Physics : charge and current Flashcards
semiconductor
number density ~10^17
not as many electrons free to conduct charge
Conductor
number density ~ 10^28
insulator
a few free conductor electrons m^-3
number density
number of free conduction electrons per unit volume
E.M.F.
Work done on charge carriers to “push” them around a circuit
(Chemical -> electrical)
Potential difference
Work done by charge carriers “on” components
(electrical -> whatever)
mean drift velocity
-average displacement travelled by e- along the wire/second
-move slowly in one direction through metal lattice when P.D. across wire -as they collide constantly w lattice
Oil drop experiment
analysed movement of electrically charged oil droplets
plates changed until oil droplet stationary
mg = electrostatic force + buoyancy
Physically increasing current
More electrons pass a point /s - higher number density/area
same number of electrons moving faster
Kirchhoff’s 2nd law
around any closed loop in a circuit, the directed sum of potential differences across components is zero
Kirchhoff’s 1st law
All current into a junction equals the current out of a junction
Electron gun
hot filament (cathode) (thermionic emission)
electrons accelerate to anode (via high P.d.)
e- focus in ring to form beam
linear particle accelerator
e- accelerate to cylindrical drift tube (+ charge)
polarity alternates to accelerate to next drift tube
electrodes get longer so timing remains ok
V=Q/V alternate form
eV = 1/2mv^2
(work done = gain in kinetic energy)
Definition of resistance
how much a component resists the flow of charge
(ratio of P.D per unit current)
relationship between resistance and temperature
increase in thermal E -> increase in KE
more internal energy -> vibrate with greater amplitude
more collisions and charge carriers do more work
Diodes
current flows in only one direction
LED emits light of a specific wavelength
Semiconductor
why does a voltmeter show 0V when connected across 2 parallel branches?
calculate P.D. of both components (should be =)
as they have the same p.ds, there is no current flowing through voltmeter
explain m.d.v. of electrons in metal wire
free e-‘s make collisions with metal ions => gives random velocity
free e-‘s drift to + end of supply along length of the wire
distance travelled/t along the length of the wire= m.d.v. of e-‘s
