Questions regarding basic electricity and unit conversion

[psyhprog]

I originally wanted to post this in the EE forum, but after seeing the sticky there, I decided that this might be a better place.

1. I got the whole thing about unit conversions (like from V to kV, move the decimal point 3 to the left -- the number becomes smaller by 10^3). Right now, we're at Classical Mechanics (motion, kinematics, static etc.) and I don't know how to convert composed units. Like how do I convert from m/s to km/h, or backwards from km/h to m/s. A step-by-step explanation would be most appreciated.

2. I took up Electricity on my own, kind of a hobby, and I started studying the textbook. So far, just the first chapter, brushed up on some old concepts. There are a few things I don't quite get though. Energy (joules), power (watts). What they really mean, why P = E * I, and all of that. Could anyone shed some light on this?

 

[glenn21]

For 1.
If you have for example 60m/s and want to convert to km/hr you do the following:

(60*1000km)/(60*60hr)=16.67km/hr. (You convert the numerator to km, and the denominator to hours).

There is also a very nice conversion factor: 60/3.6=16.67km/hr, to go the other way simply multiply by 3.6.

For 2.
Power=J/s. Basically if you have a high power in your circuit, it means that a lot of energy is being converted per second. If your circuit contains a resistor and P is high, then lots of energy is being converted to thermal energy each second. P=EI because E is in volts which is J/C and I is current which is C/s and so if you multiply them together your left with J/s which is the unit of power.

 

[psyhprog]

Thank you for your answer, I think I get it. One more thing. When you're dividing by something like 2,5 , how do you do the division? I remember that for multiplication you did it normally, you counted how many decimals each number had, you added them up and that's how.many decimals the result has.