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aqa a level physics > electricity > Flashcards

electricity Flashcards

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1
Q

what is current

A

the rate of flow of charge in a circuit

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2
Q

charge {}
what are the units for charge, give two answers

A

Q = It
coulombs (C)
or
As

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3
Q

define 1 coulomb

A

amount of charge passing in 1 second when the current is 1 ampere

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4
Q

what do we use to measure current
why must it always be connected in series

A

ammeter
so that the current through the ammeter is the same through the component

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5
Q

what is potential difference

A

work done in moving a unit charge between two points

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6
Q

what is the role of a power source in a circuit

A

does work to move charge around the circuit
charge flows through the power source
where it is “raised” through a potential and energy is transferred to its electrical potential energy

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7
Q

what is 1 volt

A

when you convert 1J of energy to move 1 coulomb of charge through a component

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8
Q

what can you use to measure potential difference
how does it work

A

a voltmeter
A voltmeter measures how much voltage is passing between two points.
It does this by measuring the difference between the positive input of voltage and the negative input of voltage.

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9
Q

what is resistance

A

measure of how difficult it is for current to flow through a component

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10
Q

resistance {}
what are the units for resistance
give two answers

A

resistance = potential difference / current
ohms
VA^-1

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11
Q

potential difference {} not V=IR
what is the unit for potential difference
give two answers

A

potential difference = work done / charge

volt
JC^-1

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12
Q

Ohm’s law

A

provided physical conditions (temp ect.) remain constant the current passing through an ohmic conductor is directly proportional to the potential difference across it

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13
Q

what are ohmic conductors

A

conductors that obey ohm’s law

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14
Q

what does the IV graph look like for a ohmic conductor

A

current against potential difference
straight line through origin
resistance is gradient, therefore is constant

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15
Q

what are some factors which can affect resistance

A

light level
temperature

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16
Q

shallower gradient on a IV graph means…

A

greater resistance

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17
Q

how can you investigate the IV characteristic of a component

A

using a variable resistor to vary resistance
which in turn changes the voltage and current across a component
take a range of current and voltage values
plot an IV graph

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18
Q

what is an ideal voltmeter

A

assumed to have infinite resistance
( which means no current flows through them )
current takes the path of least resistance
if there is infinite resistance in the voltmeter
all current will flow through the component

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19
Q

what is an ideal ammeter

A

assumed to have no resistance
(so will have no potential difference across them , so no energy is lost across it)

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20
Q

what does the IV graph for a filament look like

A

like a weird cubic
an actual cubic would be a VI graph

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21
Q

what is a filament and how does this explain its IV characteristic

A

a filament is a thin coil of metal wire

as current flows through temperature increases which increases the resistance of the lamp
this is why the curve levels off at high currents

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22
Q

why does resistance increase with temperature

A

when current flows through a metal conductor some of the electrical energy is transferred to thermal energy and causes the metal to heat up
causes particles in the metal to vibrate more
this makes it more difficult for the charge carrying electrons to get through
more difficult for current to flow so resistance increases

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23
Q

what are diodes

A

designed to allow current to flow in one direction only
made from semi-conductors

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24
Q

what is forward bias

A

the direction in which current is allowed to flow in a diode

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25
Q

what is threshold voltage

A

most diodes require around 0.6v in the forward direction before they will conduct

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26
Q

what is the reverse bias of a diode

A

where the resistance is very high and current flow is tiny

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27
Q

what is resistivity
what can it be affected by

A

measure of how much a particular material resists flow of current
it is a property of a material

structure of the material
light intensity
temperature

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28
Q

resistivity {}
what are the units

A

ρ = AR/L
Ωm = (m^2 * Ω) / m

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29
Q

how is resistance of a wire affected by the length

A

the longer the wire the more difficult it is to make current pass through it (greater resistance)
resistance is proportional to the length of the wire

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30
Q

how is resistance of a wire affected by the area

A

the greater the cross-sectional area of the wire the easier it is for electrons to pass through it
less resistance
inversely proportional

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31
Q

what does lower resistivity mean and why would this be beneficial

A

better at conducing electricity
useful in electrical engineering

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32
Q

what are semiconductors

A

group of materials that aren’t as good conductors as metals
since they have fewer charge carriers

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33
Q

what makes semiconductors useful

A

if energy is supplied to a semiconductor more charge carriers can be released
this causes the resistivity of the material to decrease
making them good sensors for detecting change in an environment

34
Q

what are three common semiconductors

A

LDRs
(light dependent resistors)
diodes
thermistors

35
Q

what is a thermistor

A

a component that has resistance depending on its temperature

36
Q

what is an NTC thermistor

how do they work

why are they useful

A

negative temperature coefficient
resistance decreases as temperature goes up

warming the thermistor gives electrons energy to escape their atoms
more charge carriers available
lower resistance

makes them good temperature sensors
high accuracy

37
Q

investigating the resistance of a thermistor

A

water bath
keep pd constant
record current for changes in temperature
calculate resistance and plot against temperature
should give a decreasing curve

38
Q

what is superconductivity

A

the ability of certain materials to conduct a direct electric current (DC) with practically zero resistance

39
Q

how can you lower the resistivity of a material

A

cooling them down

40
Q

what is the critical temperature of a material

A

if you cool the material down to below its critical temperature
resistivity disappears
becomes a superconductor
no resistance so electrical energy not turned to heat, “not wasted”

41
Q

what are 3 uses of superconductors

A

power cables that transmit energy without loss of power
strong electromagnets - MRI scanners
electrical circuits working with minimal energy loss

42
Q

what is the catch to superconductivity

A

critical temperatures tend to be very low
lowering temperature that much is expensive and difficult

43
Q

REQUIRED PRACTICAL
finding the resistivity of a material

A

find cross sectional area of wire
measure diameter with micrometer
set up wire in circuit
voltmeter across
measure current and pd through wire for different lengths
plot average resistance against length
gradient of the graph is R/L
ρ/A = R/L
so gradient * A is the resistivity

44
Q

what is current due to

A

the flow of charged particle
charge carriers
metals - electrons
salt solution - ions

45
Q

how does current flow through an insulator

A

electrons in an insulator are attached to their atoms and cannot move away
no electrons are free to move
so no current flows

46
Q

how does current flow in a metallic conductor

A

delocalised electrons act as charge carriers
when a potential difference is applied the electrons are attracted to the positive terminal of the metal

47
Q

what is the emf of a source of electricity

A

the electrical energy produced per unit charge passing through the source

48
Q

how does the resistance of an LDR vary

A

resistance decreases with increasing light intensity

49
Q

what is a battery

A

combination of cells

50
Q

what is a resistor

A

a component designed to have a certain resistance

51
Q

what is an intrinsic semiconductor

A

An intrinsic semiconductor is an undoped semiconductor, or semiconductor in the raw form without any impurities to add current carriers to the basic semiconductor material.

52
Q

how does resistance vary for a thermistor

A

resistance decreases with increasing temperature

53
Q

why is it important to keep the temperature constant when doing an experiment to find the resistivity of a material
suggest how this can be achieved

A

resistivity is dependent on temperature
as current flows through a wire, temperature increases
the temperature can be kept constant by using a small current
a switch can be used

54
Q

define power

A

rate of transfer of energy

55
Q

Power {}
give four formulae
what are its units

A

P = E/t
Watts = Joules/seconds
P = IV
P = V^2/R
P = I^2 R

56
Q

how do you calculate the total energy transferred

A

use P=E/t
E = Pt
P = IV
ect.

57
Q

what is internal resistance

A

In a battery chemical energy is used to make electrons move
when they move they collide with atoms in the battery
causes batteries and cells warm up when used

58
Q

what is e.m.f

A

electromotive force
amount of electrical energy the battery produces and transfers to each coulomb of charge

the electrical energy transferred by a power supply per unit charge

59
Q

e.m.f {}
give its units

A

ε = E/Q
volts

60
Q

terminal pd

A

the potential difference across the terminals of the cells
( when one coulomb of charge flows through the load resistance )
if there was no internal resistance would equal emf

61
Q

why does terminal pd differ from the emf

A

energy is always lost in overcoming the internal resistance of power supplies

62
Q

lost volts

A

energy wasted per coulomb overcoming the internal resistance

63
Q

what are the equations linking emf and internal resistance

A

ε = I(R + r)
ε = V + v
V - terminal pd - IR
v - lost volts - Ir

64
Q

describe a practical to investigate internal resistance and emf

A

set up circuit with variable resistor
set variable resistor to max resistance and record current and potential difference
repeat for different load resistances
plot a VI graph
gradient = -r
y intercept = emf

65
Q

what is the load resistance in a circuit

A

the total resistance of all the components in the external part of the circuit
not including the internal resistance

66
Q

what is Kirchoff’s first law

A

the total current entering a junction = the total current leaving it

accounts for conservation of charge
since current is rate of flow of charge

67
Q

what is Kirchoff’s second law

A

the total emf around a series circuit = the sum of pds in each loop of a circuit
ε = ΣIR

accounts for conservation of energy

68
Q

in a series circuit how do properties add up

A

current is the same everywhere
emf is split up proportionally based on the resistance of the components as V = IR
always adds up to emf
total resistance = ΣR

69
Q

how do properties add up in a parallel circuit

A

current split up at each junction
same pd across components in parallel
1/Rt = 1/R1 + 1/R2 + 1/R3 +…

70
Q

how can you work out the emf of a battery

A

add up the emf of each cell making up the battery

71
Q

how do two cells in parallel behave in comparison to 1 in series

A

two cells in parallel behave as a single cell (with the same emf) but with half the
internal resistance / reduced internal resistance

72
Q

what is a potential divider and what is its purpose

A

a circuit with a voltage source and a few
resistors in series

to provide a varying potential difference
or a constant specific potential difference

73
Q

why are potential dividers useful

A

they can be used to supply a specific potential difference between zero and the the pd of the supply

74
Q

potential divider {}

A

V(out) = (R2/R1+R2) * V(supply)

75
Q

in the potential divider equation which resistance is assigned to R1

A

R1 - whichever resistor the current travels through first
R2 - the resistor that you’re measuring the voltage across

76
Q

using a potential divider, how could you vary the output pd

A

replacing R1 with a variable resistor

77
Q

how are light and temperature sensors made

A

using a potential divider
use LDR or a thermistor instead of R1
V(out) varies with light levels or temperature

78
Q

what is a potentiometer

A

has a variable resistor replacing both R1 and R2 (two resistors in series)
you move a slider to adjust the relative sizes of R1 and R2
volume control on a stereo

79
Q

what is the resistance on a reverse-biased diode

A

infinite resistance

80
Q

State what is meant by an emf of 1.5 V [2]

A

work done to move a unit charge through the cell
1.5J of work is done moving 1C through the cell

81
Q

what effect does decreasing current have on the terminal pd of a cell

A

Decreased current through cell decreases lost volts
this increases the reading of terminal pd

82
Q

what is a superconductor

A

a material that has zero resistivity below a critical temperature

aqa a level physics (10 decks)

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