Kirchhoff’s 1st law
The sum of the currents entering a point in a circuit is equal to the sum of the currents leaving the same point
- conservation of charge
Equation for charge
Q = It
equation for number of electrons
Q/e = no. e
What is mean drift velocity?
The average displacement of electrons as they move through a wire per unit time.
UInits: v ms-1
What is number density?
The number of conduction electrons per unit volume.
(n)
Equation for current with cross sectional area?
I = nAve
where:
v = mean drift velocity
A = cross sectional area
e = fundamental charge
n = number density
Why are electrons slow in copper wire?
-The copper atoms are continuously vibrating, collide with delocalized electrons and slow down
- This repeats and as a result of the haphazard nature of motion the mean drift velocity is slow.
typical values of number density and mean drift velocity
METAL:
number density:
10^28 m-3
Mean drifty velocity:
2x10^-5 m s-1
INSULATOR:
number density:
10^3 m-3
Mean drifty velocity:
1x10^-31 m s-1
Effect of cross sectional area on drift velocity
cross sectional area inversely proportional to drifty velocity
A = I/nve
