Sep 9th
Some useful remainders
- If \( \Delta Q\) is the amount of passing through an area in a time interval \( t \) the average current \( I \) is: \( I = \frac{\Delta Q}{\Delta t} \).
- For instantaneous current \( \Delta t \to 0\) then \( \frac{\Delta Q}{\Delta t} \) = Coulombs per second = Amps.
- If you want to find out how much current has traveled through something, \( \Delta Q = I * \Delta t = Amps * sec = charge \) .
- "Electrons flow in the opposite direction as defined by current," pg. 7
- We don't change the equations around this fact because as long as our "positive flow" equations are sound, the inverse is proportional.
- if you ever see "electron flow" note that it is moving opposite to conventional \(I\)
- Formally speaking: \( V = \frac{J}{Q} \) where V is volts, J is joules (energy), Q is coulombs (charge). Or "A 'Volt' is a Joule per Coulomb."
- To measure power consumptions (watts): \(P = VI\) (watts = volts * amps). Example: if we have a 1.5 v circuit that draws 0.1 amps, it consumes 0.15 watts (1.5*0.1 = 0.15).
- Inversely, if you need to figure out the power draw (amps), divide the consumption (watts) by the 'pressure' (voltage). Voltage can also be deduced from with this method if you use amps instead of volts.
- Resistance (ohms)/ Ohm's Law (statement): \(R = \frac{V}{I}\) where R = ohms, V = volts, and I = amps. This rule only applies to material thats resistance remains constant under different voltages: ohmic material.
- Resistivity: \( \rho = R\frac{A}{L}\) where R = resistance, A = cross-sectional area, and L = length. Conductivity is mathematical inverse of resistivity.
Reactions
For the most part, it all seems pretty straightforward. The stuff about heat went a little over me, but other then that it was fine. I guess I want to know what to do in circuit analysis if I encounter non-ohmic materials. The section about grounding quite interesting. Other than that, I think this is a fine pace to move through the book over the following weeks.
Jim says :
Yes.
This is a bit of calculus and a few unit definitions.
I've added a few clarifications and modifications
and changed some variable names to match common usage :
Q charge in Coulombs ; 1 electron = - 1.60217657 × 10-19 Coulomb
I current in Amps = Coulomb/sec = Q / t (or dQ/dt if time varying)
1 Amp is defined as 1 Coulomb / (1 sec)
V voltage in Volts = energy/charge
1 Volt is defined as 1 Joule / (1 Coulomb)
(You can use the page history to see the old version.)
For practical electronics, power in Watts = 1 Joule / (1 sec)
is used much more often than energy itself. Energy, by
the way, has a gazillion different units :
joule MKS unit
erg cgs unit
BTU British Thermal Unit (heating)
calorie energy to raise 1gm of water 1 degree C ~ 4.2 Joules
Calorie or kcal 1000 calorie ; what they put on food labels
kW hr 1000 (Joule/sec) * 3600 sec = what's on your electric bill
...
Also note power of 10 variations :
kV mV µV
mA µA
Should also do resistance (in Ohms = Ω ) here ...
