Chapter 8 Flashcards
Current
Equation + units
Rate of flow of CHARGE
thus I = Change in charge / change in time
Columbus / second
Elementary charge
1.6 x 10^-19
Why is charge quantised
Net charge is +- n x E
Charge is quantised because It csn’t be anything, it is only integer values of e (csn’t be half an e)
What carries charge
What are electro,gets
Positive negative ions, electrons
Liquids that can carry current
How current go through metal atoms
Metal atoms have delocalised electrons due to metallic binding. This if charge is applied current can go through and connect a circuit
Conservation if charge
Charge can not be made or destroyed, total amount in universe is constant
Kirchhofs first law
Even if current is leaked?
Sum of current going into a junctions = to sum of current coming out of the junction
- to do with conservation if charge, as current is charge / time, and if current is conserved then charge is conserved as it is not crested here nor destroyed, and total amount is still constant
Thus - total number of charge carriers entering MUST = total number if charge carriers leaving
2) even if current is leaked , earth will give back the current to the circuit, so charge is conserved and Kirchhofs First law stands
Denote Kirchhofs First law
E In = EIout
If I1 is positive and I2 is negative then
EI in junction = 0
Number density ?
Number density is number of FREE electrons PER cubic metre of material .
Higher number density = higher electrons = better material
Classify materials using density how
Copper as example of conductor ?
Insulators like wax = dint allow electronsto move = low number density
Semi conductors like silicon / germanium = let more through so = higher number density
Conductors , lead zinc copper = let a lot through = so highest number density
2) copper and conductors have high magnitudes to order of 10^28
Thus semiconductors have to move faster to reach same current as normal
Equation for electric current
I = A (area) N (number density) E (electrical charge ) V (mean drift velocity )
I = ANEV
Cross sectional arrea and relationship to mean drift velocity
However?
(Remember )
If I =ANEV
THEN V = I/ANE
SO V = PROPERIONAL TO 1/ A
Such that if you decrease area you mus increase velocity so current is the same, inversely proportional
HOWEVER RESISTANCE WILL INCREASE WITH LESS AREA TOO
3) remember radius will decrease area by squared etc
explain the term mean drift be,o it’s of electrons in a metal wire
FULL THREE MARKER
Learn
- The fre electrons make collisions with the positive metal ions in wire, giving them a RANDOM VELOCITY
- free electrons drift towards the Positive end of the supply along the length of the wire
- the distance travelled per unit of time in the length of the wire is called the MEAN DRIFT VELOCITY
