Electricity 2 Flashcards
Current
The flow of charge
3 examples of current
passage of electrons along a copper wire
motion of ions between electrodes in a solution
motion of positive holes and electrons in a semiconductor
current is also known as
charge per unit time passing a point
I =
Q/t
I
resistance
Q
charge
t
time
unit of I
Ampere (A)
unit of Q
Coulumb (C)
1 ampere =
1 Coulumb/1 second
modern current name
actual current
old-style current name
conventional current
which direction does conventional current flow?
positive to negative
in which direction does actual current flow?
negative to positive
easy explanation of current
number of electrons flowing in the wire
easy explanation of voltage
ability of electrons to do a task
how to find number of electrons
total charge / charge on one electron
what did ohm notice?
potential difference was directly proportional to the current if the temperature remained constant
what is resistance
the voltage across a conductor divided by the current flowing through
3 factors affecting resistance
length
resistivity
1/cross sectional area
p=
RA/l
p
resistivity
R
resistance
A
cross sectional area
definition of resistivity
the resistivity of a conductor is the resistance of the conductor of length 1m and cross-sectional area of 1 m squared
unit of resistivity
ohm meter
cross-sectional area
pi r squared
diameter
2r
what do you measure the diameter with?
micrometer
name of thing that olds wire in place when measuring diameter
grip/teeth
what do you tighten teeth when measuring diameter
slip control
how do you measure diameter accurately on the wire
take several measurements down the wires and get the average
how to calculate resistivity in the experiment
p= R x pi r squared / l
1 more accuracy technique in experiment of measuring resistivity
avoid small values of length beacuse measuring small values of length, result in greater percentage error
length and resistance graph, whats on x-axis
length
length and resistance graph, whats on y-axis
resistance
length and resistance graph, whats the resistivity
slope x pi r squared
tiny notches on vernier scale go up in
2s
2 scales on micrometer
main scale
vernier scale
what do you have to make sure before you measure the length of a wire
make sure it’s straight and taut
what do you check on micrometer before measuring
check the zero error
(2) higher temperature=
increase in resistance
increase in diameter
how do you use a wheatstone bridge
by trial and error change the values of the resistors in the circuit until the galvanometer reads zero
wheatstone bridge culculation
R1/R2 = R3/R4
wheatstone bridge top left
R1
wheatstone bridge bottom left
R3
wheatstone bridge top right
R2
wheatstone bridge bottom right
R4
advantage of wheatstone bridge
compact
disadvantage of wheatstone bridge
expensive
how do you use a meter bridge
the sliding contact is moved along the uniform resistance wire until the galvanometer reads zero
meter bridge calculation
Rx x I2 = Ry x I1
meter bridge top left
Rx
meter bridge top right
Ry
meter bridge bottom left
I1
meter bridge bottom right
I2
advantage of meter bridge
very accurate
disadvantage of meter bridge
very large
advantage of ohmmeter
fast
disadvantage of ohmmeter
less accurate
2 practical uses wheatstone bridge
temporary control
fail safe device
explain temporary control as a wheatstone bridge
to maintain a room at a fixed temperature
how is a fail safe device a wheatstone bridge
to prevent poisonous leaking from a boiler
explain the fail safe gas boiler system
- thermistor positions above the flame
- if the flame goes out then the resitance changes and wheatstone bridge is no longer balanced
- current flows through and is used to turn off a switch which cuts off the gas supply,
why do you need a failsafe device in a gas boiler
because if the gas didnt burn it could seep into the house and it is poisonous
resistors in series
Rt = R1 + R2 + R3
resisitors in parallel
1/Rt = 1/R1 + 1/R2 + 1/R3
combining resistors in series
bigger resistance
combining resistors in parallel
smaller resistance
6 steps in in long question
- resistance in parallel
- total resistance (in series)
- find total current
- find voltage across AB
- therefore voltage across BC
- current in that one resistor
steps for calculating current flowing through a certain resistor
calculate resistance in parallel (1/Rt etc)
total resistance in series
total current (I=V/R)
voltage across resistors in series (Vab=IxR)
therefore voltage left for other resistors (total - ^^^)
current in that resistor (I = V/R)
moving the sliding contact of a potential divider up as much as possible gives a voltage of…
whatever the battery is
moving the sliding contact of a potential divider down as much as possible gives a voltage of…
0 V
ratio of voltage divided in what ratio?
the resistance values
emf
electromotive force
symbol of emf
E
unit of emf
Volt
what is emf
the same as potential difference but only for things that make their own electricity
3 sources of emf
an electric cell (battery)
ESB mains
a thermocouple
what does emf depend on in a battery
it depends on the chemicals used in the cell
what is the emf of the ESB mains?
230 V
what does the emf depend on in a thermocouple
the material of the two wires and the temperature difference between the ends on the wires
sliding contact of a potential divider
long rectangle
symbol for voltameter
V in a circle
symbol for milliammeter
mA in a circle
graph of voltage and current, which is on x-axis?`
voltage
graph of voltage and current, which is on y-axis
current
graph of voltage and current for metallic conductor (wire)
straight line graph through the origin
what must remain constant in current and voltage against a wire?
temperature must remain constant
3 factors of accuracy in metallic conductor (wire) experiment
- avoid large currents, causes wire to heat up, non linear graph
- ensure good electrical contacts, poor contacts causes variation in resistance
- check voltmeter and milliammeter for zero error and correct
symbol for a filament bulb
a rainbow shape with a circle around it
3 steps in sliding contacts experiments
- adjust sliding contact until voltmeter reads zero
record milliammeter value - adjust until voltmeter reads one and record milliammeter value
- do the same for 2V, 3V, 4V etc.
3 notes on accuracy
- avoid very low currents, bulb will blow
- good electrical contacts, or there will be variations in resistance
- check voltmeter and milliammeter for 0 error and correct if necessary
graph of filament bulb experiment
non-linear
what does it mean that filament bulb graph is non-linear?
Ohm’s law is not obeyed
why is ohm’s law not obeyed in filament bulb experiment?
current increases, temperature increases, increased thermal vibrations in the atoms of the filament
more difficult for the electrons to flow through the filament, resistance of the filament increases
To investigate the variation of the resistance of a metallic conductor with temperature
WHAT LIQUID IS IN THE BEAKER
glycerol
To investigate the variation of the resistance of a metallic conductor with temperature
THE TEMPERATURE OF THE METALLIC CONDUCTOR IS THE SAME AS
the temperature of the oil
To investigate the variation of the resistance of a metallic conductor with temperature
WHAT’S ON THE Y AXIS?
RESISTANCE
To investigate the variation of the resistance of a metallic conductor with temperature
X-AXIS
TEMPERATURE
To investigate the variation of the resistance of a metallic conductor with temperature
2 POINTS ON ACCURACY
check ohmmeter for zero error and correct if necessary
ensure good electrical contacts between the metallic conductor and the ohmmeter as poor electrical contacts cause a variation in resistance
Heat = (2)
V I t
I² R t
Power = (2)
V I
I²R
unit of energy by ESB
kilowatt hour
units of energy =
kilowatts x hours
current has a (2)
current has a heating effect
current has a magnetic effect
practical example to show the heating effect of electricity
boiling a kettle
cathode is connected to which side
cathode to negative
anode is connected to which side
anode to positive
which loses mass the cathode or the anode?
copper anode loses mass
which one gains mass the cathode or the anode?
copper cathode gains mass
electrolysis
current causing a chemical reaction
2 practical examples of electrolysis
ELECTROPLATING a less expensive metal with silver
RECHARGING your mobile phone battery
what happens to a compass when current flows through it
turns 90º from original position
experiment to verify joule’s law
time interval to allow water to heat up
3 minutes
experiment to verify joule’s law
y -axis
temperature rise
experiment to verify joule’s law
x-axis
current ²
type of graph
straight-line graph through the origin
joules law
P ∝ I ²
experiment to verify joule’s law
2 points on accuracy
make sure the interval for which the current flows is fixed and the mass of the water is constant
check ammeter for zero error and correct if necessary
showing that temperature rise ∝ I ² is equivalent to showing that
P ∝ I ²
heat generated by electricity =
heat gain of water
I ² R t =
m c Δ θ
in the power station voltage is
x1000
in the power station current is
÷1000
what multiplies the voltage in the power station
a step-up transformer
why is current ÷1000
to obey the conservation of energy
heat generated in transmission cables is
÷ 1 000 000
why is heat generated in transmission cables ÷ 1 000 000
joules law (Δθ ∝ I²)
heat losses to the air reduced by a factor of
1 000 000
what doe it mean that heat losses to the air reduced by a factor of 1 000 000
a huge reduction of energy losses and a huge saving in money terms
brown wire
live wire
blue wire
neutral wire
green/yellow wire
earth wire
ring circuit
current takes 2 paths from the fuse to the socket
current along each path in ring circuit
half the current the appliance will use
ring circuit, heat generated reduced
by a factor of 4
what is the reduced risk of overheating in a ring circuit
a definite safety factor
fuse
a protective device that cuts off the flow of current if the current is about exceed a certain value
what does the fuse limit
limits the current that can flow in a circuit
what does the fuse prevent
prevents a wire from overheating and therefore reduces the risk of a fire starting
what does a fuse not do
does not prevent a person from being electrocuted
mains voltage
230 V
I =
P/V
what fuse do you choose
with a current rating above the current you figure out with the closest value
what appliances have an earth wire
appliances with metal on their outer surface
what does an earth connection provide?
a good conducting path from the outer metal surface to the earth
if the live wire accidentally came in contact with the outer metal surface of an appliance, what would happen?
flow through the earth rather than through a person who may be in contact with the appliance
what does the earth connection protect?
protects the person from electric shock
MCB
miniature circuit breaker
what does the MCB do?
limits the current flowing in a circuit
fuse blows, MCB …
trips
when an MCB trips
can be reset by the flick of a switch
if the current flowing in an electromagnet increases…
the magnet flux density is quite large
what can increased magnetic flux density do
can be used to break the circuit by opening the switch
RCD
residual current device
current in live wire and neutral wire
should be the same
if current in live and neutral are not the same
RCD wil be activated and current will stop immediately
response of RCD
very fast
what does the RDC offer?
great protection to a person against the dangers of electrocution
bonding
if live wire touches any metal objects in the house the current will flow into the earth not a person touching them, protects from electrocution
MEASURING THE RESISTIVITY OF A NICHROME WIRE:
one way to make a reading more accurate
use a more sensitive ohmmeter
derive equation for resistors in series
Vt = V1 + V2 + V3
Rt It = R1I1 + R2I2 + R3I3
all I values are equal ->
Rt = R1 + R2 + R3
derive formula for resistors in series
Vt = V1 = V2 = V3 total current split in 3 It = I1 + I2 + I3 Vt/Rt = V1/R1 + V2/R2 + V3/R3 all V values are the same 1 / Rt = 1/R1 + 1/R2 + 1/R3
inert
unreactive
Current voltage graph for: metals
straight line through the origin
Current voltage graph for: filament bulb
curves up and across
Current voltage graph for: ionic solution (active electrodes)
straight line through the origin
Current voltage graph for: ionic solution (inert electrodes)
straight line a little bit to the right of the origin
Current voltage graph for: gases
straight line through the origin, then horizontal, then diagonal again
Current voltage graph for: vacuum
straight line through the origin, then horizontal
Current voltage graph for: semiconductor
curves right and then up
definition of semiconductor
a material with conductivity or resistivity between that of a conductor and an insulator
intrinsic conduction
when a semiconductor material starts to conduct due to heating the material
explain intrinsic conduction
as the temperature increases the atoms of silicone vibrate more vigorously, some of the bonds are broken, free electrons and positive holes now available to act as conductors
for semiconductors; the higher the temperature
the greater the conduction
LDR
light dependent resistor
as the intensity of light falling on it increases the level of conduction increases
other name for extrinsic conduction
doping
definition of extrinsic conduction
when the addition of a controlled quantity of an impurity increases conduction
what do you add in n-type doping and why?
a group V element such as phosphorous as 5 outer electrons, 4 involved in bonding and 5th acts as a conductor
majority charge carrier in n-type doping
the 5th outer electron in phosphorous
why is it called n-type doping
the majority charge carrier is negative (electron)
what is added in p-type doping and why?
a group III element such as boron is added, 3 outer electrons, creates a positive hole, available to act as a conductor
majority charge carrier in p-type doping
the positive hole
p-n junction 2 sides
a piece of silicon is p-type doped on the left and n-type doped on the right
p-n junction what diffuses across the junction?
some free electrons from the n-type to the p-type and some positive holes from the p-type to the n-type
p-n junction what happens to the 2 regions
n-type region loses electrons and becomes positively charged
p-type region gains electrons and becomes negatively charged
p-n junction what happens once the 2 sides are charged
eventually these charged regions are sufficient to prevent further diffusion or charges, a potential barrier has been set up
potential barrier
junction voltage
junction voltage for silicon
0.6 V
junction voltage for germanium
0.2V
depletion layer
there are no charge carriers on either side of the junction and this area is called the depletion layer
Pn junction is known as
diode
how do you put a diode in reverse bias
negative terminal to the p-type junction
putting a diode in reverse bias what reinforces the potential barrier
the external voltage
putting a diode in reverse bias width of the depletion layer
increases
putting a diode in reverse bias resistance
very high, the pn junction does not conduct
a current will through diode in forward bias if
the current is significant enough for the voltage to exceed the junction voltage
