Vhwpter 9 Flashcards
Cell vs battery
Remember battery it s’more than one cell (so two) connected dotted line, big line photogenen terminal, conventional current means it starts from here.
Circuits ti remember!
Led is diode with two arrows emitting
Ldris two arrows coming into it
Capacity or two lines
Fuse one going in
The volts
Pd vs emf
Defined as the energy per colourblind , jc-1
Pd is the joules/ Colombian lost
Voltages vs emf vs pd
Voltage is both umbers,a formed and emf
Pd is the energy out per colourblind, energy lost per colourblind (form equipment like lights anything that takes )
Emf is energy gained per column /from batteries supplied solar panels etc(
Essentially Kirchhoff Devon law?
Conservation of energy here
The sum of emf = sum of pd in a closed loop
Energy conserved ,
What ever goes into a closed loop must leave
Emf source s
Solar cells, dynamos panels , energy added
Pd
Pd dna Amos be work done by carriers per second
Emf is work done to csrriers permfolikn
Voltmeters
Voltmeters ideally need infinite resistance so they csn resd all of the voltage that goes through, as v= iR
Also no current do voltage is notmlost
What is an electron gun
Essentially s small metal filament thst is nested out to direct s beam of electrons if a certain velocity which cna be controlled depending on the scclerting pd thst is used to accelerate them after wards
How does an electron gun even work?
In a vacuum environment
1) a small metal filament is heater up by an electric current , ti the point where electrons gain enough KE to escape the metal . This is known as THERMIONIC EMISSION
2) then if placed in a vacuum and a high pd applied between the filament snd sn snode, the filament will act ss a cathode
- so free electrons then accelerate towards the anode gaining kinetic energy, and if there is a small hole in the snode, the beam if electrons will escape creating a beam of electrons with a specific kinetic energy
Again
1) if a metal filament is heated up by a current, eventually electrons will gain enough kinetic energy to esdcame, known as thermionic emission
2) place this metal filament in a vscuukm, and if a high accelerating Pd is applied between the filament and an anode, the filament will act a s acsthode
3) here you will see the free electrons accelerate towards the positive anode , and thus gain kinetic energy.
If the anode has a small hole in it a beam of electrons will be created, Esch with a specific kinetic energy snfnghus velocity
So what is the energy transfer here then
What is assumption
Assuming no energy slot (hopefully not in a vacuum etc)
Energy applied electrically = transferred to ke
On a single Electrom e = =vq= ve
Ev = 1/2mv2
And if kniw mass then kniw velocity
2) assumption is that the electrons DIDNT HAVE KIMERIC ENERGY IM THE FIRST PLACE HOWEGER!
So what is the mass of an electron
9.11x10^-31
So conclusion
Increase accelerating or,micnfrease ke increase velocity of the electrons
What is one electron volt
A unit of energy, it’d the amount if energy in an electron when applied by and accelrwted by 1 volt
=1x1.6x10-19
So small so we give new unit= 1ev
When using charge , what do you do if it is proton vs electron
Both times use magnitude and just value for e, so proton being accelerated would elabe it a mich lower velocity because msds Is much bigger
What is resistance
Anything that resists the flow of charge
R= V/I so it is the ratio of pd to current in a circuit
Higher Résistance, more energy it takes ti Oudh same Charger
Remember importsnt thing for fscusltimg rrsistsnce in groans
IT IS NOT INSTANTANEOUS, it is more total voltsge / total current
Ohms law
When temperature is kept constant, the voltage in a circuit is proportional to its current
Current in a meraillic wire is proprtionsl to the pd across it whilst tmelertaure kept constant
Whilst temp kept constant, current in a (metwllic’ wire is proportional to the volatrh pd across both ends
If you increase current what happens to voltage in a light bulb?
Initially as you increase current you increase voltage according to ihms law however Vater a while an increase of current increases the temperature
An jncrease in temperature increased the kinetic energy of thr metal posit be ions in the wire, which start to vibrste more
These act as an I pediment to electron flow,increasing resistance , as work is done on them when electrons collid e
So resistance?
If reade current increase viltsge and resistance increase, then increase at different rwte as temperature increases which actually increases resistance as electron flow is impeded by movin metal ions
Similar to plastic wrapped around a wire
Similar to thst, initially increase current snd volateh increases proportionally. However as the insulation created an increase of teoeftaure, the resistance increases due to impediment of electron flow, which causes a DECREASE in current, hence why the graph levels form
Eventually the temperature is constant again every, and current limited
What is conduction in terms of resistance
Conduction = 1/resistance , oppsite, 1/ v/I = I /V!
thus in an IV graoh the gradient is actually conduction, such that a stepper gradient means less resistance and shallower gradient means more resostance
IV characteristic graph for a fixed resistor
Fixed resistors designed ti have constant resistance .this
- pd is peorptinsl to current
-resistor is AN OHMIC RESISTOR BECAUSE IT OBEYS OHMS LAW
-behaves same way regardless of polarity
Straight line through the origin
OHMIC CONDUCTOR
What makes a conductor OHMIC
If it obeys ohms law
I v characteristic for filament lamp
- initially current proportional to voltage, so goes through 0,0
- however at higher voltages the current begins ti decrease, resistance begins to increase which means grsdient deference
- this because temp increase, means Impediment and resistance increases
Therefore NON OHMIC Z
SAME regardless of polarity
The curves don’t fully level off and curve DOWN EITHER
What circuit di you want to chose to get different values for I and V experimentally?
Preferably circuit 2 which is the potentiometer, as this allows a full swing of resistance and yiu can measure fully
This is because the voltage can start from 0 and go up, so full rnsge kf voltage means full range Kfz-Steuer resistance too!
Again why potentiometer
Offers full range of voltage
What else would determine the resistance
- the material as this determines number density
- the length
- the cross sectional area
How to derive resistance equation then
Resistance is propetinsk to length
And inversely to cross sectional
Thus r = P L/A
Where P is resitivity and special for eagerly material
Independent of shape and size, if length snd area 1 then it still has a value
= ohm meters
Sonwhat is resitivty
Is resitivty constant
Defined as the product of resitanceand cross sectional area divide by length of object
No resitivty often changes WITH TEMPERATURE ! And this too changes depending on thpemofmsterial
How to investigate value for resitivty in a lab
Relasitvsly all you can do is change lengths of a wire and see how current and voltage barriers, do this using a potentiometer
Then find out values for resistance by doing v/I at different lengths and plot a graph R/L
This will be proeptionsk through origin as R is proportional to length and gradient = P/A
Here find the length using meter stick, and cross sectional area using micrometer in three places
Often take averages best you can
MUST KEEP CURRENT CONSTAMT EACH TIME , thus you use portnistmter to adjust, this way the temperature doesn’t increase which causes resitivity to increases .
How does resitivity vary for materials
For insulators, HIGH rrsitivity, increases temperature shakes this worse (very resistance )
For semi conductors, medium resitivity, but when increase temperature, actually lowers (as number density electrons are released) (covalent binds break releasing electrons (
Metals have lowest resitivity and most current can go through .
Again increase temperature here, then you also increase festivity because metal ions gain ke to impede
If ehcactetisrc graoh for a diode like LED
- not proprtional, initially current very low and RESISTANCE VERY HIGH until it reaches a point , where afterwards current increases rapidly and resistance tends to 0 # not proprtional at any stage, non ohmic conductor
+ when REVERSED POLARITY, THE DIODE DOES NOT HAVE NAY CURRENT = INFITIE RESISTWNCE
What are diodes used for
Diodie inkhmletcurrent one way, so they fundamental for on off systems
LEDs used to checked if current is live in a sustem
What’s going on with the hr diode, hoe does it work
REVERSE DIRECRIOJ?
If successfully connected to any part of a circuit where the current is in the right way, then the diode will always twke its share of voltage first before anyone else, so like 0,7 v
Essentially it needs this for a threshold which is different for everyone’s thatswhy resistancenso high at the start, once reached threshold, it decreases
IN THE reverse direction it takes ALL THE VOLTSGE IN THE DYDGEM AS IT IS INFINITLEY RESISTANT! REMEMBER
Diode
Right way, takes it share and then splits
Wrong way, takes all the voltage !
Remember again fir looking at resistance wpsialky in these IV graphs what do you need to remember
Yh resistance is 1/gradient, so higher is lower but
HAS TI BE FROM 0, NOT INSTANTANEOUS!
What does negative temperature coeffecintmean
It means that is temperature increases, the resistance decreases
The negative is in relation to resistance and yiu say temp increasing
So how do NTC thermistors work?
A thermistor will have a semiconductor that is an NTC. Thus as temperature increases resistance decreases, which can be measured, and information processed to determine chmages in temperature etc
How does a resirsnceagaisnnttemoertauregraoh loom like
Amd for a PTC?
1/x graph, inversely proportional curve etc
Positive one will be proportional y= x
Where are NTC thermistors use in?
Everything, heat control, oven, iPad, pc etc
Some used fir smaller temperature changes are more expensive as precise semiconductor is needed, however over a larger range of temperatures itscheaper, such as an oven
Small temperature change found in something like incubator or thermosoftening for baby ear
IV chatacterietifnfornthermisotrs
- as current increase so does voltage at a steady rate and resistance too # but as current continues to increase, the temperature in the system increases too - an increase in t emf = decrease in resistance due to increase of number density - as a result, current increases resistance decreases
Same bith ways
Non ohmic
- looks like y3 graoh rising up,
I
Odd this time also semiconductor material , and used visible waves energy instead of infrareds, to break civslent bonds in semidcondufor and increase number density
IV for alder
Again similar,mat lowmlight very very high resistance, as soon as light chines then the resistance decreases adm current increases
Can’t really have an IV graoh fir thid, instead the resistance eve intensity is again inversely proprtional graoh, curved line symmetrical etc
How can you use for sensing circuits
Essenitlalynsay you want cooling systems
- place THERMISOR so that when heat increases, resistance decreases, meaning it takes a lower share of the voltage . Place this in series with a cooler and a fixed resitronin parallel, so that when it takes less voltsge , the cooler takes more
Similar concept with light , when light stops shining you want more light
As light shines less, resistance increase, so place this in parallel with bulbs, so that when light decreases it takes more resistance and thus kroe voktsge
How to do experiments for them bith
How to reduce any background light for invesktgating LSR
For thermistor
- use Khmer-Republik with thermistor in water bath, changin temp of water and seeigjnhoenredistsmfenberiee
For LDR
- light intensity is properinsk tindidtance
# so move the light source back and see how resistance changes
- tinminise anyaffect if background light, ensure yiu out a BLACK CARDBOARD AROUND !
Power gained lost equations
Prefer sly poeer gsinsed =vI
And power lost = I2R or v2/R
Try use these when resistance sinvikved ss that is where Vintage I lost
Why does p= I2R show why poerlinesalways drop current
They drop current because multiply current by 2 poeer by 4, thus yiu need
They step down current and increase voltage do it’s more effeicmt
Why!
Higher currents lead to higher temperatures which lead to wasted of energy due to higher resistance
Why do we not want high currents in circuits do poeelinesstop it down
High currents lead to high resistance and wastage if energy
But higher viktsged is no problem
kWh
Is another unit if energy, Equalizern to 1000x3600= 3.6MJ
As p= wd/t, wd= pt
We use bacuse electricity usage goes up so to shorten we do in kWh , where companies charge per unit a fixed rwte
What influenced why you licks. Device for saving energy
Need to see poeer and how logn yiu use it
A hairdryer with high power is used less than one if low thst will be used for more for exapmle
Okay so if you increase cross are any 3 the volume say the length
Decreases by 3
With an NTC connected to battery what happens when turn on
Initially increase current increases temp do lowers resistance er, but once all current is used, then it goes ti max temp, and currrent is now stable until external heat id applejd
What is krichoffs second law
It states the sum of emf in a closed loop is = to the sum of pd
A closed loop considered a single pathway that an electron can take
And thus the conservation fi energy, as e = vxc
Energy is conserved
First law charge was consefved
Providing current snd voktsge laws in series vs parallel
Series
According fir 1st law current netring a junction is = to current leaving the junction
Since there is only one junction, then the current is the same everyhwere
2nd law stated in a closed loop sum emf = sum pd
Again , only one closed loop here, so the whole emf on battery must be used up by the components equally , on a basis of resistance
Also Yh charge diesn’t split up or anything it just has one place tongo
Rules for circuits with more than one source of emf?
If same direction = add, if oppsite, then subtract
So if 9 and 6, then 3!
Parallel
2st law, urgent in junction= out, thus current is plot in parallel. This is dependent on the reisstances if Esch series circuit in the parallel
2nd= emf in a closed loop = pd out
Here you mow have multiple closed loops (paths that an electron can take), so emf shared between these loops
This VOLTAGE NOT NECESSARILY SPLIT IN PARALLEL,
So with parallel, how can be thought
Either collapse the resistance of parallel and split them with the series, or think as spearates loops thst need voltage tonbe split amongst the, maugthbebesdiemr
What happens when you add brsnches to a parallel circuit, does current jncreases decrease
Current increases,mbevause resistance decreases with every branch (more places for current to go and be pslit, each branch hsd less resistance, adds up to be lesd)
Deriving resistance laws series
Kirchhofs second law emf in closed loop out, for series
V= v1 +v2 etc
First law current in junction = out , so current constant , can multiply everything by I
IV = IV1 + IV2
Which becomes
R tot = R1 + R2
Parallel resistance
We know from Kirsch of first tfotal current Id dumb of Esch branch
I = I1 + I2
And from second law each loop has same emf so
I/V = I1/V + I2/V
So = conduction = 1/R
1/R = 1R1 (in each branch)
Thus tinsolve need to collapse branch then each branch
Thing to help you incase stuck, what other quantity is conserved for the total
Power of Esch component= power of total energy source!
