unit 2 electricity Flashcards
current
rate of flow of charge through a point
kirchhoff’s first law
principle conservation of charge
total current passes through a point = 0
I=nAqv
v velocity
n?
q?
n
-number of free charge carrier per unit volume
depends on material
q
- charge of free charge carrier
I=nAqv
v ?
drift velocity
rate change of distance travelled by free charge carriers along the wire per unit time
derivation of I = nAqv
pg 2
derivation of I = nAqv
homogeneous?
unit A
pg 3/4
why no current in plastic compare to copper
plastic is an insulator so n = 0
copper is a conductor n is very large
from I = nAqv , n= 0 in insulator so I = 0
for same current flow and same dimension,
drift speed of the charge carriers in semiconductor is much higher than metal
why
I = nAqv
v is proportional to 1/n
n semiconductor is much lower than metal
so drift speed of semiconductor is much higher
thin wire connected in series with thick wire made of same material
drift speed of electron in thin wire is higher because
- thin wire smaller cross sectional area
- connected in series, same current flow
- same material so same n and Q
- from I = nAqv
- drift speed, v is directly proportional to 1/A
- thinner wire higher drift speed
drift velocity
charged particles move faster through the wire with smaller diameter
why?
because it has a larger potential difference applied to it
explain the different in resistance of conductor with bigger diameter and conductor with smaller diameter
in drift velocity
- drift velocity greater for conductor with smaller diameter. therefore electrons gain more kinetic energy btwn collision with lattice ions
- more frequent collisions with lattice ions
- more energy lost in a given time in collision with lattice ions
- greater p.d required for a given current ( V =E/Q )
- resistance of conductor with the smaller diameter is greater than big diameter conductor ( R=V/I )
as the temperature of the wire increases, ions in the lattice gain higher thermal energy
what happens to drift speed
higher thermal energy, vibrate with larger amplitude
rate of collision of electrons with vibrating ions increase
drift speed decreases
motion of charged carriers in wire
- the charges on the plates of a cell
- attract and repel charge carries in wire - these forces make charge carriers accelerate ( due to electric force ) until they collide with atoms / ions within material
- transfer of energy
- increasing in temperature - charge carrier continues to accelerate as before
- as a result of this cycle of acceleration and collision
- charge carriers settle into constant average speed - they are continually gaining energy from cell and losing energy to material
graph pg 5
potential difference
work done when unit charge flow from one point to another point
electrical energy converted to other form of energy per unit charge
v= work done / charge v= power / current
electric potential energy
E = QV
ideal voltmeter
connected in parallel
infinite internal resistance
no current flows through voltmeter
potential diff across component remain constant
ideal ammeter
connected in series
no resistance
no potential diff across ammeter , current remains constant
electromotive force
the work done by cell to bring unit positive charge through complete circuit
e= work done by cell/ charge
unit : volt
involves change in chemical energy to electric energy per unit charge
kirchhoff’s second law
principle conservation of energy
sum of emf through a complete circuit is equal to sum of potential differences
emf = IR
resistance
potential diff across component per unit of current pass through the component
definition of R: V = IR ( not pd )
R= V/I
unit ohm
ohmmeter
disadvantage
disadvantage
- not very accurate for low resistance component
- reading depends on cell in ohmmeter which may flat as use for long time
while using prevent contact resistance
- resistance to current flow due to surface conditions
- cause high voltage drop in system
contact corrosion leads to
- power loss
- heat generation
ohmmeter
not accurate
so what else can we use
use ammeter and voltmeter to get readings and use formula R = V/I
advantage
- more accurate
disadvantage
- high current heats up component
- change resistance
- may change few values of V and I
if plot V vs I
straight line through origin , gradient = resistance
ohmmeter
advantage
advantage
- very small current flows through the component
- resistance remains constant
ohmmeter
not accurate
so what else can we use
R is not gradient except for ohmic conductor
so how to get R for non ohmic
R=V/I
NOT R=change in V/ change in R
gradient = R=change in V/ change in R = ohmic conductor only , gradient passes through origin
ohmic conductor
conductor that obeys the ohm’s law
define
ohm’s law
- potential difference across ohmic conductor is proportional to the current provided external conditions
- eg. temp remains constant
-for object to obey ohm’s law
resistance remains constant at any values of pd / current
-copper at low current flow obeys ohm’s law cause temp constant
filament lamp an ohmic conductor ?
no
working temp very high
filament lamp NOT an ohmic conductor
explain how resistance change indicates in graph pg 10
- at low voltage the resistance is constant / constant gradient
- when voltage increases resistance increases - at low voltage, temp of lamp constant, reisistance constant
- as voltage increases, current increases
- current heats up lamp temp increases
- resistance of lamp increases
conclusion : as v increases , for equal increases in V leads to smaller increase in I , resistance increases
- do not talk about gradient cause resistance not equal to gradient
filament lamp NOT an ohmic conductor
explain in microscopic term for the increase in resistance
metal, temp increases resistance increases
- R due to collision btwn electrons and ions
- V increases I increases and temp increase
- ions in lattice more thermal energy , vibrate with larger amplitude
- increase frequency of collision
- offer more resistance to the flow of electrons so resistance increases
filament lamp NOT an ohmic conductor
why the filament is more likely to fail when being switch on rather than at other times
initially temp is low , current is high
resistance of lamp increases as temp increases
current falls to steady calue when temp constant
max current, hence max heating when lamp is switched on
filament breaks due to melting caused by temp rise
thermistor NTC
NON ohmic conductor
negative temp coefficient means
when temp rises
resistance of thermistor decreases
thermistor NTC
NON ohmic conductor
graph and symbol
pg 11
graph is a reverse S shaped through origin
symbol rectangle and a hockey stick right through the middle
thermistor NTC
NON ohmic conductor
explain how R change indicates in graph pg 11
as voltage increases, current increases
current heats up the thermistor
temp of thermistor increases
resistance of thermistor decreases
as V increases, for equal increase in V leads to larger increase in I so resistance decreases
thermistor NTC
NON ohmic conductor
explain in microscopic term for the decrease in resistance
semiconductor, temp increases resistance decreases
- V increases I increases and temp increase
- ions in lattice more thermal energy , vibrate with larger amplitude
- more charged carrier set free , n increases
- I = nAqv, I increases, R=V/I resistance decreases
- same time rate of collision bten ions and charge carriers increase
- offer more resistance to the flow of charge carriers so resistance increases
- the decrease in R is much larger than the increase in R so overall effect is resistance decrease
diode including LED
NON OHMIC CONDUCTOR
forward biased , current flows
reverse biased, no current flow
function and pratical use
only allow current to flow in one direction
usage
- rectification change AC to DC
- produce DC supply
- protect components
- power indicator light ( LED )
extra note on ohm’s law
which 2 equations are the representation of resistance R
V=IR
R=V/I (non ohmic conductor ‘s resistance can be found using this ) , but value is not a constant
note: not representation of ohm’s law
resistivity
symbol: p
defined as / formula
unit
( resistance times cross-sectional area )
over
length
unit : ohm metre
resisitivity of a material is also equal to
resistivity depends on
the resistance btwn 2 opposite faces of a cube of the material whose sides are of unit length
resistivity depends on material
