electricity Flashcards
reason why increasing resistance may not affect voltage
voltmeter may not be sensitive enough
steel increases
mechanical strength cables
why superconducting wire increase efficiency
no resistance no power loss
state and explain what happens to brightness of lamp if connected to power supply w/ internal resistance(2)
- lamp less bright
- power now wasted
application superconductor and why (2)
MRI / strong magnetic field
application superconductor and why (2)
MRI / strong magnetic field
application superconductor and why (2)
transformer - no energy dissipation
what’s a superconductor and how does it become a superconductor (3)
superconductivity means a material has zero resistivity/resistance (1)
resistivity decreases with temperature or idea of cooling (1)
becomes superconducting when you reach the critical/certain/
transition temperature (1)
what happens to resistance of cable when embedded filaments are made superconducting (3)
the resistance decreases (to zero) (1) copper still has resistance (1) but this is in parallel with filaments (which have zero resistance) (1) hence total resistance is zero (1) current goes through filaments (1)
explain why resistance NTC decreases as temp rises (2)
- increased temp = release electrons
- current increases as charge carrier density increases
2 heating element at 230V 500W connected to 230V supply in series + parallel
explain why only one will give power output 1KW(6)
parallell
- lower resistance = higher current = more power
- parallel voltage across each constant whereas in series split between the 2
in potential divider questions ALWAYS
give p.d. affect on all resistors / components
to say __ of parallel branch say
resistance in parallel combination decreases
what give largest resistance
series
define emf
emf is the work done / energy transferred by a voltage source / battery / cell ✓per unit charge✓
explain why voltage on graph always less than emf - why difference voltage emf becomes larger w/ increasing current
-internal resistance
-terminal pd = emf - lost volts
-lost volts increase as current increases
-
if gives 2 requirements for something to work and asks if it works
state both
why voltmeter has a high resistance
so current is 0
remember 2 cells =
double emf
1 reason for rechargeable battery to have low internal resistance
charges quicker as less energy wasted
define emf
joules per coulomb (of charge)/work done per unit charge
where charge moved (whole way) round circuit
lost volts
emf - terminal pd
2 points for why voltage measured less than emf
- work done in battery due to internal resistance
- lost volts
why would ammeter reading be greater if motor in series is lifting a load
battery has to provide more energy / power
why something will not start with higher internal resistance e
terminal pd will not be high enough
when talking about effect of switch always say
when switch closed CURRENT FLOWS then rest
asks how values obtained from internal resistance practical
by altering variable resistor
what two statements to say when voltmeter < emf
- current through battery
- pd lost across internal resistance
- work done to lose pd
why does pd between two points in circuit not change even if cell replaced w/ cell of internal resistance (2)
- terminal pd decreases
- BUT ratio of resistors remain constant OR voltmeter not sensitive enough to measure change
potential divider equation
combination resistors in series connected to voltage source
3 points to make in potential divider 3 marker about pd across each component (3)
- change in resistance one = pd change
- pd across component + pd other = supply pd (in series)
- so change in pd on other component
never give fractional ! always
decimals
2 marker potential divider question
- resistance (__) decrease = pd decrease
- so greater (PROPORTION/SHARE) PD to (_)
2 marker on why pd wouldn’t change if resistance of different parallel branch changed
no change (1) constant pd across parallel branches (1)
define 1 volt
1 joule per couloumb charge
advantage to high value resistance in potential divider circuit
reduced current
compare potential divider in controlling current across resistor to variable resistor circuit (2)
- potential divider = sensitive control current
- variable resistor = larger current but not near 0A
why resistance NTC decreases w/ temp increases (3)
- more charge carriers available
- internal energy used to liberate electrons
- more significant than vibrations lattice ions
for power and resistance equation
V squared / R
why does resistance of thermistor decrease as temperature rises (3)
-extra charge carriers released as temperature rises
-increased thermal agitation of atoms resists flow of
charge carriers
- 1st effect overwhelms 2nd
brightness of lamps w/ same current
same
advantage and disadvantage to potential divider method of Current (2)
- adv- better control 0- to max
- disadvan-power wasted lower half resistor always carries
adv and disas to rehostat method
-adv- easier to connect
disad- minimum current through bulb never 0
temp measured at
voltmeter
3 points for how voltmeter reading across LDR changes as light increases - DIFFERENT AS ACROSS LDR
LDR resistance drops
voltmeter reading decreases
because more conduction electrons/charge
carriers released
why resistance LDR decreases at higher temp (2)
- more charge carriers released - liberated electrons
- increases current
application superconductor and reason
- maglev trains / MRI - strong magnet
- no thermal energy produced
how to get readings for reverse bias
reverse cell
why non ohmic
resistance not constant
when asked how results obtained always say ( and in 6 markers)
variable resistor
for filament lamp 4 marker say
current heats filament
same in reverse direction / mirror image
practical use diode
protect against current surges
ohms law definition
VI directly proportional if temp constant
on emf graph questions - for value internal resistance
say modulus gradient is r
when pd across component is 0 say
no current flows
component placed in parallel between two components of equal pd =
0V as diff between two components NOT SAME
is bulb filament breaks
ignore from circuit
energy transformed across battery
from EMF
energy dissipate across battery
internal r
in describe filament IV curve for increasing temp say
increasing RATE of collisions
graph relationship power and current
P=I^2R therefore y=x^2 graph
on VI graph for resistance NEVER
find gradient tangent for point - only ever read off points
if voltmeter put in branch in series w/ resistor in parallel circuit current through that branch is
0A as voltmeter infinite resistance - therefore discard any resistance in that branch as no current flows through it
when a resistor in added in circuit - 2 marker explain as to why
increase resistance
therefore limit max current
therefore prevent cell from overheating ( current too high) = short circuit
if given gradient min and grad max how to find grad
mean
why would 2 cells in series go out faster than 2 cells in parallel
- 2 cells in parallel receive double the amount of energy per second
- more charge per second passes through cells in series
what would happen If use battery with much higher internal resistance to start car
- wouldn’t start
- lower terminal pd
if light intensity 450 W m-2 with 0.15 efficiency use
450 x 0.15
how to take measurements emf and V
Voltmeter connected across cell terminals ✓
Switch open, voltmeter records ε
Switch closed, voltmeter records V
for potential divider never divide PD it’s
SPLITTING OF PD
