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aqa a level physics > electricity > Flashcards

electricity Flashcards

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
what is threshold voltage
most diodes require around 0.6v in the forward direction before they will conduct
26
what is the reverse bias of a diode
where the resistance is very high and current flow is tiny
27
what is resistivity what can it be affected by
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
28
resistivity {} what are the units
ρ = AR/L Ωm = (m^2 * Ω) / m
29
how is resistance of a wire affected by the length
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
30
how is resistance of a wire affected by the area
the greater the cross-sectional area of the wire the easier it is for electrons to pass through it less resistance inversely proportional
31
what does lower resistivity mean and why would this be beneficial
better at conducing electricity useful in electrical engineering
32
what are semiconductors
group of materials that aren't as good conductors as metals since they have fewer charge carriers
33
what makes semiconductors useful
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
what are three common semiconductors
LDRs (light dependent resistors) diodes thermistors
35
what is a thermistor
a component that has resistance depending on its temperature
36
what is an NTC thermistor how do they work why are they useful
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
investigating the resistance of a thermistor
water bath keep pd constant record current for changes in temperature calculate resistance and plot against temperature should give a decreasing curve
38
what is superconductivity
the ability of certain materials to conduct a direct electric current (DC) with practically zero resistance
39
how can you lower the resistivity of a material
cooling them down
40
what is the critical temperature of a material
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
what are 3 uses of superconductors
power cables that transmit energy without loss of power strong electromagnets - MRI scanners electrical circuits working with minimal energy loss
42
what is the catch to superconductivity
critical temperatures tend to be very low lowering temperature that much is expensive and difficult
43
REQUIRED PRACTICAL finding the resistivity of a material
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
what is current due to
the flow of charged particle charge carriers metals - electrons salt solution - ions
45
how does current flow through an insulator
electrons in an insulator are attached to their atoms and cannot move away no electrons are free to move so no current flows
46
how does current flow in a metallic conductor
delocalised electrons act as charge carriers when a potential difference is applied the electrons are attracted to the positive terminal of the metal
47
what is the emf of a source of electricity
the electrical energy produced per unit charge passing through the source
48
how does the resistance of an LDR vary
resistance decreases with increasing light intensity
49
what is a battery
combination of cells
50
what is a resistor
a component designed to have a certain resistance
51
what is an intrinsic semiconductor
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
how does resistance vary for a thermistor
resistance decreases with increasing temperature
53
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
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
define power
rate of transfer of energy
55
Power {} give four formulae what are its units
P = E/t Watts = Joules/seconds P = IV P = V^2/R P = I^2 R
56
how do you calculate the total energy transferred
use P=E/t E = Pt P = IV ect.
57
what is internal resistance
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
what is e.m.f
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
e.m.f {} give its units
ε = E/Q volts
60
terminal pd
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
why does terminal pd differ from the emf
energy is always lost in overcoming the internal resistance of power supplies
62
lost volts
energy wasted per coulomb overcoming the internal resistance
63
what are the equations linking emf and internal resistance
ε = I(R + r) ε = V + v V - terminal pd - IR v - lost volts - Ir
64
describe a practical to investigate internal resistance and emf
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
what is the load resistance in a circuit
the total resistance of all the components in the external part of the circuit not including the internal resistance
66
what is Kirchoff's first law
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
what is Kirchoff's second law
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
in a series circuit how do properties add up
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
how do properties add up in a parallel circuit
current split up at each junction same pd across components in parallel 1/Rt = 1/R1 + 1/R2 + 1/R3 +...
70
how can you work out the emf of a battery
add up the emf of each cell making up the battery
71
how do two cells in parallel behave in comparison to 1 in series
two cells in parallel behave as a single cell (with the same emf) but with half the internal resistance / reduced internal resistance
72
what is a potential divider and what is its purpose
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
why are potential dividers useful
they can be used to supply a specific potential difference between zero and the the pd of the supply
74
potential divider {}
V(out) = (R2/R1+R2) * V(supply)
75
in the potential divider equation which resistance is assigned to R1
R1 - whichever resistor the current travels through first R2 - the resistor that you're measuring the voltage across
76
using a potential divider, how could you vary the output pd
replacing R1 with a variable resistor
77
how are light and temperature sensors made
using a potential divider use LDR or a thermistor instead of R1 V(out) varies with light levels or temperature
78
what is a potentiometer
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
what is the resistance on a reverse-biased diode
infinite resistance
80
State what is meant by an emf of 1.5 V [2]
work done to move a unit charge through the cell 1.5J of work is done moving 1C through the cell
81
what effect does decreasing current have on the terminal pd of a cell
Decreased current through cell decreases lost volts this increases the reading of terminal pd
82
what is a superconductor
a material that has zero resistivity below a critical temperature

aqa a level physics (10 decks)

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