Module 4 Electrons: 4.1 - 4.3 Flashcards
(108 cards)
1
Q
what is meant by electric current?
A
the rate of flow of charge
2
Q
what is electric current measured in?
A
amperes or amps
3
Q
what equation can you use to work out the electric current?
A
I=ΔQ/Δt
I= current in amperes
ΔQ= change in charge in coulombs
Δt= time taken in seconds
4
Q
what is the defintion for one coulomb?
A
the amount of charge that passes 1 second when the current is 1 ampere
5
Q
what type of electrical component can you use to measure the current flowing through part of a circuit?
A
an ammeter
6
Q
what is electric charge measured in?
A
coulombs (C)
7
Q
what is the elementary charge?
A
the smalles unit that charge comes in and is quantised
e= 1.60 x 10-19 C
8
Q
gives examples of relative charges
A
Sodium ions have a relative charge of +1
chlorine ions have a relative charge of -1
9
Q
what is 12kA converted into coulombs per second?
A
12 000 Cs-1
10
Q
what is the eqaution used for working out the net charge of an object?
A
Q= ±ne
11
Q
what does it suggest when it said that the charge of an object is quantised?
A
- this means that the charge of the object can only have certain values.
- these calues must be interger multiples of e (1.60 x 10-19)
12
Q
give the definition for current in a metal and in an electrolyte
A
current is the movement of electrons in a metal
current is the movement of ions in an electrolyte
13
Q
answer these questions on charge and current
A
14
Q
describe Milikan’s oil drop experiment (the discovery of the quantisation of charge)
A
- in 1990 Robert Millikan
- managed to levitate charged oil droplets between two oppositely charged metal plates by precisely balancing the weight of the negatively charged droplet acting downwards with an upwards attractive force from the positively charged plate
- by taking very precise measurements he was able to determine the charge on each droplet, and did this for may droplets
- he found the charge on the drops was quantised: it did not take just any value, but only values that were multiples of elementary charge
- Using his data Millikan calculated e to be -1.59 x 10-19
15
Q
what may a larger current be due to?
A
- a greater number of electrons moving past a given point each second
- the same number of electrons moving faster through the metal
16
Q
what is meant by a conventional current?
A
- a model used to describe electric current in a circuit
- conventional current travels from positive to negative
- it is the direction which positive charges would travel
17
Q
what do you call liquids that can carry an electric current?
A
electrolytes
18
Q
why should an ammeter have the lowest possible resistance?
A
so they do not reduce the current
19
Q
answer these questions on moving charges
A
20
Q
what does the conservation of charge state?
A
- the conservation of charge states that electric charge can neither be created nor destroyed
- the total amount of electric charge in the universe is constant
21
Q
what does Kirchhoff’s first law state?
A
it states that for any point in an electrical circuit, the sum of currents into that point is eqaul to the sum of current out of that point
22
Q
what is the eqaution for Kirchhoff’s law?
A
ΣIin = ΣIout
ΣIin= sum off current into a point
ΣIin = sum of current out of that point
23
Q
what does kirchhoff’s second law state? what eqaution can you use to show this?
A
the sum of the e.m.fs around any closed loop of a circuit is equal to the sum of the potential difference
ΣE=ΣV
ΣE is the sum of the e.m.f
ΣV is the sum of potential difference (voltage)
24
Q
answer these questions on Kirchhoff’s first law
A
25
what is meant by the mean drift velocity?
the average velocity of the charge carriers (and is much less than the electron's actual speed)
26
when talking about mean drift velocity what eqaution can you use to work out current?
I= Anev
I= current
A= cross-sectional area (m2)
n= number density of electrons (m-3) (number per unit volume)
e= size of charge on one electron
27
if you double the number of electrons what happens to the current?
the current doubles
28
what happens to the current if your double the area?
the current doubles
29
what happens to the current if the electrons move twice as fast?
the current doubles twice as many electrons move past a point in the same amount of time
30
what happens to the mean drift velocity when the electrons move from a wider (greater cross-sectional area) to a narrow space (smaller cross-sectional area)
when the electrons are in a wider area (larger cross-sectional area) the electrons move at a lower drift velocity so when they move to a narrower area (smaller cross-sectional area) the electrons move at a higher drift velocity in order for the current (the rate of flow of charge) to be the same.
the mean drift velocity is inversely proportional to the cross-sectional area of the wire.
31
what is the difference between conductors, semiconducts and insulators?
conductors have a very high number density (so more delocalised electrons per m3 so the best electrical conductor)
semiconductors then come next with a lower number of density
and then insulators have a very low value in number density
32
why is that a semiconductor may increase in temperature a lot more than a normal conductor (metal)?
- semiconductors have a much lower number density than metals, so in order to carry the same current the electrons in semiconductors need to move much faster
- this increases the temperature of the semiconductor
33
show the derivation of I = Anev
1. )from defintion electric (I) is given by I= ΔQ/Δt
2. ) the number of electrons in a given volume V of the conductor is nV (where n is the number density
the total charge of the electrons in this volume of conductor (Q) is now neV (where e is the elementary charge) this gives:
I= neV/Δt
3.) when there is an electric current in the conduct, a certain volume of charge carriers passes a given point each second. This volume depends on the cross-sectional area A of the conductor and the mean drift velocity v of the charge carriers
V/Δt = Av
4.) substituting this into our previous eqaution for electric current gives
I= neV/Δt= neAv
which is also I= Anev
34
answer these questions on mean drift velocity
answers
35
what is meant by a battery?
two or more cells connected end-to-end, or in series.
(on the symbol the longer terminal represents the positive terminal whlist the smaller terminal represents the negative one)
36
give the name for each circuit symbol shown
a= switch (open)
b= switch (closed)
c= cell
d= battery
e= diode
f= resistor
g= variable resistor
h= lamp (bulb)
i= fuse
j= voltmeter
k= ammeter
l= thermistor
m= LDR
n= LED
o= capacitor
37
draw a circuit diagram using the correct symbols of a power supply connected to two resistors connected in series
38
draw a circuit diagram using the correct symbols of a battery connected to a resistor and an ammeter in series
39
draw a circuit diagram using the correct symbols of a single cell connected in series with a filament lamp
40
what is another name for potential difference?
voltage
41
what is potential difference of?
potential difference is a measure of the transfer of energy by charged carriers
42
what is the definition of potential difference?
what eqaution is used to show this?
the work done per unit charge moved (energy transferred per unit charge)
W= VQ rearranged to
V= W/Q
were V is potential difference/volts
W is work done in Joules
Q is the total charge in Coulombs
43
define the volt
the potential difference across a component is 1 volt (V) when you do 1 joule of work moving 1 coulomb of charge through the component
1 V = 1 JC-1
44
what is used to measure the p.d in a circuit and how would you see it arranged in a circuit?
- a voltmeter is used to measure the p.d
- voltmeters are always connected parallel across a particular compent
45
what would be the ideal voltmeter?
An ideal voltmeter would have infinite resistance so thagt when connected, no current passes through the voltmeter itself
46
eqaution that gives you the velocity of a single electron accelerated through a potential difference
eV= 1/2mv2
47
what is meant by electromotive force and when is the term e.m.f used?
- electromotive force is defind as the energy transferred from chemical energy (or another form) to electrical energy per unit charge
- used when charged carriers gain energy from a source
48
what is the difference between e.m.f and p.d?
e. m.f (electromotive force) is used to describe the work done on charged carriers and is found across the cell, battery, power pack etc
p. d is used to describe the work done by charged carriers and is found acrross other components
49
what is the eqaution for e.m.f?
ε= W/Q
where ε is e.m.f measured in volts
W is energy transferred (work done) in Jules by charge Q
50
state the eqautions that can be used for E.m.f and iternal resistance questions
ε= E.m.f
V= p.d (voltage)
*v*= lost volts
I= current
R= load resistance
*r*= internal resistance
51
for cells in series, how do you calculate the total e.m.f of the cells?
you add their individual e.m.f.s
(because each charge goes through each of the cells and so gains e.m.f from each one)
εtotal= ε1 + ε2 + ε3 + ...
52
for identical cells in parrallel how do you calculate the total e.m.f?
εtotal= ε1 = ε2 = ε3 = ...
this is because the current will split eqaully between identical cells
53
how does an increase in p.d affect electron speed?
the greater the p.d the more energy is transferred to the electrons and so the faster they move
54
answer these summary questions on potential differece and electromotive force
55
answer these summary questions on the electron gun
56
what is meant by resistance?
a property of a component calculated by dividing the potential difference across it by the current in it
its units is ohm Ω
57
what is the eqaution for working out resistance from current and voltage?
R= V/I
58
define the ohm
the resistance of a component when p.d of 1V is produced oer ampere of current
1Ω = 1 V A-1
59
what is ohm's law show?
provided the temperature is constant, the current through an ohmic conductor is directly proportional to the potential difference across it (that's V= IR)
60
explain the shape of this graph
- the p.d (voltage) across the wire remains constant at 1.%V, but the current in the wire decreases with time
- the graph curves because the resistance of the wire increasing with time (you can see this at t=0s and t= 4s)
- the current in the circuit changes because the temperature of the wire increases over time as a result of heating caused by the current.
- as the wire gets hotter the resistance increases this is due to the fact that the positive ions in the wire have more internal energy and vibrate with greater amplitude about their mean positions.
- the collision frequency between the charge carriers (delocalised electrons) and the positive ions increase, and so the charge carriers do more work (more work done as they travel through the wire)
61
what other 3 things determine resistance?
1. ) length (L)- the longer the wire the more difficult it is to make a current flow
2. ) Area (A). the wider the wire the easier it will be for electrons to pass along it
3. ) resistivity (*p which is rho*)
62
answer these summary questions on resistance
63
what do I-V graphs show?
the current-potential difference characterisitic for any electrical component shows the relationbship between the electric current I in a component and the potential difference V across it
64
name ohmic and non-ohmic components
ohmic components
- metallic conductor
- resistor
non-ohmic conductors
- filament lamp
- thermistor
- LDR (light-dependant resistor)
- Diode
65
what does the I-V characteristic graph look like for a filament lamp?
what does the graph show?
the graph shows that as the p.d increases the resistance also increases
this is because current flowing through a lamp increases the temperature increasing the resistance
66
what are NTC thermistor and what do they mean?
NTC thermistor stand for 'Negative Temperature Coefficient' this means that the resistance decreases as the temperature goes up
67
what does a I-V graph look like for a NTC thermistor
explain the change in resistance as the temperature is increased
- increasing the current thrugh the thermistor increases its temperature
- this graph shows you that resistance is decresing as the thermistor heats up (as current increases)
- the increase in temperature warms up the themistor gives more electrons enough energy to escape from their atoms. This means that there ae more charged carriers (delocalised electrons) available so resistance is lower
68
what is meant by forward bias?
this is the direction in which the current is allowed to flow
69
what happens to resistance as light intensity increases in an LED (light emitting diode) ?
why is this?
- the resistance decreases as light increases increases
- this happena because light provides more energy that releases more electrons, so more charge cariers are available meaning a lower resistance
70
what do diodes do?
-dioded only allow allow a current in one particular direction
71
what is meant by forward and reverse bias?
- forward bias is the direction in which the current is allowed to flow
- reverse bias is where the resistance of the diode is very high so the the current that flows through is very tiny
72
what is meant by the threshold voltage?
the minimum potential difference at which a diode begins to conduct
73
what is usually the threshold voltage needed for a diode to conduct?
0.6-0.7V
74
what does the I-V graph look like for a diode?
- at point A the resistance of the diode is very high, with the p.d. in this reverse direction, the diode does not conduct
- at B the p.d increases, the resistance gradually starts to drop
- at point C the resistance drops sharply for every small increase in p.d above this point the diode has very little resistance
-
75
the shallower the gradient of a I-V characteristic graph, the _______ the resistance of the compound
the shallower the gradient of a I-V characteristic graph, the greater the resistance of the compound
76
give the eqaution that defines resistance
R= V/I
77
answer these summary questions on diodes
78
what is the defintion for the resistivity of a material?
give the eqaution that shows this
the resistance of a 1 m length with a 1 m2 cross-sectional area,
* p* = RA/L
* p* is resistivity in ohm metres (Ωm(
R= resistance in ohms (Ω)
L= length in metres (m)
A= cross sectional area in m2
79
what is the relationship between the resistance and length?
give a expression to show this
as the resistance R of a wire is directly proportional to its length L
R ∝ L
80
what is the relationship between the resistance of a wire and it's cross-sectional area A and why is this?
give an expression to show this
the resistance of a wire R is inversely proportional to its cross-sectional area A.
(this is because when the cross sectional area increases the resistance does the opposite
doubling the cross-sectional area will double the current in the wire, so the resistance must have halved)
R ∝ 1/A
81
what is the eqaution for resitivity of a particular material at a given constant temperature?
*p*= RA/L
resistivity= resitance x cross-sectiona area/ length
82
draw a circuit that would be used where the resitivity of a wire is being measured and state the other calculations that would have to be done to work it out.
- using the circuit you can obtain values of the p.d. across different lenghts of wire
- if the current in each wire is measured too, we can use R= V/I to calculate the resistance for each length
- then you can plot a graph of resistance (y-axis) against length (x-axis) (the graph should be a straight line)
- the gradient of the graph is p/A so then you rearange to find restivity
p= gradient x A
83
what is meant by power (P)?
power (P) is the rate of doing work
84
# define the watt
give the eqaution for this
1 watt is equivalent to 1 joule of work done per second
P= W/t
85
what is the eqaution for power in electrical circuits?
P= VI
86
when you're interested in the total energy transferred you just multiply the power by the time
P= VI , P= V2/R, P=I2R
so to get the total energy trannsferred (work done) you multiply the power eqautions by time
W= VIt, W= (V2/R)t, W= I2Rt
87
answer these summary questions on electrical energy and power
88
what type of unit do electricity companies use?
kilowatt-hour (kWh)
89
define the kilowatt-hour (kWh)
the energy transferred by a device with a power of 1 kW operating for a time of 1 hour
1 kWh is equivalent to 3.6 MJ (3.6 million joules)
90
what is the word eqaution for working out the kWh?
work done (kWh)= Power (kW) x time (h)
91
what is the eqaution for cost?
Cost= No. of units x price per unit
92
answer these summary questions on paying for electricity
93
what is meant by internal resistance
the resistance of source of e.m.f (e.g a cell) due to its construction, which causes a loss in energy/voltage as the charge passes through the same source
symbol: r
unit: ohm Ω
94
what causes batteries and calls to warm up when they're used?
internal resistance
95
what is meant by 'lost volts'
the potential difference across the internal resistor of a source of e.m.f
(the p.d measures at the terminals of the power source- the terminal p.d- is less than the actual e.m.f, we call this the difference lost volts)
96
what is meant load resistance/ external resistance?
the symbol for load resistance is R
thia is the total resistance of all the components in the external circuit.
97
state an eqaution for kirchhoff's second law
electromotive force= terminal p.d + lost volts
98
how does increasing the current affect the amount of lost volts?
increasing the current means that more charges travel through the cell each second so more work done is by the charged carriers, increasing the volts. This lowers the terminal p.d (the p.d measure at the terminals of the power source)
99
how can you calculate the e.m.f from p.d and velocity?
e.m.f= p.d + velocity
100
what is meant by load resistance/ external resistance?
this is the total resistance of all the components in the external circuit
101
describe an experiment for investigating internal resistance and the e.m.f
- show the type of circuit you would need
- what graph you would draw
1. ) vary the current in the circuit by chaging the value of the load resistance (R) using the variable resistor.
2. ) measure the p.d (V) for several different values of current (I).
3. ) record data for V and I in a table, and plot the results in a graph of V against I.
to find the e.m.f and internal resistance of the cell, start with the eqaution:
e. m.f= p.d + current x internal resistance
1. ) ε= V x Ir (the rearrange to give: V= -rI + e)
2. ) since ε and r are constants, thats just the eqaution of a straight line: y= mx + c
3. ) so the intercept of the vertical axis is ε and the gradient -r
102
whats another easier way of measuring the e.m.f?
An easier eay to measure the e.m.f of a power source;
- connect a voltmeter across its terminals
- Voltmeters have a very high resitance, but a small current will still flow through them, so there must be some losts volts
- which means you measure a value very slightly less than the e.m.f
103
what is the difference between kirchoff's first and second law?
Kirchoff's first law: the total current entering a junction= the total current leaving it
Kirchoff's second law: the total e.m.f around a series circuit = the sum of the p,d.s across each component
ε= ΣVR
104
what is a potential divider?
a potential divider is a circuit with a voltage source and a couple of resistors in series
potential dividers to divide the p.d to give any value you require up to the maximum supplied from the power source
105
- the p.d across each resistor in a potential divider depends on their resistance
- if they have the same resistance then the p.d is shared eqaully between them
- if one has twice the resistance of the other, then this one will recieve two-thirds of the total p.d
how can this be expressed in an equation?
V1/V2 - R1/R2
106
so how would you work out the V out in this circuit?
you use the eqaution
107
techniques and procedures used to investigate potential divider circuits which may include a sensor such as a thermistor or an LDR
108
