ch 10 - resistance and resistivity Flashcards
I-V characteristic
a graph of current against voltage for a particular component of an electrical circuit
resistance formula
R = V/I
resistance on an IV graph (if its straight line through origin)
reciprocal of gradient
CHECK AXIS IN CASE THEY ARE NOT SWITCHED
if connections to a resistor is switched
pd and current become opposite/negative
what is a resistor
an electical component whose resistance in a circuit remains constant, is independent of current or pd
Ohm’s law is obeyed when
current in a conductor is directly proportional to the pd across its ends
IV characteristic will show straight line through origin
ohmic component
a component that obeys Ohm’s laws
non ohmic component
a component that does not obey Ohm’s laws
filament lamp is (ohmic/non)
non ohmic
filament lamp graph
. line passes through origin
. roughly straight line for voltages close to zero
. curves towards x axis for very high and very low voltages, more resistance
thermistors are (ohmic/non)
non ohmic
thermistors
. made from metal oxides, eg. manganese and nickel
TYPES:–
. PTC - positive temperature coefficient (more T = more resistance) UNNEEDED
. NTC - negative temperature coefficient (more T = less resistance)
USES:–
. ice sensors on aircraft wings
. water temperature sensors in cars
. baby breathing monitors
. fire sensors
. overload protection in electric razor sockets
NTC thermistor
a device whose resistance decreases rapidly when the temperature increases
semiconductor diode is (ohmic/non)
non ohmic
diode
a component that allows current in one direction
diode details
. LED is a type, gives out light when it conducts
. graph : near 0 until positive V value (usually 6), when it shoots up
. when forward biased - conducts, fairly low resistance
. when reverse biased - tiny current, infinite resistance
. starts to conduct at threshold voltage
. used as rectifiers ( I in only one direction)
. AC to. DC
USUALLY
. made from silicon
. threshold V is 0.6
diode details
. LED is a type, gives out light when it conducts
. graph : near 0 until positive V value (usually 6), when it shoots up
. when forward biased - conducts, fairly low resistance
. when reverse biased - tiny current, infinite resistance
. starts to conduct at threshold voltage
. used as rectifiers ( I in only one direction)
. AC to. DC
USUALLY
. made from silicon
. threshold V is 0.6
threshold voltage
the minimum forward bias potential different across a LED when is starts to conduct and emit light
0.6
when forward biased
conducts, fairly low resistance
when reverse biased
tiny current, infinite resistance
LEDs replacing filament lamps
. more costs for manufacture
. more energy efficient
LDR
light dependent resistor
component whose resistance decreases when the light intensity increases
LDR
. made of high resistance semiconductor
. if light falling is of high enough frequency, photons are absorbed by the semiconductor
. electrons thus releases
. better conduction
. resistance reduced
more light = less r
factors affecting resistance
MORE
. temperature
. presence of impurities
MORE
resistance
semiconductor conduction
more temperature = more conduction so less r
in wire, r depends on
. length L
. cross sectional area A
. material wire is made from
. temperature of wire
R formulas and proportionalities
R prop. to L
R prop. to 1/A
R prop. to L/A
R = pL / A
what is resistivity
. a property of a material
. measure of its electrical resistance defined by p = RA/L
. units - ohm m
more temperature =
more resistivity =
more resistance
double L in circuit is like
R plus R, R increases
double A in a circuit is like
parallel, R decreases
