.

7. Jun 21, 2004

### Alkatran

Well, it turns out we were given the (unbelievable) luxury of all the equations and constants printed out on a page at the back of the exam. Imagine my surprise when I came upon it as I finished :rofl: .

Anyways, the whole thing went well. I got an 89 (yes, our teacher marks THAT fast), highest in the class. Thanks for your help.